11843862461

Committed 14 Nov 2024 06:30PM UTC coverage: 88.91% (-0.001%) from 88.911%

Build # 11843862461

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Commit Message

Fix process-based non-determinism in `SparsePauliOp.to_matrix` (#13439) (#13440)

* Fix process-based non-determinism in `SparsePauliOp.to_matrix`

The simplification step of the summed Pauli terms in
`SparsePauliOp.to_matrix` had introduced a non-determinism via hash-map
iteration.  In practice, the base hash seed was set per initialisation
of the extension module, then stayed the same.  This is hard to detect
from within tests, but caused unexpected numerical behaviour on
different invocations of the same script.

* Use ahash::RandomState

---------

Co-authored-by: Kevin Hartman <kevin@hart.mn>
(cherry picked from commit 74b32c9d2)

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

Run Details

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94.02

/crates/qasm2/src/expr.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.

//! An operator-precedence subparser used by the main parser for handling parameter expressions.
//! Instances of this subparser are intended to only live for as long as it takes to parse a single
//! parameter.

use core::f64;

use hashbrown::HashMap;
use pyo3::prelude::*;
use pyo3::types::PyTuple;

use crate::bytecode;
use crate::error::{
    message_bad_eof, message_generic, message_incorrect_requirement, Position, QASM2ParseError,
};
use crate::lex::{Token, TokenContext, TokenStream, TokenType};
use crate::parse::{GateSymbol, GlobalSymbol, ParamId};

/// Enum representation of the builtin OpenQASM 2 functions.  The built-in Qiskit parser adds the
/// inverse trigonometric functions, but these are an extension to the version as given in the
/// arXiv paper describing OpenQASM 2.  This enum is essentially just a subset of the [TokenType]
/// enum, to allow for better pattern-match checking in the Rust compiler.
pub enum Function {
    Cos,
    Exp,
    Ln,
    Sin,
    Sqrt,
    Tan,
}

impl From<TokenType> for Function {
    fn from(value: TokenType) -> Self {
        match value {
            TokenType::Cos => Function::Cos,
            TokenType::Exp => Function::Exp,
            TokenType::Ln => Function::Ln,
            TokenType::Sin => Function::Sin,
            TokenType::Sqrt => Function::Sqrt,
            TokenType::Tan => Function::Tan,
            _ => panic!(),
        }
    }
}

impl From<Function> for bytecode::UnaryOpCode {
    fn from(value: Function) -> Self {
        match value {
            Function::Cos => Self::Cos,
            Function::Exp => Self::Exp,
            Function::Ln => Self::Ln,
            Function::Sin => Self::Sin,
            Function::Sqrt => Self::Sqrt,
            Function::Tan => Self::Tan,
        }
    }
}

/// An operator symbol used in the expression parsing.  This is essentially just a subset of the
/// [TokenType] enum (albeit with resolved names) to allow for better pattern-match semantics in
/// the Rust compiler.
#[derive(Clone, Copy)]
enum Op {
    Plus,
    Minus,
    Multiply,
    Divide,
    Power,
}

impl Op {
    fn text(&self) -> &'static str {
        match self {
            Self::Plus => "+",
            Self::Minus => "-",
            Self::Multiply => "*",
            Self::Divide => "/",
            Self::Power => "^",
        }
    }
}

impl From<TokenType> for Op {
    fn from(value: TokenType) -> Self {
        match value {
            TokenType::Plus => Op::Plus,
            TokenType::Minus => Op::Minus,
            TokenType::Asterisk => Op::Multiply,
            TokenType::Slash => Op::Divide,
            TokenType::Caret => Op::Power,
            _ => panic!(),
        }
    }
}

/// An atom of the operator-precedence expression parsing.  This is a stripped-down version of the
/// [Token] and [TokenType] used in the main parser.  We can use a data enum here because we do not
/// need all the expressive flexibility in expecting and accepting many different token types as
/// we do in the main parser; it does not significantly harm legibility to simply do
///
/// ```rust
/// match atom {
///     Atom::Const(val) => (),
///     Atom::Parameter(index) => (),
///     // ...
/// }
/// ```
///
/// where required.
enum Atom {
    LParen,
    RParen,
    Function(Function),
    CustomFunction(PyObject, usize),
    Op(Op),
    Const(f64),
    Parameter(ParamId),
}

/// A tree representation of parameter expressions in OpenQASM 2.  The expression
/// operator-precedence parser will do complete constant folding on operations that only involve
/// floating-point numbers, so these will simply be evaluated into a `Constant` variant rather than
/// represented in full tree form.  For references to the gate parameters, we just store the index
/// of which parameter it is.
pub enum Expr {
    Constant(f64),
    Parameter(ParamId),
    Negate(Box<Expr>),
    Add(Box<Expr>, Box<Expr>),
    Subtract(Box<Expr>, Box<Expr>),
    Multiply(Box<Expr>, Box<Expr>),
    Divide(Box<Expr>, Box<Expr>),
    Power(Box<Expr>, Box<Expr>),
    Function(Function, Box<Expr>),
    CustomFunction(PyObject, Vec<Expr>),
}

impl IntoPy<PyObject> for Expr {
    fn into_py(self, py: Python<'_>) -> PyObject {
        match self {
            Expr::Constant(value) => bytecode::ExprConstant { value }.into_py(py),
            Expr::Parameter(index) => bytecode::ExprArgument { index }.into_py(py),
            Expr::Negate(expr) => bytecode::ExprUnary {
                opcode: bytecode::UnaryOpCode::Negate,
                argument: expr.into_py(py),
            }
            .into_py(py),
            Expr::Add(left, right) => bytecode::ExprBinary {
                opcode: bytecode::BinaryOpCode::Add,
                left: left.into_py(py),
                right: right.into_py(py),
            }
            .into_py(py),
            Expr::Subtract(left, right) => bytecode::ExprBinary {
                opcode: bytecode::BinaryOpCode::Subtract,
                left: left.into_py(py),
                right: right.into_py(py),
            }
            .into_py(py),
            Expr::Multiply(left, right) => bytecode::ExprBinary {
                opcode: bytecode::BinaryOpCode::Multiply,
                left: left.into_py(py),
                right: right.into_py(py),
            }
            .into_py(py),
            Expr::Divide(left, right) => bytecode::ExprBinary {
                opcode: bytecode::BinaryOpCode::Divide,
                left: left.into_py(py),
                right: right.into_py(py),
            }
            .into_py(py),
            Expr::Power(left, right) => bytecode::ExprBinary {
                opcode: bytecode::BinaryOpCode::Power,
                left: left.into_py(py),
                right: right.into_py(py),
            }
            .into_py(py),
            Expr::Function(func, expr) => bytecode::ExprUnary {
                opcode: func.into(),
                argument: expr.into_py(py),
            }
            .into_py(py),
            Expr::CustomFunction(func, exprs) => bytecode::ExprCustom {
                callable: func,
                arguments: exprs.into_iter().map(|expr| expr.into_py(py)).collect(),
            }
            .into_py(py),
        }
    }
}

/// Calculate the binding power of an [Op] when used in a prefix position.  Returns [None] if the
/// operation cannot be used in the prefix position.  The binding power is on the same scale as
/// those returned by [binary_power].
fn prefix_power(op: Op) -> Option<u8> {
    match op {
        Op::Plus | Op::Minus => Some(5),
        _ => None,
    }
}

/// Calculate the binding power of an [Op] when used in an infix position.  The differences between
/// left- and right-binding powers represent the associativity of the operation.
fn binary_power(op: Op) -> (u8, u8) {
    // For new binding powers, use the odd number as the "base" and the even number one larger than
    // it to represent the associativity.  Left-associative operators bind more strongly to the
    // operand on their right (i.e. in `a + b + c`, the first `+` binds to the `b` more tightly
    // than the second, so we get the left-associative form), and right-associative operators bind
    // more strongly to the operand of their left.  The separation of using the odd--even pair is
    // so there's no clash between different operator levels, even accounting for the associativity
    // distinction.
    //
    // All powers should be greater than zero; we need zero free to be the base case in the
    // entry-point to the precedence parser.
    match op {
        Op::Plus | Op::Minus => (1, 2),
        Op::Multiply | Op::Divide => (3, 4),
        Op::Power => (8, 7),
    }
}

/// A subparser used to do the operator-precedence part of the parsing for individual parameter
/// expressions.  The main parser creates a new instance of this struct for each expression it
/// expects, and the instance lives only as long as is required to parse that expression, because
/// it takes temporary responsibility for the [TokenStream] that backs the main parser.
pub struct ExprParser<'a> {
    pub tokens: &'a mut Vec<TokenStream>,
    pub context: &'a mut TokenContext,
    pub gate_symbols: &'a HashMap<String, GateSymbol>,
    pub global_symbols: &'a HashMap<String, GlobalSymbol>,
    pub strict: bool,
}

impl<'a> ExprParser<'a> {
    /// 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 > 1 {
            let out = self.tokens[pointer].next(self.context)?;
            if out.is_some() {
                return Ok(out);
            }
            self.tokens.pop();
            pointer -= 1;
        }
        self.tokens[0].next(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 > 1 && self.tokens[pointer].peek(self.context)?.is_none() {
            pointer -= 1;
        }
        self.tokens[pointer].peek(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
    }

    /// Expect a token of the correct [TokenType].  This is a direct analogue of
    /// [parse::State::expect].  The error variant of the result contains a suitable error message
    /// if the expectation is violated.
    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(),
            )))
        }
    }

    /// Peek the next token from the stream, and consume and return it only if it has the correct
    /// type.
    fn accept(&mut self, acceptable: TokenType) -> PyResult<Option<Token>> {
        match self.peek_token()? {
            Some(Token { ttype, .. }) if *ttype == acceptable => self.next_token(),
            _ => Ok(None),
        }
    }

    /// Apply a prefix [Op] to the current [expression][Expr].  If the current expression is a
    /// constant floating-point value the application will be eagerly constant-folded, otherwise
    /// the resulting [Expr] will have a tree structure.
    fn apply_prefix(&mut self, prefix: Op, expr: Expr) -> PyResult<Expr> {
        match prefix {
            Op::Plus => Ok(expr),
            Op::Minus => match expr {
                Expr::Constant(val) => Ok(Expr::Constant(-val)),
                _ => Ok(Expr::Negate(Box::new(expr))),
            },
            _ => panic!(),
        }
    }

    /// Apply a binary infix [Op] to the current [expression][Expr].  If both operands have
    /// constant floating-point values the application will be eagerly constant-folded, otherwise
    /// the resulting [Expr] will have a tree structure.
    fn apply_infix(&mut self, infix: Op, lhs: Expr, rhs: Expr, op_token: &Token) -> PyResult<Expr> {
        if let (Expr::Constant(val), Op::Divide) = (&rhs, infix) {
            if *val == 0.0 {
                return Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        op_token.line,
                        op_token.col,
                    )),
                    "cannot divide by zero",
                )));
            }
        };
        if let (Expr::Constant(val_l), Expr::Constant(val_r)) = (&lhs, &rhs) {
            // Eagerly constant-fold if possible.
            match infix {
                Op::Plus => Ok(Expr::Constant(val_l + val_r)),
                Op::Minus => Ok(Expr::Constant(val_l - val_r)),
                Op::Multiply => Ok(Expr::Constant(val_l * val_r)),
                Op::Divide => Ok(Expr::Constant(val_l / val_r)),
                Op::Power => Ok(Expr::Constant(val_l.powf(*val_r))),
            }
        } else {
            // If not, we have to build a tree.
            let id_l = Box::new(lhs);
            let id_r = Box::new(rhs);
            match infix {
                Op::Plus => Ok(Expr::Add(id_l, id_r)),
                Op::Minus => Ok(Expr::Subtract(id_l, id_r)),
                Op::Multiply => Ok(Expr::Multiply(id_l, id_r)),
                Op::Divide => Ok(Expr::Divide(id_l, id_r)),
                Op::Power => Ok(Expr::Power(id_l, id_r)),
            }
        }
    }

    /// Apply a "scientific calculator" built-in function to an [expression][Expr].  If the operand
    /// is a constant, the function will be constant-folded to produce a new constant expression,
    /// otherwise a tree-form [Expr] is returned.
    fn apply_function(&mut self, func: Function, expr: Expr, token: &Token) -> PyResult<Expr> {
        match expr {
            Expr::Constant(val) => match func {
                Function::Cos => Ok(Expr::Constant(val.cos())),
                Function::Exp => Ok(Expr::Constant(val.exp())),
                Function::Ln => {
                    if val > 0.0 {
                        Ok(Expr::Constant(val.ln()))
                    } else {
                        Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                token.line,
                                token.col,
                            )),
                            &format!(
                                "failure in constant folding: cannot take ln of non-positive {}",
                                val
                            ),
                        )))
                    }
                }
                Function::Sin => Ok(Expr::Constant(val.sin())),
                Function::Sqrt => {
                    if val >= 0.0 {
                        Ok(Expr::Constant(val.sqrt()))
                    } else {
                        Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                token.line,
                                token.col,
                            )),
                            &format!(
                                "failure in constant folding: cannot take sqrt of negative {}",
                                val
                            ),
                        )))
                    }
                }
                Function::Tan => Ok(Expr::Constant(val.tan())),
            },
            _ => Ok(Expr::Function(func, Box::new(expr))),
        }
    }

    fn apply_custom_function(
        &mut self,
        callable: PyObject,
        exprs: Vec<Expr>,
        token: &Token,
    ) -> PyResult<Expr> {
        if exprs.iter().all(|x| matches!(x, Expr::Constant(_))) {
            // We can still do constant folding with custom user classical functions, we're just
            // going to have to acquire the GIL and call the Python object the user gave us right
            // now.  We need to explicitly handle any exceptions that might occur from that.
            Python::with_gil(|py| {
                let args = PyTuple::new_bound(
                    py,
                    exprs.iter().map(|x| {
                        if let Expr::Constant(val) = x {
                            *val
                        } else {
                            unreachable!()
                        }
                    }),
                );
                match callable.call1(py, args) {
                    Ok(retval) => {
                        match retval.extract::<f64>(py) {
                            Ok(fval) => Ok(Expr::Constant(fval)),
                            Err(inner) => {
                                let error = QASM2ParseError::new_err(message_generic(
                                Some(&Position::new(self.current_filename(), token.line, token.col)),
                                "user-defined function returned non-float during constant folding",
                            ));
                                error.set_cause(py, Some(inner));
                                Err(error)
                            }
                        }
                    }
                    Err(inner) => {
                        let error = QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(
                                self.current_filename(),
                                token.line,
                                token.col,
                            )),
                            "caught exception when constant folding with user-defined function",
                        ));
                        error.set_cause(py, Some(inner));
                        Err(error)
                    }
                }
            })
        } else {
            Ok(Expr::CustomFunction(callable, exprs))
        }
    }

    /// 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(()),
        }
    }

    /// Convert the given general [Token] into the expression-parser-specific [Atom], if possible.
    /// Not all [Token]s have a corresponding [Atom]; if this is the case, the return value is
    /// `Ok(None)`.  The error variant is returned if the next token is grammatically valid, but
    /// not semantically, such as an identifier for a value of an incorrect type.
    fn try_atom_from_token(&self, token: &Token) -> PyResult<Option<Atom>> {
        match token.ttype {
            TokenType::LParen => Ok(Some(Atom::LParen)),
            TokenType::RParen => Ok(Some(Atom::RParen)),
            TokenType::Minus
            | TokenType::Plus
            | TokenType::Asterisk
            | TokenType::Slash
            | TokenType::Caret => Ok(Some(Atom::Op(token.ttype.into()))),
            TokenType::Cos
            | TokenType::Exp
            | TokenType::Ln
            | TokenType::Sin
            | TokenType::Sqrt
            | TokenType::Tan => Ok(Some(Atom::Function(token.ttype.into()))),
            // This deliberately parses an _integer_ token as a float, since all OpenQASM 2.0
            // integers can be interpreted as floats, and doing that allows us to gracefully handle
            // cases where a huge float would overflow a `usize`.  Never mind that in such a case,
            // there's almost certainly precision loss from the floating-point representing
            // having insufficient mantissa digits to faithfully represent the angle mod 2pi;
            // that's not our fault in the parser.
            TokenType::Real | TokenType::Integer => Ok(Some(Atom::Const(token.real(self.context)))),
            TokenType::Pi => Ok(Some(Atom::Const(f64::consts::PI))),
            TokenType::Id => {
                let id = token.text(self.context);
                match self.gate_symbols.get(id) {
                    Some(GateSymbol::Parameter { index }) => Ok(Some(Atom::Parameter(*index))),
                    Some(GateSymbol::Qubit { .. }) => {
                        Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(self.current_filename(), token.line, token.col)),
                            &format!("'{}' is a gate qubit, not a parameter", id),
                        )))
                    }
                    None => {
                        match self.global_symbols.get(id) {
                            Some(GlobalSymbol::Classical { callable, num_params }) => {
                                Ok(Some(Atom::CustomFunction(callable.clone(), *num_params)))
                            }
                            _ =>  {
                            Err(QASM2ParseError::new_err(message_generic(
                            Some(&Position::new(self.current_filename(), token.line, token.col)),
                            &format!(
                                "'{}' is not a parameter or custom instruction defined in this scope",
                                id,
                            ))))
                            }
                    }
                }
            }
            }
            _ => Ok(None),
        }
    }

    /// Peek at the next [Atom] (and backing [Token]) if the next token exists and can be converted
    /// into a valid [Atom].  If it can't, or if we are at the end of the input, the `None` variant
    /// is returned.
    fn peek_atom(&mut self) -> PyResult<Option<(Atom, Token)>> {
        if let Some(&token) = self.peek_token()? {
            if let Ok(Some(atom)) = self.try_atom_from_token(&token) {
                Ok(Some((atom, token)))
            } else {
                Ok(None)
            }
        } else {
            Ok(None)
        }
    }

    /// The main recursive worker routing of the operator-precedence parser.  This evaluates a
    /// series of binary infix operators that have binding powers greater than the input
    /// `power_min`, and unary prefixes on the left-hand operand.  For example, if `power_min`
    /// starts out at `2` (such as it would when evaluating the right-hand side of a binary `+`
    /// expression), then as many `*` and `^` operations as appear would be evaluated by this loop,
    /// and its parsing would finish when it saw the next `+` binary operation.  For initial entry,
    /// the `power_min` should be zero.
    fn eval_expression(&mut self, power_min: u8, cause: &Token) -> PyResult<Expr> {
        let token = self.next_token()?.ok_or_else(|| {
            QASM2ParseError::new_err(message_bad_eof(
                Some(&Position::new(
                    self.current_filename(),
                    cause.line,
                    cause.col,
                )),
                if power_min == 0 {
                    "an expression"
                } else {
                    "a missing operand"
                },
            ))
        })?;
        let atom = self.try_atom_from_token(&token)?.ok_or_else(|| {
            QASM2ParseError::new_err(message_incorrect_requirement(
                if power_min == 0 {
                    "an expression"
                } else {
                    "a missing operand"
                },
                &token,
                self.current_filename(),
            ))
        })?;
        // First evaluate the "left-hand side" of a (potential) sequence of binary infix operators.
        // This might be a simple value, a unary operator acting on a value, or a bracketed
        // expression (either the operand of a function, or just plain parentheses).  This can also
        // invoke a recursive call; the parenthesis components feel naturally recursive, and the
        // unary operator component introduces a new precedence level that requires a recursive
        // call to evaluate.
        let mut lhs = match atom {
            Atom::LParen => {
                let out = self.eval_expression(0, cause)?;
                self.expect(TokenType::RParen, "a closing parenthesis", &token)?;
                Ok(out)
            }
            Atom::RParen => {
                if power_min == 0 {
                    Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            token.line,
                            token.col,
                        )),
                        "did not find an expected expression",
                    )))
                } else {
                    Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            token.line,
                            token.col,
                        )),
                        "the parenthesis closed, but there was a missing operand",
                    )))
                }
            }
            Atom::Function(func) => {
                let lparen_token =
                    self.expect(TokenType::LParen, "an opening parenthesis", &token)?;
                let argument = self.eval_expression(0, &token)?;
                let comma = self.accept(TokenType::Comma)?;
                self.check_trailing_comma(comma.as_ref())?;
                self.expect(TokenType::RParen, "a closing parenthesis", &lparen_token)?;
                Ok(self.apply_function(func, argument, &token)?)
            }
            Atom::CustomFunction(callable, num_params) => {
                let lparen_token =
                    self.expect(TokenType::LParen, "an opening parenthesis", &token)?;
                let mut arguments = Vec::<Expr>::with_capacity(num_params);
                let mut comma = None;
                loop {
                    // There are breaks at the start and end of this loop, because we might be
                    // breaking because there are _no_ parameters, because there's a trailing
                    // comma before the closing parenthesis, or because we didn't see a comma after
                    // an expression so we _need_ a closing parenthesis.
                    if let Some((Atom::RParen, _)) = self.peek_atom()? {
                        break;
                    }
                    arguments.push(self.eval_expression(0, &token)?);
                    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)?;
                if arguments.len() == num_params {
                    Ok(self.apply_custom_function(callable, arguments, &token)?)
                } else {
                    Err(QASM2ParseError::new_err(message_generic(
                        Some(&Position::new(
                            self.current_filename(),
                            token.line,
                            token.col,
                        )),
                        &format!(
                            "custom function argument-count mismatch: expected {}, saw {}",
                            num_params,
                            arguments.len(),
                        ),
                    )))
                }
            }
            Atom::Op(op) => match prefix_power(op) {
                Some(power) => {
                    let expr = self.eval_expression(power, &token)?;
                    Ok(self.apply_prefix(op, expr)?)
                }
                None => Err(QASM2ParseError::new_err(message_generic(
                    Some(&Position::new(
                        self.current_filename(),
                        token.line,
                        token.col,
                    )),
                    &format!("'{}' is not a valid unary operator", op.text()),
                ))),
            },
            Atom::Const(val) => Ok(Expr::Constant(val)),
            Atom::Parameter(val) => Ok(Expr::Parameter(val)),
        }?;
        // Now loop over a series of infix operators.  We can continue as long as we're just
        // looking at operators that bind more tightly than the `power_min` passed to this
        // function.  Once they're the same power or less, we have to return, because the calling
        // evaluator needs to bind its operator before we move on to the next infix operator.
        while let Some((Atom::Op(op), peeked_token)) = self.peek_atom()? {
            let (power_l, power_r) = binary_power(op);
            if power_l < power_min {
                break;
            }
            self.next_token()?; // Skip peeked operator.
            let rhs = self.eval_expression(power_r, &peeked_token)?;
            lhs = self.apply_infix(op, lhs, rhs, &peeked_token)?;
        }
        Ok(lhs)
    }

    /// Parse a single expression completely. This is the only public entry point to the
    /// operator-precedence parser.
    ///
    /// .. note::
    ///
    ///     This evaluates in a floating-point context, including evaluating integer tokens, since
    ///     the only places that expressions are valid in OpenQASM 2 is during gate applications.
    pub fn parse_expression(&mut self, cause: &Token) -> PyResult<Expr> {
        self.eval_expression(0, cause)
    }
}

Qiskit / qiskit / 11843862461

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