discoveryjs / scan-git / 11604860525

// https://git-scm.com/docs/pack-format

// offset encoding: n bytes with MSB set in all but the last one.

// The offset is then the number constructed by

// concatenating the lower 7 bit of each byte, and

// for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))

// to the result.

export function readEncodedOffset(reader: BufferCursor) {

    let result = -1;

    let byte = 0;

    do {

        byte = reader.readUInt8();

        // Note: An encoded offset can be greater than int32, so we can't use binary ops

        // for the result to avoid an overflow. As an alternative a BigInt for the result

        // might be used but it involves unwanted BigInt<->Number conversions.

        //

        // The expression is equivalent to:

        //   result = ((result + 1) << 7) | (byte & 0b0111111)

        result = (result + 1) * 128 + (byte & 0b01111111);

    } while (byte & 0b10000000);

    return result;

}

export function readVarIntLE(reader: BufferCursor) {

    let result = 0;

    let shift = 0;

    let byte = 0;

    do {

        byte = reader.readUInt8();

        result |= (byte & 0b01111111) << shift;

        shift += 7;

    } while (byte & 0b10000000);

    return result;

}

export class BufferCursor {

    offset = 0;

    constructor(public buffer: Buffer) {}

    get eof() {

        return this.offset >= this.buffer.length;

    }

    get bytesLeft() {

        return this.buffer.byteLength - this.offset;

    }

    slice(length: number) {

        return this.buffer.slice(this.offset, (this.offset += length));

    }

    toString(enc: BufferEncoding, length: number) {

    write(value: string, length: number, enc?: BufferEncoding) {

    copyFrom(source: Buffer, sourceStart?: number, sourceEnd?: number) {

    copyTo(target: Buffer, targetStart = 0, length = this.bytesLeft) {

        const copiedBytes = this.buffer.copy(

            target,

            targetStart,

            this.offset,

            this.offset + length

        );

        this.offset += copiedBytes;

        return copiedBytes;

    }

    readUInt8() {

        const r = this.buffer.readUInt8(this.offset);

        this.offset += 1;

        return r;

    }

    readUInt32BE() {

}