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Fix empty-barrier handling in OpenQASM 2 (#10469) (#10491)

The new parser would allow a `barrier;` statement, implicitly
broadcasting it across all qubits in scope.  This is technically not
supported by the OpenQASM 2 specification, but is a useful
quality-of-life extension to the specification (in the same way that
Qiskit interprets barriers, and the OpenQASM 3 specification defines the
`barrier;` statement).  The precise rule is added to the new parser's
`strict` mode.

The OpenQASM 2 _exporter_ similarly should not have been putting out
`barrier;` statements.  These could only occur in Qiskit when a barrier
was explicitly constructed with zero elements (as opposed to the call
`QuantumCircuit.barrier()`, which has the all-in-scope behaviour), and
consequently have no actual meaning or effect.  The exporter is modified
to simply skip such instructions, for as long as Qiskit permits the
qubitless barrier statement.

(cherry picked from commit e75893d45)

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

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/crates/qasm2/src/parse.rs

// This code is part of Qiskit.
//
// (C) Copyright IBM 2023
//
// This code is licensed under the Apache License, Version 2.0. You may
// obtain a copy of this license in the LICENSE.txt file in the root directory
// of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
//
// Any modifications or derivative works of this code must retain this
// copyright notice, and modified files need to carry a notice indicating
// that they have been altered from the originals.

//! The core of the parsing algorithm.  This module contains the core logic for the
//! recursive-descent parser, which handles all statements of OpenQASM 2.  In places where we have
//! to evaluate a mathematical expression on parameters, we instead swap to a short-lived
//! operator-precedence parser.

use hashbrown::{HashMap, HashSet};
use pyo3::prelude::{PyObject, PyResult, Python};

use crate::bytecode::InternalBytecode;
use crate::error::{
    message_bad_eof, message_generic, message_incorrect_requirement, Position, QASM2ParseError,
};
use crate::expr::{Expr, ExprParser};
use crate::lex::{Token, TokenContext, TokenStream, TokenType, Version};
use crate::{CustomClassical, CustomInstruction};

/// The number of gates that are built in to the OpenQASM 2 language.  This is U and CX.
const N_BUILTIN_GATES: usize = 2;
/// The "qelib1.inc" special include.  The elements of the tuple are the gate name, the number of
/// parameters it takes, and the number of qubits it acts on.
const QELIB1: [(&str, usize, usize); 23] = [
    ("u3", 3, 1),
    ("u2", 2, 1),
    ("u1", 1, 1),
    ("cx", 0, 2),
    ("id", 0, 1),
    ("x", 0, 1),
    ("y", 0, 1),
    ("z", 0, 1),
    ("h", 0, 1),
    ("s", 0, 1),
    ("sdg", 0, 1),
    ("t", 0, 1),
    ("tdg", 0, 1),
    ("rx", 1, 1),
    ("ry", 1, 1),
    ("rz", 1, 1),
    ("cz", 0, 2),
    ("cy", 0, 2),
    ("ch", 0, 2),
    ("ccx", 0, 3),
    ("crz", 1, 2),
    ("cu1", 1, 2),
    ("cu3", 3, 2),
];

const BUILTIN_CLASSICAL: [&str; 6] = ["cos", "exp", "ln", "sin", "sqrt", "tan"];

/// Define a simple newtype that just has a single non-public `usize` field, has a `new`
/// constructor, and implements `Copy` and `IntoPy`.  The first argument is the name of the type,
/// the second is whether to also define addition to make offsetting the newtype easier.
macro_rules! newtype_id {
    ($id:ident, false) => {
        #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
        pub struct $id(usize);

        impl $id {
            pub fn new(value: usize) -> Self {
                Self(value)
            }
        }

        impl pyo3::IntoPy<PyObject> for $id {
            fn into_py(self, py: Python<'_>) -> PyObject {
                self.0.into_py(py)
            }
        }
    };

    ($id:ident, true) => {
        newtype_id!($id, false);

        impl std::ops::Add<usize> for $id {
            type Output = Self;

            fn add(self, rhs: usize) -> Self {
                Self::new(self.0 + rhs)
            }
        }
    };
}

newtype_id!(GateId, false);
newtype_id!(CregId, false);
newtype_id!(ParamId, false);
newtype_id!(QubitId, true);
newtype_id!(ClbitId, true);

/// A symbol in the global symbol table.  Parameters and individual qubits can't be in the global
/// symbol table, as there is no way for them to be defined.
pub enum GlobalSymbol {
    Qreg {
        size: usize,
        start: QubitId,
    },
    Creg {
        size: usize,
        start: ClbitId,
        index: CregId,
    },
    Gate {
        num_params: usize,
        num_qubits: usize,
        index: GateId,
    },
    Classical {
        callable: PyObject,
        num_params: usize,
    },
}

impl GlobalSymbol {
    pub fn describe(&self) -> &'static str {
        match self {
            Self::Qreg { .. } => "a quantum register",
            Self::Creg { .. } => "a classical register",
            Self::Gate { .. } => "a gate",
            Self::Classical { .. } => "a custom classical function",
        }
    }
}

/// Information about a gate that permits a new definition in the OQ3 file only in the case that
/// the number of parameters and qubits matches.  This is used when the user specifies custom gates
/// that are not
struct OverridableGate {
    num_params: usize,
    num_qubits: usize,
    index: GateId,
}

impl From<OverridableGate> for GlobalSymbol {
    fn from(value: OverridableGate) -> Self {
        Self::Gate {
            num_params: value.num_params,
            num_qubits: value.num_qubits,
            index: value.index,
        }
    }
}

/// A symbol in the scope of a single gate definition.  This only includes the symbols that are
/// specifically gate-scoped.  The rest are part of [GlobalSymbol].
pub enum GateSymbol {
    Qubit { index: QubitId },
    Parameter { index: ParamId },
}

impl GateSymbol {
    pub fn describe(&self) -> &'static str {
        match self {
            Self::Qubit { .. } => "a qubit",
            Self::Parameter { .. } => "a parameter",
        }
    }
}

/// An operand for an instruction.  This can be both quantum or classical.  Classical operands only
/// occur in the `measure` operation.  `Single` variants are what we mostly expect to see; these
/// happen in gate definitions (always), and in regular applications when registers are indexed.
/// The `Range` operand only occurs when a register is used as an operation target.
enum Operand<T> {
    Single(T),
    Range(usize, T),
}

/// The available types for the arrays of parameters in a gate call.  The `Constant` variant is for
/// general applications, whereas the more general `Expression` variant is for gate bodies, where
/// there might be mathematics occurring on symbolic parameters.  We have the special case for the
/// far more common `Constant` form; in this case we immediately unwrap the result of the
/// `ExprParser`, and we won't have to make a new vector with the conversion later.
enum GateParameters {
    Constant(Vec<f64>),
    Expression(Vec<Expr>),
}

/// An equality condition from an `if` statement.  These can condition gate applications, measures
/// and resets, although in practice they're basically only ever used on gates.
struct Condition {
    creg: CregId,
    value: usize,
}

/// Find the first match for the partial [filename] in the directories along [path].  Returns
/// `None` if the cannot be found.
fn find_include_path(
    filename: &std::path::Path,
    path: &[std::path::PathBuf],
) -> Option<std::path::PathBuf> {
    for directory in path.iter() {
        let mut absolute_path = directory.clone();
        absolute_path.push(filename);
        if absolute_path.is_file() {
            return Some(absolute_path);
        }
    }
    None
}

/// The state of the parser (but not its output).  This struct is opaque to the rest of the
/// program; only its associated functions ever need to modify its internals.  The counts of
/// qubits, clbits, classical registers and gates are necessary to efficiently assign index keys to
/// new symbols as they arise.  We don't need to track quantum registers like this because no part
/// of the output instruction set requires a reference to a quantum register, since we resolve any
/// broadcast gate applications from within Rust.
pub struct State {
    tokens: Vec<TokenStream>,
    /// The context object that owns all the text strings that back the tokens seen so far.  This
    /// needs to be given as a read-only reference to the [Token] methods that extract information
    /// based on the text they came from.
    context: TokenContext,
    include_path: Vec<std::path::PathBuf>,
    /// Mapping of name to global-scoped symbols.
    symbols: HashMap<String, GlobalSymbol>,
    /// Mapping of name to gate-scoped symbols.  This object only logically lasts for the duration
    /// of the parsing of one gate definition, but we can save allocations by re-using the same
    /// object between calls.
    gate_symbols: HashMap<String, GateSymbol>,
    /// Gate names that can accept a definition, even if they are already in the symbol table as
    /// gates. For example, if a user defines a custom gate as a `builtin`, we don't error out if
    /// we see a compatible definition later.  Regardless of whether the symbol was already in the
    /// symbol table or not, any gate-based definition of this symbol must match the signature we
    /// expect for it.
    overridable_gates: HashMap<String, OverridableGate>,
    num_qubits: usize,
    num_clbits: usize,
    num_cregs: usize,
    num_gates: usize,
    /// Whether a version statement is allowed in this position.
    allow_version: bool,
    /// Whether we're in strict mode or (the default) more permissive parse.
    strict: bool,
}

impl State {
    /// Create and initialise a state for the parser.
    pub fn new(
        tokens: TokenStream,
        include_path: Vec<std::path::PathBuf>,
        custom_instructions: &[CustomInstruction],
        custom_classical: &[CustomClassical],
        strict: bool,
    ) -> PyResult<Self> {
        let mut state = State {
            tokens: vec![tokens],
            context: TokenContext::new(),
            include_path,
            // For Qiskit-created circuits, all files will have the builtin gates and `qelib1.inc`,
            // so we allocate with that in mind.  There may well be overlap between libraries and
            // custom instructions, but this is small-potatoes allocation and we'd rather not have
            // to reallocate.
            symbols: HashMap::with_capacity(
                N_BUILTIN_GATES + QELIB1.len() + custom_instructions.len() + custom_classical.len(),
            ),
            gate_symbols: HashMap::new(),
            overridable_gates: HashMap::new(),
            num_qubits: 0,
            num_clbits: 0,
            num_cregs: 0,
            num_gates: 0,
            allow_version: true,
            strict,
        };
        for inst in custom_instructions {
            if state.symbols.contains_key(&inst.name)
                || state.overridable_gates.contains_key(&inst.name)
            {
                return Err(QASM2ParseError::new_err(message_generic(
                    None,
                    &format!("duplicate custom instruction '{}'", inst.name),
                )));
            }
            state.overridable_gates.insert(
                inst.name.clone(),
                OverridableGate {
                    num_params: inst.num_params,
                    num_qubits: inst.num_qubits,
                    index: GateId::new(state.num_gates),
                },
            );
            if inst.builtin {
                state.symbols.insert(
                    inst.name.clone(),
                    GlobalSymbol::Gate {
                        num_params: inst.num_params,
                        num_qubits: inst.num_qubits,
                        index: GateId::new(state.num_gates),
                    },
                );
            }
            state.num_gates += 1;
        }
        state.define_gate(None, "U".to_owned(), 3, 1)?;
        state.define_gate(None, "CX".to_owned(), 0, 2)?;
        for classical in custom_classical {
            if BUILTIN_CLASSICAL.contains(&&*classical.name) {
                return Err(QASM2ParseError::new_err(message_generic(
                    None,
                    &format!(
                        "cannot override builtin classical function '{}'",
                        &classical.name
                    ),
                )));
            }
            match state.symbols.insert(
                classical.name.clone(),
                GlobalSymbol::Classical {
                    num_params: classical.num_params,
                    callable: classical.callable.clone(),
                },
            ) {
                Some(GlobalSymbol::Gate { .. }) => {
                    let message = match classical.name.as_str() {
                        "U" | "CX" => format!(
                            "custom classical instructions cannot shadow built-in gates, but got '{}'",
                            &classical.name,
                        ),
                        _ => format!(
                            "custom classical instruction '{}' has a naming clash with a custom gate",
                            &classical.name,
                        ),
                    };
                    return Err(QASM2ParseError::new_err(message_generic(None, &message)));
                }
                Some(GlobalSymbol::Classical { .. }) => {
                    return Err(QASM2ParseError::new_err(message_generic(
                        None,
                        &format!("duplicate custom classical function '{}'", &classical.name,),
                    )));
                }
                _ => (),
            }
        }
        Ok(state)
    }

    /// Get the next token available in the stack of token streams, popping and removing any
    /// complete streams, except the base case.  Will only return `None` once all streams are
    /// exhausted.
    fn next_token(&mut self) -> PyResult<Option<Token>> {
        let mut pointer = self.tokens.len() - 1;
        while pointer > 0 {
            let out = self.tokens[pointer].next(&mut self.context)?;
            if out.is_some() {
                return Ok(out);
            }
            self.tokens.pop();
            pointer -= 1;
        }
        self.tokens[0].next(&mut self.context)
    }

    /// Peek the next token in the stack of token streams.  This does not remove any complete
    /// streams yet.  Will only return `None` once all streams are exhausted.
    fn peek_token(&mut self) -> PyResult<Option<&Token>> {
        let mut pointer = self.tokens.len() - 1;
        while pointer > 0 && self.tokens[pointer].peek(&mut self.context)?.is_none() {
            pointer -= 1;
        }
        self.tokens[pointer].peek(&mut self.context)
    }

    /// Get the filename associated with the currently active token stream.
    fn current_filename(&self) -> &std::ffi::OsStr {
        &self.tokens[self.tokens.len() - 1].filename
    }

    /// Take a token from the stream that is known to be present and correct, generally because it
    /// has already been peeked.  Panics if the token type is not correct.
    fn expect_known(&mut self, expected: TokenType) -> Token {
        let out = self.next_token().unwrap().unwrap();
        if out.ttype != expected {
            panic!(
                "expected '{}' but got '{}'",
                expected.describe(),
                out.ttype.describe()
            )
        }
        out
    }

    /// Take the next token from the stream, expecting that it is of a particular type because it
    /// is required to be in order for the input program to be valid OpenQASM 2.  This returns the
    /// token if successful, and a suitable error message if the token type is incorrect, or the
    /// end of the file is reached.
    fn expect(&mut self, expected: TokenType, required: &str, cause: &Token) -> PyResult<Token> {
        let token = match self.next_token()? {
            None => {
                return Err(QASM2ParseError::new_err(message_bad_eof(
                    Some(&Position::new(
                        self.current_filename(),
                        cause.line,
                        cause.col,
                    )),
                    required,
                )))
            }
            Some(token) => token,
        };
        if token.ttype == expected {
            Ok(token)
        } else {
            Err(QASM2ParseError::new_err(message_incorrect_requirement(
                required,
                &token,
                self.current_filename(),
            )))
        }
    }

    /// Take the next token from the stream, if it is of the correct type.  Returns `None` and
    /// leaves the next token in the underlying iterator if it does not match.
    fn accept(&mut self, expected: TokenType) -> PyResult<Option<Token>> {
        let peeked = self.peek_token()?;
        if peeked.is_some() && peeked.unwrap().ttype == expected {
            self.next_token()
        } else {
            Ok(None)
        }
    }

    /// True if the next token in the stream matches the given type, and false if it doesn't.
    fn next_is(&mut self, expected: TokenType) -> PyResult<bool> {
        let peeked = self.peek_token()?;
        Ok(peeked.is_some() && peeked.unwrap().ttype == expected)
    }

    /// If in `strict` mode, and we have a trailing comma, emit a suitable error message.
    fn check_trailing_comma(&self, comma: Option<&Token>) -> PyResult<()> {
        match (self.strict, comma) {
            (true, Some(token)) => Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    token.line,
                    token.col,
                )),
                "[strict] trailing commas in parameter and qubit lists are forbidden",
            ))),
            _ => Ok(()),
        }
    }

    /// Take a complete quantum argument from the token stream, if the next token is an identifier.
    /// This includes resolving any following subscript operation.  Returns an error variant if the
    /// next token _is_ an identifier, but the symbol represents something other than a quantum
    /// register, or isn't defined.  This can also be an error if the subscript is opened, but
    /// cannot be completely resolved due to a typing error or other invalid parse.  `Ok(None)` is
    /// returned if the next token in the stream does not match a possible quantum argument.
    fn accept_qarg(&mut self) -> PyResult<Option<Operand<QubitId>>> {
        let (name, name_token) = match self.accept(TokenType::Id)? {
            None => return Ok(None),
            Some(token) => (token.id(&self.context), token),
        };
        let (register_size, register_start) = match self.symbols.get(&name) {
            Some(GlobalSymbol::Qreg { size, start }) => (*size, *start),
            Some(symbol) => {
                return Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        name_token.line,
                        name_token.col,
                    )),
                    &format!(
                        "'{}' is {}, not a quantum register",
                        name,
                        symbol.describe()
                    ),
                )))
            }
            None => {
                return Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        name_token.line,
                        name_token.col,
                    )),
                    &format!("'{}' is not defined in this scope", name),
                )))
            }
        };
        self.complete_operand(&name, register_size, register_start)
            .map(Some)
    }

    /// Take a complete quantum argument from the stream, if it matches.  This is for use within
    /// gates, and so the only valid type of quantum argument is a single qubit.
    fn accept_qarg_gate(&mut self) -> PyResult<Option<Operand<QubitId>>> {
        let (name, name_token) = match self.accept(TokenType::Id)? {
            None => return Ok(None),
            Some(token) => (token.id(&self.context), token),
        };
        match self.gate_symbols.get(&name) {
            Some(GateSymbol::Qubit { index }) => Ok(Some(Operand::Single(*index))),
            Some(GateSymbol::Parameter { .. }) => Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    name_token.line,
                    name_token.col,
                )),
                &format!("'{}' is a parameter, not a qubit", name),
            ))),
            None => {
                if let Some(symbol) = self.symbols.get(&name) {
                    Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            name_token.line,
                            name_token.col,
                        )),
                        &format!("'{}' is {}, not a qubit", name, symbol.describe()),
                    )))
                } else {
                    Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            name_token.line,
                            name_token.col,
                        )),
                        &format!("'{}' is not defined in this scope", name),
                    )))
                }
            }
        }
    }

    /// Take a complete quantum argument from the token stream, returning an error message if one
    /// is not present.
    fn require_qarg(&mut self, instruction: &Token) -> PyResult<Operand<QubitId>> {
        match self.peek_token()?.map(|tok| tok.ttype) {
            Some(TokenType::Id) => self.accept_qarg().map(Option::unwrap),
            Some(_) => {
                let token = self.next_token()?;
                Err(QASM2ParseError::new_err(message_incorrect_requirement(
                    "a quantum argument",
                    &token.unwrap(),
                    self.current_filename(),
                )))
            }
            None => Err(QASM2ParseError::new_err(message_bad_eof(
                Some(&Position::new(
                    self.current_filename(),
                    instruction.line,
                    instruction.col,
                )),
                "a quantum argument",
            ))),
        }
    }

    /// Take a complete classical argument from the token stream, if the next token is an
    /// identifier.  This includes resolving any following subscript operation.  Returns an error
    /// variant if the next token _is_ an identifier, but the symbol represents something other
    /// than a classical register, or isn't defined.  This can also be an error if the subscript is
    /// opened, but cannot be completely resolved due to a typing error or other invalid parse.
    /// `Ok(None)` is returned if the next token in the stream does not match a possible classical
    /// argument.
    fn accept_carg(&mut self) -> PyResult<Option<Operand<ClbitId>>> {
        let (name, name_token) = match self.accept(TokenType::Id)? {
            None => return Ok(None),
            Some(token) => (token.id(&self.context), token),
        };
        let (register_size, register_start) = match self.symbols.get(&name) {
            Some(GlobalSymbol::Creg { size, start, .. }) => (*size, *start),
            Some(symbol) => {
                return Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        name_token.line,
                        name_token.col,
                    )),
                    &format!(
                        "'{}' is {}, not a classical register",
                        name,
                        symbol.describe()
                    ),
                )))
            }
            None => {
                return Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        name_token.line,
                        name_token.col,
                    )),
                    &format!("'{}' is not defined in this scope", name),
                )))
            }
        };
        self.complete_operand(&name, register_size, register_start)
            .map(Some)
    }

    /// Take a complete classical argument from the token stream, returning an error message if one
    /// is not present.
    fn require_carg(&mut self, instruction: &Token) -> PyResult<Operand<ClbitId>> {
        match self.peek_token()?.map(|tok| tok.ttype) {
            Some(TokenType::Id) => self.accept_carg().map(Option::unwrap),
            Some(_) => {
                let token = self.next_token()?;
                Err(QASM2ParseError::new_err(message_incorrect_requirement(
                    "a classical argument",
                    &token.unwrap(),
                    self.current_filename(),
                )))
            }
            None => Err(QASM2ParseError::new_err(message_bad_eof(
                Some(&Position::new(
                    self.current_filename(),
                    instruction.line,
                    instruction.col,
                )),
                "a classical argument",
            ))),
        }
    }

    /// Evaluate a possible subscript on a register into a final [Operand], consuming the tokens
    /// (if present) from the stream.  Can return error variants if the subscript cannot be
    /// completed or if there is a parse error while reading the subscript.
    fn complete_operand<T>(
        &mut self,
        name: &str,
        register_size: usize,
        register_start: T,
    ) -> PyResult<Operand<T>>
    where
        T: std::ops::Add<usize, Output = T>,
    {
        let lbracket_token = match self.accept(TokenType::LBracket)? {
            Some(token) => token,
            None => return Ok(Operand::Range(register_size, register_start)),
        };
        let index_token = self.expect(TokenType::Integer, "an integer index", &lbracket_token)?;
        let index = index_token.int(&self.context);
        self.expect(TokenType::RBracket, "a closing bracket", &lbracket_token)?;
        if index < register_size {
            Ok(Operand::Single(register_start + index))
        } else {
            Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    index_token.line,
                    index_token.col,
                )),
                &format!(
                    "index {} is out-of-range for register '{}' of size {}",
                    index, name, register_size
                ),
            )))
        }
    }

    /// Parse an `OPENQASM <version>;` statement completely.  This function does not need to take
    /// the bytecode stream because the version information has no actionable effects for Qiskit
    /// to care about.  We simply error if the version supplied by the file is not the version of
    /// OpenQASM that we are able to support.  This assumes that the `OPENQASM` token is still in
    /// the stream.
    fn parse_version(&mut self) -> PyResult<usize> {
        let openqasm_token = self.expect_known(TokenType::OpenQASM);
        let version_token = self.expect(TokenType::Version, "version number", &openqasm_token)?;
        match version_token.version(&self.context) {
            Version {
                major: 2,
                minor: Some(0) | None,
            } => Ok(()),
            _ => Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    version_token.line,
                    version_token.col,
                )),
                &format!(
                    "can only handle OpenQASM 2.0, but given {}",
                    version_token.text(&self.context),
                ),
            ))),
        }?;
        self.expect(TokenType::Semicolon, ";", &openqasm_token)?;
        Ok(0)
    }

    /// Parse a complete gate definition (including the body of the definition).  This assumes that
    /// the `gate` token is still in the scheme.  This function will likely result in many
    /// instructions being pushed onto the bytecode stream; one for the start and end of the gate
    /// definition, and then one instruction each for the gate applications in the body.
    fn parse_gate_definition(&mut self, bc: &mut Vec<Option<InternalBytecode>>) -> PyResult<usize> {
        let gate_token = self.expect_known(TokenType::Gate);
        let name_token = self.expect(TokenType::Id, "an identifier", &gate_token)?;
        let name = name_token.id(&self.context);
        // Parse the gate parameters (if any) into the symbol take.
        let mut num_params = 0usize;
        if let Some(lparen_token) = self.accept(TokenType::LParen)? {
            let mut comma = None;
            while let Some(param_token) = self.accept(TokenType::Id)? {
                let param_name = param_token.id(&self.context);
                if let Some(symbol) = self.gate_symbols.insert(
                    param_name.to_owned(),
                    GateSymbol::Parameter {
                        index: ParamId::new(num_params),
                    },
                ) {
                    return Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            param_token.line,
                            param_token.col,
                        )),
                        &format!(
                            "'{}' is already defined as {}",
                            param_name,
                            symbol.describe()
                        ),
                    )));
                }
                num_params += 1;
                comma = self.accept(TokenType::Comma)?;
                if comma.is_none() {
                    break;
                }
            }
            self.check_trailing_comma(comma.as_ref())?;
            self.expect(TokenType::RParen, "a closing parenthesis", &lparen_token)?;
        }
        // Parse the quantum parameters into the symbol table.
        let mut num_qubits = 0usize;
        let mut comma = None;
        while let Some(qubit_token) = self.accept(TokenType::Id)? {
            let qubit_name = qubit_token.id(&self.context).to_owned();
            if let Some(symbol) = self.gate_symbols.insert(
                qubit_name.to_owned(),
                GateSymbol::Qubit {
                    index: QubitId::new(num_qubits),
                },
            ) {
                return Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        qubit_token.line,
                        qubit_token.col,
                    )),
                    &format!(
                        "'{}' is already defined as {}",
                        qubit_name,
                        symbol.describe()
                    ),
                )));
            }
            num_qubits += 1;
            comma = self.accept(TokenType::Comma)?;
            if comma.is_none() {
                break;
            }
        }
        self.check_trailing_comma(comma.as_ref())?;
        if num_qubits == 0 {
            let eof = self.peek_token()?.is_none();
            let position = Position::new(self.current_filename(), gate_token.line, gate_token.col);
            return if eof {
                Err(QASM2ParseError::new_err(message_bad_eof(
                    Some(&position),
                    "a qubit identifier",
                )))
            } else {
                Err(QASM2ParseError::new_err(message_generic(
                    Some(&position),
                    "gates must act on at least one qubit",
                )))
            };
        }
        let lbrace_token = self.expect(TokenType::LBrace, "a gate body", &gate_token)?;
        bc.push(Some(InternalBytecode::DeclareGate {
            name: name.clone(),
            num_qubits,
        }));
        // The actual body of the gate.  Most of this is devolved to [Self::parse_gate_application]
        // to do the right thing.
        let mut statements = 0usize;
        loop {
            match self.peek_token()?.map(|tok| tok.ttype) {
                Some(TokenType::Id) => statements += self.parse_gate_application(bc, None, true)?,
                Some(TokenType::Barrier) => {
                    statements += self.parse_barrier(bc, Some(num_qubits))?
                }
                Some(TokenType::RBrace) => {
                    self.expect_known(TokenType::RBrace);
                    break;
                }
                Some(_) => {
                    let token = self.next_token()?.unwrap();
                    return Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            token.line,
                            token.col,
                        )),
                        &format!(
                            "only gate applications are valid within a 'gate' body, but saw {}",
                            token.text(&self.context)
                        ),
                    )));
                }
                None => {
                    return Err(QASM2ParseError::new_err(message_bad_eof(
                        Some(&Position::new(
                            self.current_filename(),
                            lbrace_token.line,
                            lbrace_token.col,
                        )),
                        "a closing brace '}' of the gate body",
                    )))
                }
            }
        }
        bc.push(Some(InternalBytecode::EndDeclareGate {}));
        self.gate_symbols.clear();
        self.define_gate(Some(&gate_token), name, num_params, num_qubits)?;
        Ok(statements + 2)
    }

    /// Parse an `opaque` statement.  This assumes that the `opaque` token is still in the token
    /// stream we are reading from.
    fn parse_opaque_definition(
        &mut self,
        bc: &mut Vec<Option<InternalBytecode>>,
    ) -> PyResult<usize> {
        let opaque_token = self.expect_known(TokenType::Opaque);
        let name = self
            .expect(TokenType::Id, "an identifier", &opaque_token)?
            .text(&self.context)
            .to_owned();
        let mut num_params = 0usize;
        if let Some(lparen_token) = self.accept(TokenType::LParen)? {
            let mut comma = None;
            while self.accept(TokenType::Id)?.is_some() {
                num_params += 1;
                comma = self.accept(TokenType::Comma)?;
                if comma.is_none() {
                    break;
                }
            }
            self.check_trailing_comma(comma.as_ref())?;
            self.expect(TokenType::RParen, "closing parenthesis", &lparen_token)?;
        }
        let mut num_qubits = 0usize;
        let mut comma = None;
        while self.accept(TokenType::Id)?.is_some() {
            num_qubits += 1;
            comma = self.accept(TokenType::Comma)?;
            if comma.is_none() {
                break;
            }
        }
        self.check_trailing_comma(comma.as_ref())?;
        self.expect(TokenType::Semicolon, ";", &opaque_token)?;
        if num_qubits == 0 {
            return Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    opaque_token.line,
                    opaque_token.col,
                )),
                "gates must act on at least one qubit",
            )));
        }
        bc.push(Some(InternalBytecode::DeclareOpaque {
            name: name.clone(),
            num_qubits,
        }));
        self.define_gate(Some(&opaque_token), name, num_params, num_qubits)?;
        Ok(1)
    }

    /// Parse a gate application into the bytecode stream.  This resolves any broadcast over
    /// registers into a series of bytecode instructions, rather than leaving Qiskit to do it,
    /// which would involve much slower Python execution.  This assumes that the identifier token
    /// is still in the token stream.
    fn parse_gate_application(
        &mut self,
        bc: &mut Vec<Option<InternalBytecode>>,
        condition: Option<Condition>,
        in_gate: bool,
    ) -> PyResult<usize> {
        let name_token = self.expect_known(TokenType::Id);
        let name = name_token.id(&self.context);
        let (index, num_params, num_qubits) = match self.symbols.get(&name) {
            Some(GlobalSymbol::Gate {
                num_params,
                num_qubits,
                index,
            }) => Ok((*index, *num_params, *num_qubits)),
            Some(symbol) => Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    name_token.line,
                    name_token.col,
                )),
                &format!("'{}' is {}, not a gate", name, symbol.describe()),
            ))),
            None => {
                let pos = Position::new(self.current_filename(), name_token.line, name_token.col);
                let message = if self.overridable_gates.contains_key(&name) {
                    format!(
                        "cannot use non-builtin custom instruction '{}' before definition",
                        name,
                    )
                } else {
                    format!("'{}' is not defined in this scope", name)
                };
                Err(QASM2ParseError::new_err(message_generic(
                    Some(&pos),
                    &message,
                )))
            }
        }?;
        let parameters = self.expect_gate_parameters(&name_token, num_params, in_gate)?;
        let mut qargs = Vec::<Operand<QubitId>>::with_capacity(num_qubits);
        let mut comma = None;
        if in_gate {
            while let Some(qarg) = self.accept_qarg_gate()? {
                qargs.push(qarg);
                comma = self.accept(TokenType::Comma)?;
                if comma.is_none() {
                    break;
                }
            }
        } else {
            while let Some(qarg) = self.accept_qarg()? {
                qargs.push(qarg);
                comma = self.accept(TokenType::Comma)?;
                if comma.is_none() {
                    break;
                }
            }
        }
        self.check_trailing_comma(comma.as_ref())?;
        if qargs.len() != num_qubits {
            return match self.peek_token()?.map(|tok| tok.ttype) {
                Some(TokenType::Semicolon) => Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        name_token.line,
                        name_token.col,
                    )),
                    &format!(
                        "'{}' takes {} quantum argument{}, but got {}",
                        name,
                        num_qubits,
                        if num_qubits == 1 { "" } else { "s" },
                        qargs.len()
                    ),
                ))),
                Some(_) => Err(QASM2ParseError::new_err(message_incorrect_requirement(
                    "the end of the argument list",
                    &name_token,
                    self.current_filename(),
                ))),
                None => Err(QASM2ParseError::new_err(message_bad_eof(
                    Some(&Position::new(
                        self.current_filename(),
                        name_token.line,
                        name_token.col,
                    )),
                    "the end of the argument list",
                ))),
            };
        }
        self.expect(TokenType::Semicolon, "';'", &name_token)?;
        self.emit_gate_application(bc, &name_token, index, parameters, &qargs, condition)
    }

    /// Parse the parameters (if any) from a gate application.
    fn expect_gate_parameters(
        &mut self,
        name_token: &Token,
        num_params: usize,
        in_gate: bool,
    ) -> PyResult<GateParameters> {
        let lparen_token = match self.accept(TokenType::LParen)? {
            Some(lparen_token) => lparen_token,
            None => {
                return Ok(if in_gate {
                    GateParameters::Expression(vec![])
                } else {
                    GateParameters::Constant(vec![])
                });
            }
        };
        let mut seen_params = 0usize;
        let mut comma = None;
        // This code duplication is to avoid duplication of allocation when parsing the far more
        // common case of expecting constant parameters for a gate application in the body of the
        // OQ2 file.
        let parameters = if in_gate {
            let mut parameters = Vec::<Expr>::with_capacity(num_params);
            while !self.next_is(TokenType::RParen)? {
                let mut expr_parser = ExprParser {
                    tokens: &mut self.tokens,
                    context: &mut self.context,
                    gate_symbols: &self.gate_symbols,
                    global_symbols: &self.symbols,
                    strict: self.strict,
                };
                parameters.push(expr_parser.parse_expression(&lparen_token)?);
                seen_params += 1;
                comma = self.accept(TokenType::Comma)?;
                if comma.is_none() {
                    break;
                }
            }
            self.expect(TokenType::RParen, "')'", &lparen_token)?;
            GateParameters::Expression(parameters)
        } else {
            let mut parameters = Vec::<f64>::with_capacity(num_params);
            while !self.next_is(TokenType::RParen)? {
                let mut expr_parser = ExprParser {
                    tokens: &mut self.tokens,
                    context: &mut self.context,
                    gate_symbols: &self.gate_symbols,
                    global_symbols: &self.symbols,
                    strict: self.strict,
                };
                match expr_parser.parse_expression(&lparen_token)? {
                    Expr::Constant(value) => parameters.push(value),
                    _ => {
                        return Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                lparen_token.line,
                                lparen_token.col,
                            )),
                            "non-constant expression in program body",
                        )))
                    }
                }
                seen_params += 1;
                comma = self.accept(TokenType::Comma)?;
                if comma.is_none() {
                    break;
                }
            }
            self.expect(TokenType::RParen, "')'", &lparen_token)?;
            GateParameters::Constant(parameters)
        };
        self.check_trailing_comma(comma.as_ref())?;
        if seen_params != num_params {
            return Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    name_token.line,
                    name_token.col,
                )),
                &format!(
                    "'{}' takes {} parameter{}, but got {}",
                    &name_token.text(&self.context),
                    num_params,
                    if num_params == 1 { "" } else { "s" },
                    seen_params
                ),
            )));
        }
        Ok(parameters)
    }

    /// Emit the bytecode for the application of a gate.  This involves resolving any broadcasts
    /// in the operands of the gate (i.e. if one or more of them is a register).
    fn emit_gate_application(
        &self,
        bc: &mut Vec<Option<InternalBytecode>>,
        instruction: &Token,
        gate_id: GateId,
        parameters: GateParameters,
        qargs: &[Operand<QubitId>],
        condition: Option<Condition>,
    ) -> PyResult<usize> {
        // Fast path for most common gate patterns that don't need broadcasting.
        if let Some(qubits) = match qargs {
            [Operand::Single(index)] => Some(vec![*index]),
            [Operand::Single(left), Operand::Single(right)] => {
                if *left == *right {
                    return Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            instruction.line,
                            instruction.col,
                        )),
                        "duplicate qubits in gate application",
                    )));
                }
                Some(vec![*left, *right])
            }
            [] => Some(vec![]),
            _ => None,
        } {
            return match parameters {
                GateParameters::Constant(parameters) => {
                    self.emit_single_global_gate(bc, gate_id, parameters, qubits, &condition)
                }
                GateParameters::Expression(parameters) => {
                    self.emit_single_gate_gate(bc, gate_id, parameters, qubits)
                }
            };
        };
        // If we're here we either have to broadcast or it's a 3+q gate - either way, we're not as
        // worried about performance.
        let mut qubits = HashSet::<QubitId>::with_capacity(qargs.len());
        let mut broadcast_length = 0usize;
        for qarg in qargs {
            match qarg {
                Operand::Single(index) => {
                    if !qubits.insert(*index) {
                        return Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                instruction.line,
                                instruction.col,
                            )),
                            "duplicate qubits in gate application",
                        )));
                    }
                }
                Operand::Range(size, start) => {
                    if broadcast_length != 0 && broadcast_length != *size {
                        return Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                instruction.line,
                                instruction.col,
                            )),
                            "cannot resolve broadcast in gate application",
                        )));
                    }
                    for offset in 0..*size {
                        if !qubits.insert(*start + offset) {
                            return Err(QASM2ParseError::new_err(message_generic(
                                Some(&Position::new(
                                    self.current_filename(),
                                    instruction.line,
                                    instruction.col,
                                )),
                                "duplicate qubits in gate application",
                            )));
                        }
                    }
                    broadcast_length = *size;
                }
            }
        }
        if broadcast_length == 0 {
            let qubits = qargs
                .iter()
                .filter_map(|qarg| {
                    if let Operand::Single(index) = qarg {
                        Some(*index)
                    } else {
                        None
                    }
                })
                .collect::<Vec<_>>();
            if qubits.len() < qargs.len() {
                // We're broadcasting against at least one empty register.
                return Ok(0);
            }
            return match parameters {
                GateParameters::Constant(parameters) => {
                    self.emit_single_global_gate(bc, gate_id, parameters, qubits, &condition)
                }
                GateParameters::Expression(parameters) => {
                    self.emit_single_gate_gate(bc, gate_id, parameters, qubits)
                }
            };
        }
        for i in 0..broadcast_length {
            let qubits = qargs
                .iter()
                .map(|qarg| match qarg {
                    Operand::Single(index) => *index,
                    Operand::Range(_, start) => *start + i,
                })
                .collect::<Vec<_>>();
            match parameters {
                GateParameters::Constant(ref parameters) => {
                    self.emit_single_global_gate(
                        bc,
                        gate_id,
                        parameters.clone(),
                        qubits,
                        &condition,
                    )?;
                }
                // Gates used in gate-body definitions can't ever broadcast, because their only
                // operands are single qubits.
                _ => unreachable!(),
            }
        }
        Ok(broadcast_length)
    }

    /// Emit the bytecode for a single gate application in the global scope.  This could
    /// potentially have a classical condition.
    fn emit_single_global_gate(
        &self,
        bc: &mut Vec<Option<InternalBytecode>>,
        gate_id: GateId,
        arguments: Vec<f64>,
        qubits: Vec<QubitId>,
        condition: &Option<Condition>,
    ) -> PyResult<usize> {
        if let Some(condition) = condition {
            bc.push(Some(InternalBytecode::ConditionedGate {
                id: gate_id,
                arguments,
                qubits,
                creg: condition.creg,
                value: condition.value,
            }));
        } else {
            bc.push(Some(InternalBytecode::Gate {
                id: gate_id,
                arguments,
                qubits,
            }));
        }
        Ok(1)
    }

    /// Emit the bytecode for a single gate application in a gate-definition body.  These are not
    /// allowed to be conditioned, because otherwise the containing `gate` would not be unitary.
    fn emit_single_gate_gate(
        &self,
        bc: &mut Vec<Option<InternalBytecode>>,
        gate_id: GateId,
        arguments: Vec<Expr>,
        qubits: Vec<QubitId>,
    ) -> PyResult<usize> {
        bc.push(Some(InternalBytecode::GateInBody {
            id: gate_id,
            arguments,
            qubits,
        }));
        Ok(1)
    }

    /// Parse a complete conditional statement, including the operation that follows the condition
    /// (though this work is delegated to the requisite other grammar rule).  This assumes that the
    /// `if` token is still on the token stream.
    fn parse_conditional(&mut self, bc: &mut Vec<Option<InternalBytecode>>) -> PyResult<usize> {
        let if_token = self.expect_known(TokenType::If);
        let lparen_token = self.expect(TokenType::LParen, "'('", &if_token)?;
        let name_token = self.expect(TokenType::Id, "classical register", &if_token)?;
        self.expect(TokenType::Equals, "'=='", &if_token)?;
        let value = self
            .expect(TokenType::Integer, "an integer", &if_token)?
            .int(&self.context);
        self.expect(TokenType::RParen, "')'", &lparen_token)?;
        let name = name_token.id(&self.context);
        let creg = match self.symbols.get(&name) {
            Some(GlobalSymbol::Creg { index, .. }) => Ok(*index),
            Some(symbol) => Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    name_token.line,
                    name_token.col,
                )),
                &format!(
                    "'{}' is {}, not a classical register",
                    name,
                    symbol.describe()
                ),
            ))),
            None => Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    name_token.line,
                    name_token.col,
                )),
                &format!("'{}' is not defined in this scope", name),
            ))),
        }?;
        let condition = Some(Condition { creg, value });
        match self.peek_token()?.map(|tok| tok.ttype) {
            Some(TokenType::Id) => self.parse_gate_application(bc, condition, false),
            Some(TokenType::Measure) => self.parse_measure(bc, condition),
            Some(TokenType::Reset) => self.parse_reset(bc, condition),
            Some(_) => {
                let token = self.next_token()?;
                Err(QASM2ParseError::new_err(message_incorrect_requirement(
                    "a gate application, measurement or reset",
                    &token.unwrap(),
                    self.current_filename(),
                )))
            }
            None => Err(QASM2ParseError::new_err(message_bad_eof(
                Some(&Position::new(
                    self.current_filename(),
                    if_token.line,
                    if_token.col,
                )),
                "a gate, measurement or reset to condition",
            ))),
        }
    }

    /// Parse a barrier statement.  This assumes that the `barrier` token is still in the token
    /// stream.
    fn parse_barrier(
        &mut self,
        bc: &mut Vec<Option<InternalBytecode>>,
        num_gate_qubits: Option<usize>,
    ) -> PyResult<usize> {
        let barrier_token = self.expect_known(TokenType::Barrier);
        let qubits = if !self.next_is(TokenType::Semicolon)? {
            let mut qubits = Vec::new();
            let mut used = HashSet::<QubitId>::new();
            let mut comma = None;
            while let Some(qarg) = if num_gate_qubits.is_some() {
                self.accept_qarg_gate()?
            } else {
                self.accept_qarg()?
            } {
                match qarg {
                    Operand::Single(index) => {
                        if used.insert(index) {
                            qubits.push(index)
                        }
                    }
                    Operand::Range(size, start) => qubits.extend((0..size).filter_map(|offset| {
                        let index = start + offset;
                        if used.insert(index) {
                            Some(index)
                        } else {
                            None
                        }
                    })),
                }
                comma = self.accept(TokenType::Comma)?;
                if comma.is_none() {
                    break;
                }
            }
            self.check_trailing_comma(comma.as_ref())?;
            qubits
        } else if self.strict {
            return Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    barrier_token.line,
                    barrier_token.col,
                )),
                "[strict] barrier statements must have at least one argument",
            )));
        } else if let Some(num_gate_qubits) = num_gate_qubits {
            (0..num_gate_qubits).map(QubitId::new).collect::<Vec<_>>()
        } else {
            (0..self.num_qubits).map(QubitId::new).collect::<Vec<_>>()
        };
        self.expect(TokenType::Semicolon, "';'", &barrier_token)?;
        if qubits.is_empty() {
            Ok(0)
        } else {
            // The qubits are empty iff the only operands are zero-sized registers.  If there's no
            // quantum arguments at all, then `qubits` will contain everything.
            bc.push(Some(InternalBytecode::Barrier { qubits }));
            Ok(1)
        }
    }

    /// Parse a measurement operation into bytecode.  This resolves any broadcast in the
    /// measurement statement into a series of bytecode instructions, so we do more of the work in
    /// Rust space than in Python space.
    fn parse_measure(
        &mut self,
        bc: &mut Vec<Option<InternalBytecode>>,
        condition: Option<Condition>,
    ) -> PyResult<usize> {
        let measure_token = self.expect_known(TokenType::Measure);
        let qarg = self.require_qarg(&measure_token)?;
        self.expect(TokenType::Arrow, "'->'", &measure_token)?;
        let carg = self.require_carg(&measure_token)?;
        self.expect(TokenType::Semicolon, "';'", &measure_token)?;
        if let Some(Condition { creg, value }) = condition {
            match (qarg, carg) {
                (Operand::Single(qubit), Operand::Single(clbit)) => {
                    bc.push(Some(InternalBytecode::ConditionedMeasure {
                        qubit,
                        clbit,
                        creg,
                        value,
                    }));
                    Ok(1)
                }
                (Operand::Range(q_size, q_start), Operand::Range(c_size, c_start))
                    if q_size == c_size =>
                {
                    bc.extend((0..q_size).map(|i| {
                        Some(InternalBytecode::ConditionedMeasure {
                            qubit: q_start + i,
                            clbit: c_start + i,
                            creg,
                            value,
                        })
                    }));
                    Ok(q_size)
                }
                _ => Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        measure_token.line,
                        measure_token.col,
                    )),
                    "cannot resolve broadcast in measurement",
                ))),
            }
        } else {
            match (qarg, carg) {
                (Operand::Single(qubit), Operand::Single(clbit)) => {
                    bc.push(Some(InternalBytecode::Measure { qubit, clbit }));
                    Ok(1)
                }
                (Operand::Range(q_size, q_start), Operand::Range(c_size, c_start))
                    if q_size == c_size =>
                {
                    bc.extend((0..q_size).map(|i| {
                        Some(InternalBytecode::Measure {
                            qubit: q_start + i,
                            clbit: c_start + i,
                        })
                    }));
                    Ok(q_size)
                }
                _ => Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        measure_token.line,
                        measure_token.col,
                    )),
                    "cannot resolve broadcast in measurement",
                ))),
            }
        }
    }

    /// Parse a single reset statement.  This resolves any broadcast in the statement, i.e. if the
    /// target is a register rather than a single qubit.  This assumes that the `reset` token is
    /// still in the token stream.
    fn parse_reset(
        &mut self,
        bc: &mut Vec<Option<InternalBytecode>>,
        condition: Option<Condition>,
    ) -> PyResult<usize> {
        let reset_token = self.expect_known(TokenType::Reset);
        let qarg = self.require_qarg(&reset_token)?;
        self.expect(TokenType::Semicolon, "';'", &reset_token)?;
        if let Some(Condition { creg, value }) = condition {
            match qarg {
                Operand::Single(qubit) => {
                    bc.push(Some(InternalBytecode::ConditionedReset {
                        qubit,
                        creg,
                        value,
                    }));
                    Ok(1)
                }
                Operand::Range(size, start) => {
                    bc.extend((0..size).map(|offset| {
                        Some(InternalBytecode::ConditionedReset {
                            qubit: start + offset,
                            creg,
                            value,
                        })
                    }));
                    Ok(size)
                }
            }
        } else {
            match qarg {
                Operand::Single(qubit) => {
                    bc.push(Some(InternalBytecode::Reset { qubit }));
                    Ok(0)
                }
                Operand::Range(size, start) => {
                    bc.extend((0..size).map(|offset| {
                        Some(InternalBytecode::Reset {
                            qubit: start + offset,
                        })
                    }));
                    Ok(size)
                }
            }
        }
    }

    /// Parse a declaration of a classical register, emitting the relevant bytecode and adding the
    /// definition to the relevant parts of the internal symbol tables in the parser state.  This
    /// assumes that the `creg` token is still in the token stream.
    fn parse_creg(&mut self, bc: &mut Vec<Option<InternalBytecode>>) -> PyResult<usize> {
        let creg_token = self.expect_known(TokenType::Creg);
        let name_token = self.expect(TokenType::Id, "a valid identifier", &creg_token)?;
        let name = name_token.id(&self.context);
        let lbracket_token = self.expect(TokenType::LBracket, "'['", &creg_token)?;
        let size = self
            .expect(TokenType::Integer, "an integer", &lbracket_token)?
            .int(&self.context);
        self.expect(TokenType::RBracket, "']'", &lbracket_token)?;
        self.expect(TokenType::Semicolon, "';'", &creg_token)?;
        let symbol = GlobalSymbol::Creg {
            size,
            start: ClbitId::new(self.num_clbits),
            index: CregId::new(self.num_cregs),
        };
        if self.symbols.insert(name.clone(), symbol).is_none() {
            self.num_clbits += size;
            self.num_cregs += 1;
            bc.push(Some(InternalBytecode::DeclareCreg { name, size }));
            Ok(1)
        } else {
            Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    name_token.line,
                    name_token.col,
                )),
                &format!("'{}' is already defined", name_token.id(&self.context)),
            )))
        }
    }

    /// Parse a declaration of a quantum register, emitting the relevant bytecode and adding the
    /// definition to the relevant parts of the internal symbol tables in the parser state.  This
    /// assumes that the `qreg` token is still in the token stream.
    fn parse_qreg(&mut self, bc: &mut Vec<Option<InternalBytecode>>) -> PyResult<usize> {
        let qreg_token = self.expect_known(TokenType::Qreg);
        let name_token = self.expect(TokenType::Id, "a valid identifier", &qreg_token)?;
        let name = name_token.id(&self.context);
        let lbracket_token = self.expect(TokenType::LBracket, "'['", &qreg_token)?;
        let size = self
            .expect(TokenType::Integer, "an integer", &lbracket_token)?
            .int(&self.context);
        self.expect(TokenType::RBracket, "']'", &lbracket_token)?;
        self.expect(TokenType::Semicolon, "';'", &qreg_token)?;
        let symbol = GlobalSymbol::Qreg {
            size,
            start: QubitId::new(self.num_qubits),
        };
        if self.symbols.insert(name.clone(), symbol).is_none() {
            self.num_qubits += size;
            bc.push(Some(InternalBytecode::DeclareQreg { name, size }));
            Ok(1)
        } else {
            Err(QASM2ParseError::new_err(message_generic(
                Some(&Position::new(
                    self.current_filename(),
                    name_token.line,
                    name_token.col,
                )),
                &format!("'{}' is already defined", name_token.id(&self.context)),
            )))
        }
    }

    /// Parse an include statement.  This currently only actually handles includes of `qelib1.inc`,
    /// which aren't actually parsed; the parser has a built-in version of the file that it simply
    /// updates its state with (and the Python side of the parser does the same) rather than
    /// re-parsing the same file every time.  This assumes that the `include` token is still in the
    /// token stream.
    fn parse_include(&mut self, bc: &mut Vec<Option<InternalBytecode>>) -> PyResult<usize> {
        let include_token = self.expect_known(TokenType::Include);
        let filename_token =
            self.expect(TokenType::Filename, "a filename string", &include_token)?;
        self.expect(TokenType::Semicolon, "';'", &include_token)?;
        let filename = filename_token.filename(&self.context);
        if filename == "qelib1.inc" {
            self.symbols.reserve(QELIB1.len());
            let mut indices = Vec::with_capacity(QELIB1.len());
            for (i, (name, num_params, num_qubits)) in QELIB1.iter().enumerate() {
                if self.define_gate(
                    Some(&include_token),
                    name.to_string(),
                    *num_params,
                    *num_qubits,
                )? {
                    indices.push(i);
                }
            }
            bc.push(Some(InternalBytecode::SpecialInclude { indices }));
            Ok(1)
        } else {
            let base_filename = std::path::PathBuf::from(&filename);
            let absolute_filename = find_include_path(&base_filename, &self.include_path)
                .ok_or_else(|| {
                    QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            filename_token.line,
                            filename_token.col,
                        )),
                        &format!(
                            "unable to find '{}' in the include search path",
                            base_filename.display()
                        ),
                    ))
                })?;
            let new_stream =
                TokenStream::from_path(absolute_filename, self.strict).map_err(|err| {
                    QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            filename_token.line,
                            filename_token.col,
                        )),
                        &format!("unable to open file '{}' for reading: {}", &filename, err),
                    ))
                })?;
            self.tokens.push(new_stream);
            self.allow_version = true;
            Ok(0)
        }
    }

    /// Update the parser state with the definition of a particular gate.  This does not emit any
    /// bytecode because not all gate definitions need something passing to Python.  For example,
    /// the Python parser initialises its state including the built-in gates `U` and `CX`, and
    /// handles the `qelib1.inc` include specially as well.
    fn define_gate(
        &mut self,
        owner: Option<&Token>,
        name: String,
        num_params: usize,
        num_qubits: usize,
    ) -> PyResult<bool> {
        let already_defined = |state: &Self, name: String| {
            let pos = owner.map(|tok| Position::new(state.current_filename(), tok.line, tok.col));
            Err(QASM2ParseError::new_err(message_generic(
                pos.as_ref(),
                &format!("'{}' is already defined", name),
            )))
        };
        let mismatched_definitions = |state: &Self, name: String, previous: OverridableGate| {
            let plural = |count: usize, singular: &str| {
                let mut out = format!("{} {}", count, singular);
                if count != 1 {
                    out.push('s');
                }
                out
            };
            let from_custom = format!(
                "{} and {}",
                plural(previous.num_params, "parameter"),
                plural(previous.num_qubits, "qubit")
            );
            let from_program = format!(
                "{} and {}",
                plural(num_params, "parameter"),
                plural(num_qubits, "qubit")
            );
            let pos = owner.map(|tok| Position::new(state.current_filename(), tok.line, tok.col));
            Err(QASM2ParseError::new_err(message_generic(
                pos.as_ref(),
                &format!(
                    concat!(
                        "custom instruction '{}' is mismatched with its definition: ",
                        "OpenQASM program has {}, custom has {}",
                    ),
                    name, from_program, from_custom
                ),
            )))
        };

        if let Some(symbol) = self.overridable_gates.remove(&name) {
            if num_params != symbol.num_params || num_qubits != symbol.num_qubits {
                return mismatched_definitions(self, name, symbol);
            }
            match self.symbols.get(&name) {
                None => {
                    self.symbols.insert(name, symbol.into());
                    self.num_gates += 1;
                    Ok(true)
                }
                Some(GlobalSymbol::Gate { .. }) => {
                    self.symbols.insert(name, symbol.into());
                    Ok(false)
                }
                _ => already_defined(self, name),
            }
        } else if self.symbols.contains_key(&name) {
            already_defined(self, name)
        } else {
            self.symbols.insert(
                name,
                GlobalSymbol::Gate {
                    num_params,
                    num_qubits,
                    index: GateId::new(self.num_gates),
                },
            );
            self.num_gates += 1;
            Ok(true)
        }
    }

    /// Parse the next OpenQASM 2 statement in the program into a series of bytecode instructions.
    ///
    /// This is the principal public worker function of the parser.  One call to this function
    /// parses a single OpenQASM 2 statement, which may expand to several bytecode instructions if
    /// there is any broadcasting to resolve, or if the statement is a gate definition.  A call to
    /// this function that returns `Some` will always have pushed at least one instruction to the
    /// bytecode stream (the number is included).  A return of `None` signals the end of the
    /// iterator.
    pub fn parse_next(
        &mut self,
        bc: &mut Vec<Option<InternalBytecode>>,
    ) -> PyResult<Option<usize>> {
        if self.strict && self.allow_version {
            match self.peek_token()?.map(|tok| tok.ttype) {
                Some(TokenType::OpenQASM) => self.parse_version(),
                Some(_) => {
                    let token = self.next_token()?.unwrap();
                    Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            token.line,
                            token.col,
                        )),
                        "[strict] the first statement must be 'OPENQASM 2.0;'",
                    )))
                }
                None => Err(QASM2ParseError::new_err(message_generic(
                    None,
                    "[strict] saw an empty token stream, but needed a version statement",
                ))),
            }?;
            self.allow_version = false;
        }
        let allow_version = self.allow_version;
        self.allow_version = false;
        while let Some(ttype) = self.peek_token()?.map(|tok| tok.ttype) {
            let emitted = match ttype {
                TokenType::Id => self.parse_gate_application(bc, None, false)?,
                TokenType::Creg => self.parse_creg(bc)?,
                TokenType::Qreg => self.parse_qreg(bc)?,
                TokenType::Include => self.parse_include(bc)?,
                TokenType::Measure => self.parse_measure(bc, None)?,
                TokenType::Reset => self.parse_reset(bc, None)?,
                TokenType::Barrier => self.parse_barrier(bc, None)?,
                TokenType::If => self.parse_conditional(bc)?,
                TokenType::Opaque => self.parse_opaque_definition(bc)?,
                TokenType::Gate => self.parse_gate_definition(bc)?,
                TokenType::OpenQASM => {
                    if allow_version {
                        self.parse_version()?
                    } else {
                        let token = self.next_token()?.unwrap();
                        return Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                token.line,
                                token.col,
                            )),
                            "only the first statement may be a version declaration",
                        )));
                    }
                }
                TokenType::Semicolon => {
                    let token = self.next_token()?.unwrap();
                    if self.strict {
                        return Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                token.line,
                                token.col,
                            )),
                            "[strict] empty statements and/or extra semicolons are forbidden",
                        )));
                    } else {
                        0
                    }
                }
                _ => {
                    let token = self.next_token()?.unwrap();
                    return Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            token.line,
                            token.col,
                        )),
                        &format!(
                            "needed a start-of-statement token, but instead got {}",
                            token.text(&self.context)
                        ),
                    )));
                }
            };
            if emitted > 0 {
                return Ok(Some(emitted));
            }
        }
        Ok(None)
    }
}

Qiskit / qiskit-terra / 5650588439

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