slice-source/dist/slice-source.js

A readable stream reader that reads a desired number of bytes.

// https://github.com/mbostock/slice-source Version 0.4.1. Copyright 2016 Mike Bostock.
(function (global, factory) {
  typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
  typeof define === 'function' && define.amd ? define(factory) :
  (global.sources = global.sources || {}, global.sources.slice = factory());
}(this, (function () { 'use strict';

var empty = new Uint8Array(0);

function slice_cancel() {
  return this._source.cancel();
}

function concat(a, b) {
  if (!a.length) return b;
  if (!b.length) return a;
  var c = new Uint8Array(a.length + b.length);
  c.set(a);
  c.set(b, a.length);
  return c;
}

function slice_read() {
  var that = this, array = that._array.subarray(that._index);
  return that._source.read().then(function(result) {
    that._array = empty;
    that._index = 0;
    return result.done ? (array.length > 0
        ? {done: false, value: array}
        : {done: true, value: undefined})
        : {done: false, value: concat(array, result.value)};
  });
}

function slice_slice(length) {
  if ((length |= 0) < 0) throw new Error("invalid length");
  var that = this, index = this._array.length - this._index;

  // If the request fits within the remaining buffer, resolve it immediately.
  if (this._index + length <= this._array.length) {
    return Promise.resolve(this._array.subarray(this._index, this._index += length));
  }

  // Otherwise, read chunks repeatedly until the request is fulfilled.
  var array = new Uint8Array(length);
  array.set(this._array.subarray(this._index));
  return (function read() {
    return that._source.read().then(function(result) {

      // When done, it’s possible the request wasn’t fully fullfilled!
      // If so, the pre-allocated array is too big and needs slicing.
      if (result.done) {
        that._array = empty;
        that._index = 0;
        return index > 0 ? array.subarray(0, index) : null;
      }

      // If this chunk fulfills the request, return the resulting array.
      if (index + result.value.length >= length) {
        that._array = result.value;
        that._index = length - index;
        array.set(result.value.subarray(0, length - index), index);
        return array;
      }

      // Otherwise copy this chunk into the array, then read the next chunk.
      array.set(result.value, index);
      index += result.value.length;
      return read();
    });
  })();
}

function slice(source) {
  return typeof source.slice === "function" ? source :
      new SliceSource(typeof source.read === "function" ? source
          : source.getReader());
}

function SliceSource(source) {
  this._source = source;
  this._array = empty;
  this._index = 0;
}

SliceSource.prototype.read = slice_read;
SliceSource.prototype.slice = slice_slice;
SliceSource.prototype.cancel = slice_cancel;

return slice;

})));