music21j-port/src/ext/midijs/inc/jasmid/stream.js

A toolkit for computer-aided musicology, Javascript version

/* Wrapper for accessing strings through sequential reads */
/* now an ES6 class -- MSC 2019 */

export class Stream {
	constructor(str) {
		this.str = str;
		this.position = 0;
	}

	read(length) {
		const result = this.str.substr(this.position, length);
		this.position += length;
		return result;
	}
	
	/* read a big-endian 32-bit integer */
	readInt32() {
		const result = (
			(this.str.charCodeAt(this.position) << 24)
			+ (this.str.charCodeAt(this.position + 1) << 16)
			+ (this.str.charCodeAt(this.position + 2) << 8)
			+ this.str.charCodeAt(this.position + 3));
		this.position += 4;
		return result;
	}

	/* read a big-endian 16-bit integer */
	readInt16() {
		const result = (
			(this.str.charCodeAt(this.position) << 8)
			+ this.str.charCodeAt(this.position + 1));
		this.position += 2;
		return result;
	}
	
	/* read an 8-bit integer */
	readInt8(signed) {
		let result = this.str.charCodeAt(this.position);
		if (signed && result > 127) {
			result -= 256;
		}
		this.position += 1;
		return result;
	}
	
	eof() {
		return this.position >= this.str.length;
	}
	
	/* read a MIDI-style variable-length integer
		(big-endian value in groups of 7 bits,
		with top bit set to signify that another byte follows)
	*/
	readVarInt() {
		let result = 0;
		while (true) {
			let b = this.readInt8();
			// noinspection JSBitwiseOperatorUsage
			if (b & 0x80) {
				result += (b & 0x7f);
				result <<= 7;
			} else {
				/* b is the last byte */
				return result + b;
			}
		}
	}
}