buffer/index.js

"use strict";

const Promise = require("bluebird");
const propagateAbort = require("@promistream/propagate-abort");
const unreachable = require("@joepie91/unreachable")("@promistream/buffer");

module.exports = function bufferStream() {
	let buffer;
	let bufferPeeked;

	function attemptRead(readFunc) {
		return Promise.try(() => {
			return readFunc();
		}).then((result) => {
			if (Array.isArray(result)) {
				if (result.length > 0) {
					buffer = result;
					bufferPeeked = result.map(() => false);
				} else {
					return attemptRead(readFunc);
				}
			} else {
				throw new Error("Buffered stream produced a non-array value");
			}
		});
	}

	return {
		_promistreamVersion: 0,
		description: "buffer stream",
		abort: propagateAbort,
		peek: function peekValue_bufferedSourceStream(source) {
			if (buffer == null) {
				return source.peek();
			} else {
				let unpeekedIndex = bufferPeeked.indexOf(false);

				if (unpeekedIndex > 0) {
					bufferPeeked[unpeekedIndex] = true;
					return true;
				} else {
					return source.peek();
				}
			}
		},
		read: function produceValue_bufferedSourceStream(source) {
			return Promise.try(() => {
				if (buffer == null) {
					return attemptRead(source.read);
				}
			}).then(() => {
				if (buffer != null) {
					let value = buffer.shift();
					bufferPeeked.shift();
			
					if (buffer.length === 0) {
						buffer = null;
						bufferPeeked = null;
					}
			
					return value;
				} else {
					unreachable("No buffer was present");
				}
			});
		}
	};
};