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lib/decoding.js

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+/**
+ * @module decoding
+ */
+
+/* global Buffer */
+
+import * as globals from './globals.js'
+
+/**
+ * A Decoder handles the decoding of an ArrayBuffer.
+ */
+export class Decoder {
+  /**
+   * @param {ArrayBuffer} buffer Binary data to decode
+   */
+  constructor (buffer) {
+    this.arr = new Uint8Array(buffer)
+    this.pos = 0
+  }
+}
+
+/**
+ * @function
+ * @param {ArrayBuffer} buffer
+ * @return {Decoder}
+ */
+export const createDecoder = buffer => new Decoder(buffer)
+
+/**
+ * @function
+ * @param {Decoder} decoder
+ * @return {boolean}
+ */
+export const hasContent = decoder => decoder.pos !== decoder.arr.length
+
+/**
+ * Clone a decoder instance.
+ * Optionally set a new position parameter.
+ *
+ * @function
+ * @param {Decoder} decoder The decoder instance
+ * @param {number} [newPos] Defaults to current position
+ * @return {Decoder} A clone of `decoder`
+ */
+export const clone = (decoder, newPos = decoder.pos) => {
+  let _decoder = createDecoder(decoder.arr.buffer)
+  _decoder.pos = newPos
+  return _decoder
+}
+
+/**
+ * Read `len` bytes as an ArrayBuffer.
+ * @function
+ * @param {Decoder} decoder The decoder instance
+ * @param {number} len The length of bytes to read
+ * @return {ArrayBuffer}
+ */
+export const readArrayBuffer = (decoder, len) => {
+  const arrayBuffer = globals.createUint8ArrayFromLen(len)
+  const view = globals.createUint8ArrayFromBuffer(decoder.arr.buffer, decoder.pos, len)
+  arrayBuffer.set(view)
+  decoder.pos += len
+  return arrayBuffer.buffer
+}
+
+/**
+ * Read variable length payload as ArrayBuffer
+ * @function
+ * @param {Decoder} decoder
+ * @return {ArrayBuffer}
+ */
+export const readPayload = decoder => readArrayBuffer(decoder, readVarUint(decoder))
+
+/**
+ * Read the rest of the content as an ArrayBuffer
+ * @function
+ * @param {Decoder} decoder
+ * @return {ArrayBuffer}
+ */
+export const readTail = decoder => readArrayBuffer(decoder, decoder.arr.length - decoder.pos)
+
+/**
+ * Skip one byte, jump to the next position.
+ * @function
+ * @param {Decoder} decoder The decoder instance
+ * @return {number} The next position
+ */
+export const skip8 = decoder => decoder.pos++
+
+/**
+ * Read one byte as unsigned integer.
+ * @function
+ * @param {Decoder} decoder The decoder instance
+ * @return {number} Unsigned 8-bit integer
+ */
+export const readUint8 = decoder => decoder.arr[decoder.pos++]
+
+/**
+ * Read 4 bytes as unsigned integer.
+ *
+ * @function
+ * @param {Decoder} decoder
+ * @return {number} An unsigned integer.
+ */
+export const readUint32 = decoder => {
+  let uint =
+    decoder.arr[decoder.pos] +
+    (decoder.arr[decoder.pos + 1] << 8) +
+    (decoder.arr[decoder.pos + 2] << 16) +
+    (decoder.arr[decoder.pos + 3] << 24)
+  decoder.pos += 4
+  return uint
+}
+
+/**
+ * Look ahead without incrementing position.
+ * to the next byte and read it as unsigned integer.
+ *
+ * @function
+ * @param {Decoder} decoder
+ * @return {number} An unsigned integer.
+ */
+export const peekUint8 = decoder => decoder.arr[decoder.pos]
+
+/**
+ * Read unsigned integer (32bit) with variable length.
+ * 1/8th of the storage is used as encoding overhead.
+ *  * numbers < 2^7 is stored in one bytlength
+ *  * numbers < 2^14 is stored in two bylength
+ *
+ * @function
+ * @param {Decoder} decoder
+ * @return {number} An unsigned integer.length
+ */
+export const readVarUint = decoder => {
+  let num = 0
+  let len = 0
+  while (true) {
+    let r = decoder.arr[decoder.pos++]
+    num = num | ((r & 0b1111111) << len)
+    len += 7
+    if (r < 1 << 7) {
+      return num >>> 0 // return unsigned number!
+    }
+    if (len > 35) {
+      throw new Error('Integer out of range!')
+    }
+  }
+}
+
+/**
+ * Read string of variable length
+ * * varUint is used to store the length of the string
+ *
+ * Transforming utf8 to a string is pretty expensive. The code performs 10x better
+ * when String.fromCodePoint is fed with all characters as arguments.
+ * But most environments have a maximum number of arguments per functions.
+ * For effiency reasons we apply a maximum of 10000 characters at once.
+ *
+ * @function
+ * @param {Decoder} decoder
+ * @return {String} The read String.
+ */
+export const readVarString = decoder => {
+  let remainingLen = readVarUint(decoder)
+  let encodedString = ''
+  while (remainingLen > 0) {
+    const nextLen = remainingLen < 10000 ? remainingLen : 10000
+    const bytes = new Array(nextLen)
+    for (let i = 0; i < nextLen; i++) {
+      bytes[i] = decoder.arr[decoder.pos++]
+    }
+    encodedString += String.fromCodePoint.apply(null, bytes)
+    remainingLen -= nextLen
+  }
+  return decodeURIComponent(escape(encodedString))
+}
+
+/**
+ * Look ahead and read varString without incrementing position
+ *
+ * @function
+ * @param {Decoder} decoder
+ * @return {string}
+ */
+export const peekVarString = decoder => {
+  let pos = decoder.pos
+  let s = readVarString(decoder)
+  decoder.pos = pos
+  return s
+}