Buffer(缓冲器)

于 2020-09-29 17:32:20 发布
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分类专栏： nodejs 文章标签： node.js
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本文链接：https://blog.youkuaiyun.com/smile_team/article/details/108872490
版权
本文详细介绍了Node.js中的Buffer对象，包括其与字符编码的关系，如UTF-8、UTF-16LE和Latin1的使用；Buffer与TypedArray的交互，如slice()和subarray()的区别；以及Buffer如何支持迭代器和创建新实例的方法。

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Buffer(缓冲器)

源码

'use strict';

const {
  Array,
  ArrayIsArray,
  Error,
  MathFloor,
  MathMin,
  MathTrunc,
  NumberIsNaN,
  NumberMAX_SAFE_INTEGER,
  NumberMIN_SAFE_INTEGER,
  ObjectCreate,
  ObjectDefineProperties,
  ObjectDefineProperty,
  ObjectGetOwnPropertyDescriptor,
  ObjectGetPrototypeOf,
  ObjectSetPrototypeOf,
  SymbolSpecies,
  SymbolToPrimitive,
  Uint8ArrayPrototype,
} = primordials;

const {
  byteLengthUtf8,
  compare: _compare,
  compareOffset,
  createFromString,
  fill: bindingFill,
  indexOfBuffer,
  indexOfNumber,
  indexOfString,
  swap16: _swap16,
  swap32: _swap32,
  swap64: _swap64,
  kMaxLength,
  kStringMaxLength,
  zeroFill: bindingZeroFill
} = internalBinding('buffer');
const {
  getOwnNonIndexProperties,
  propertyFilter: {
    ALL_PROPERTIES,
    ONLY_ENUMERABLE
  },
} = internalBinding('util');
const {
  customInspectSymbol,
  isInsideNodeModules,
  normalizeEncoding,
  kIsEncodingSymbol
} = require('internal/util');
const {
  isAnyArrayBuffer,
  isArrayBufferView,
  isUint8Array
} = require('internal/util/types');
const {
  inspect: utilInspect
} = require('internal/util/inspect');
const { encodings } = internalBinding('string_decoder');

const {
  codes: {
    ERR_BUFFER_OUT_OF_BOUNDS,
    ERR_INVALID_ARG_TYPE,
    ERR_INVALID_ARG_VALUE,
    ERR_INVALID_BUFFER_SIZE,
    ERR_INVALID_OPT_VALUE,
    ERR_OUT_OF_RANGE,
    ERR_UNKNOWN_ENCODING
  },
  hideStackFrames
} = require('internal/errors');
const {
  validateBuffer,
  validateInteger,
  validateString
} = require('internal/validators');
// Provide validateInteger() but with kMaxLength as the default maximum value.
const validateOffset = (value, name, min = 0, max = kMaxLength) =>
  validateInteger(value, name, min, max);

const {
  FastBuffer,
  markAsUntransferable,
  addBufferPrototypeMethods
} = require('internal/buffer');

const TypedArrayPrototype = ObjectGetPrototypeOf(Uint8ArrayPrototype);

const TypedArrayProto_byteLength =
      ObjectGetOwnPropertyDescriptor(TypedArrayPrototype,
                                     'byteLength').get;
const TypedArrayFill = TypedArrayPrototype.fill;

FastBuffer.prototype.constructor = Buffer;
Buffer.prototype = FastBuffer.prototype;
addBufferPrototypeMethods(Buffer.prototype);

const constants = ObjectDefineProperties({}, {
  MAX_LENGTH: {
    value: kMaxLength,
    writable: false,
    enumerable: true
  },
  MAX_STRING_LENGTH: {
    value: kStringMaxLength,
    writable: false,
    enumerable: true
  }
});

Buffer.poolSize = 8 * 1024;
let poolSize, poolOffset, allocPool;

// A toggle used to access the zero fill setting of the array buffer allocator
// in C++.
// |zeroFill| can be undefined when running inside an isolate where we
// do not own the ArrayBuffer allocator.  Zero fill is always on in that case.
const zeroFill = bindingZeroFill || [0];

const encodingsMap = ObjectCreate(null);
for (let i = 0; i < encodings.length; ++i)
  encodingsMap[encodings[i]] = i;

function createUnsafeBuffer(size) {
  zeroFill[0] = 0;
  try {
    return new FastBuffer(size);
  } finally {
    zeroFill[0] = 1;
  }
}

function createPool() {
  poolSize = Buffer.poolSize;
  allocPool = createUnsafeBuffer(poolSize).buffer;
  markAsUntransferable(allocPool);
  poolOffset = 0;
}
createPool();

function alignPool() {
  // Ensure aligned slices
  if (poolOffset & 0x7) {
    poolOffset |= 0x7;
    poolOffset++;
  }
}

let bufferWarningAlreadyEmitted = false;
let nodeModulesCheckCounter = 0;
const bufferWarning = 'Buffer() is deprecated due to security and usability ' +
                      'issues. Please use the Buffer.alloc(), ' +
                      'Buffer.allocUnsafe(), or Buffer.from() methods instead.';

function showFlaggedDeprecation() {
  if (bufferWarningAlreadyEmitted ||
      ++nodeModulesCheckCounter > 10000 ||
      (!require('internal/options').getOptionValue('--pending-deprecation') &&
       isInsideNodeModules())) {
    // We don't emit a warning, because we either:
    // - Already did so, or
    // - Already checked too many times whether a call is coming
    //   from node_modules and want to stop slowing down things, or
    // - We aren't running with `--pending-deprecation` enabled,
    //   and the code is inside `node_modules`.
    return;
  }

  process.emitWarning(bufferWarning, 'DeprecationWarning', 'DEP0005');
  bufferWarningAlreadyEmitted = true;
}

function toInteger(n, defaultVal) {
  n = +n;
  if (!NumberIsNaN(n) &&
      n >= NumberMIN_SAFE_INTEGER &&
      n <= NumberMAX_SAFE_INTEGER) {
    return ((n % 1) === 0 ? n : MathFloor(n));
  }
  return defaultVal;
}

function _copy(source, target, targetStart, sourceStart, sourceEnd) {
  if (!isUint8Array(source))
    throw new ERR_INVALID_ARG_TYPE('source', ['Buffer', 'Uint8Array'], source);
  if (!isUint8Array(target))
    throw new ERR_INVALID_ARG_TYPE('target', ['Buffer', 'Uint8Array'], target);

  if (targetStart === undefined) {
    targetStart = 0;
  } else {
    targetStart = toInteger(targetStart, 0);
    if (targetStart < 0)
      throw new ERR_OUT_OF_RANGE('targetStart', '>= 0', targetStart);
  }

  if (sourceStart === undefined) {
    sourceStart = 0;
  } else {
    sourceStart = toInteger(sourceStart, 0);
    if (sourceStart < 0)
      throw new ERR_OUT_OF_RANGE('sourceStart', '>= 0', sourceStart);
  }

  if (sourceEnd === undefined) {
    sourceEnd = source.length;
  } else {
    sourceEnd = toInteger(sourceEnd, 0);
    if (sourceEnd < 0)
      throw new ERR_OUT_OF_RANGE('sourceEnd', '>= 0', sourceEnd);
  }

  if (targetStart >= target.length || sourceStart >= sourceEnd)
    return 0;

  if (sourceStart > source.length) {
    throw new ERR_OUT_OF_RANGE('sourceStart',
                               `<= ${source.length}`,
                               sourceStart);
  }

  return _copyActual(source, target, targetStart, sourceStart, sourceEnd);
}

function _copyActual(source, target, targetStart, sourceStart, sourceEnd) {
  if (sourceEnd - sourceStart > target.length - targetStart)
    sourceEnd = sourceStart + target.length - targetStart;

  let nb = sourceEnd - sourceStart;
  const targetLen = target.length - targetStart;
  const sourceLen = source.length - sourceStart;
  if (nb > targetLen)
    nb = targetLen;
  if (nb > sourceLen)
    nb = sourceLen;

  if (sourceStart !== 0 || sourceEnd < source.length)
    source = new Uint8Array(source.buffer, source.byteOffset + sourceStart, nb);

  target.set(source, targetStart);

  return nb;
}

/**
 * The Buffer() constructor is deprecated in documentation and should not be
 * used moving forward. Rather, developers should use one of the three new
 * factory APIs: Buffer.from(), Buffer.allocUnsafe() or Buffer.alloc() based on
 * their specific needs. There is no runtime deprecation because of the extent
 * to which the Buffer constructor is used in the ecosystem currently -- a
 * runtime deprecation would introduce too much breakage at this time. It's not
 * likely that the Buffer constructors would ever actually be removed.
 * Deprecation Code: DEP0005
 */
function Buffer(arg, encodingOrOffset, length) {
  showFlaggedDeprecation();
  // Common case.
  if (typeof arg === 'number') {
    if (typeof encodingOrOffset === 'string') {
      throw new ERR_INVALID_ARG_TYPE('string', 'string', arg);
    }
    return Buffer.alloc(arg);
  }
  return Buffer.from(arg, encodingOrOffset, length);
}

ObjectDefineProperty(Buffer, SymbolSpecies, {
  enumerable: false,
  configurable: true,
  get() { return FastBuffer; }
});

/**
 * Functionally equivalent to Buffer(arg, encoding) but throws a TypeError
 * if value is a number.
 * Buffer.from(str[, encoding])
 * Buffer.from(array)
 * Buffer.from(buffer)
 * Buffer.from(arrayBuffer[, byteOffset[, length]])
 */
Buffer.from = function from(value, encodingOrOffset, length) {
  if (typeof value === 'string')
    return fromString(value, encodingOrOffset);

  if (typeof value === 'object' && value !== null) {
    if (isAnyArrayBuffer(value))
      return fromArrayBuffer(value, encodingOrOffset, length);

    const valueOf = value.valueOf && value.valueOf();
    if (valueOf != null &&
        valueOf !== value &&
        (typeof valueOf === 'string' || typeof valueOf === 'object')) {
      return from(valueOf, encodingOrOffset, length);
    }

    const b = fromObject(value);
    if (b)
      return b;

    if (typeof value[SymbolToPrimitive] === 'function') {
      const primitive = value[SymbolToPrimitive]('string');
      if (typeof primitive === 'string') {
        return fromString(primitive, encodingOrOffset);
      }
    }
  }

  throw new ERR_INVALID_ARG_TYPE(
    'first argument',
    ['string', 'Buffer', 'ArrayBuffer', 'Array', 'Array-like Object'],
    value
  );
};

// Identical to the built-in %TypedArray%.of(), but avoids using the deprecated
// Buffer() constructor. Must use arrow function syntax to avoid automatically
// adding a `prototype` property and making the function a constructor.
//
// Refs: https://tc39.github.io/ecma262/#sec-%typedarray%.of
// Refs: https://esdiscuss.org/topic/isconstructor#content-11
const of = (...items) => {
  const newObj = createUnsafeBuffer(items.length);
  for (let k = 0; k < items.length; k++)
    newObj[k] = items[k];
  return newObj;
};
Buffer.of = of;

ObjectSetPrototypeOf(Buffer, Uint8Array);

// The 'assertSize' method will remove itself from the callstack when an error
// occurs. This is done simply to keep the internal details of the
// implementation from bleeding out to users.
const assertSize = hideStackFrames((size) => {
  if (typeof size !== 'number') {
    throw new ERR_INVALID_ARG_TYPE('size', 'number', size);
  }
  if (!(size >= 0 && size <= kMaxLength)) {
    throw new ERR_INVALID_OPT_VALUE.RangeError('size', size);
  }
});

/**
 * Creates a new filled Buffer instance.
 * alloc(size[, fill[, encoding]])
 */
Buffer.alloc = function alloc(size, fill, encoding) {
  assertSize(size);
  if (fill !== undefined && fill !== 0 && size > 0) {
    const buf = createUnsafeBuffer(size);
    return _fill(buf, fill, 0, buf.length, encoding);
  }
  return new FastBuffer(size);
};

/**
 * Equivalent to Buffer(num), by default creates a non-zero-filled Buffer
 * instance. If `--zero-fill-buffers` is set, will zero-fill the buffer.
 */
Buffer.allocUnsafe = function allocUnsafe(size) {
  assertSize(size);
  return allocate(size);
};

/**
 * Equivalent to SlowBuffer(num), by default creates a non-zero-filled
 * Buffer instance that is not allocated off the pre-initialized pool.
 * If `--zero-fill-buffers` is set, will zero-fill the buffer.
 */
Buffer.allocUnsafeSlow = function allocUnsafeSlow(size) {
  assertSize(size);
  return createUnsafeBuffer(size);
};

// If --zero-fill-buffers command line argument is set, a zero-filled
// buffer is returned.
function SlowBuffer(length) {
  assertSize(length);
  return createUnsafeBuffer(length);
}

ObjectSetPrototypeOf(SlowBuffer.prototype, Uint8Array.prototype);
ObjectSetPrototypeOf(SlowBuffer, Uint8Array);

function allocate(size) {
  if (size <= 0) {
    return new FastBuffer();
  }
  if (size < (Buffer.poolSize >>> 1)) {
    if (size > (poolSize - poolOffset))
      createPool();
    const b = new FastBuffer(allocPool, poolOffset, size);
    poolOffset += size;
    alignPool();
    return b;
  }
  return createUnsafeBuffer(size);
}

function fromStringFast(string, ops) {
  const length = ops.byteLength(string);

  if (length >= (Buffer.poolSize >>> 1))
    return createFromString(string, ops.encodingVal);

  if (length > (poolSize - poolOffset))
    createPool();
  let b = new FastBuffer(allocPool, poolOffset, length);
  const actual = ops.write(b, string, 0, length);
  if (actual !== length) {
    // byteLength() may overestimate. That's a rare case, though.
    b = new FastBuffer(allocPool, poolOffset, actual);
  }
  poolOffset += actual;
  alignPool();
  return b;
}

function fromString(string, encoding) {
  let ops;
  if (typeof encoding !== 'string' || encoding.length === 0) {
    if (string.length === 0)
      return new FastBuffer();
    ops = encodingOps.utf8;
    encoding = undefined;
  } else {
    ops = getEncodingOps(encoding);
    if (ops === undefined)
      throw new ERR_UNKNOWN_ENCODING(encoding);
    if (string.length === 0)
      return new FastBuffer();
  }
  return fromStringFast(string, ops);
}

function fromArrayBuffer(obj, byteOffset, length) {
  // Convert byteOffset to integer
  if (byteOffset === undefined) {
    byteOffset = 0;
  } else {
    byteOffset = +byteOffset;
    if (NumberIsNaN(byteOffset))
      byteOffset = 0;
  }

  const maxLength = obj.byteLength - byteOffset;

  if (maxLength < 0)
    throw new ERR_BUFFER_OUT_OF_BOUNDS('offset');

  if (length === undefined) {
    length = maxLength;
  } else {
    // Convert length to non-negative integer.
    length = +length;
    if (length > 0) {
      if (length > maxLength)
        throw new ERR_BUFFER_OUT_OF_BOUNDS('length');
    } else {
      length = 0;
    }
  }

  return new FastBuffer(obj, byteOffset, length);
}

function fromArrayLike(obj) {
  if (obj.length <= 0)
    return new FastBuffer();
  if (obj.length < (Buffer.poolSize >>> 1)) {
    if (obj.length > (poolSize - poolOffset))
      createPool();
    const b = new FastBuffer(allocPool, poolOffset, obj.length);
    b.set(obj, 0);
    poolOffset += obj.length;
    alignPool();
    return b;
  }
  return new FastBuffer(obj);
}

function fromObject(obj) {
  if (obj.length !== undefined || isAnyArrayBuffer(obj.buffer)) {
    if (typeof obj.length !== 'number') {
      return new FastBuffer();
    }
    return fromArrayLike(obj);
  }

  if (obj.type === 'Buffer' && ArrayIsArray(obj.data)) {
    return fromArrayLike(obj.data);
  }
}

// Static methods

Buffer.isBuffer = function isBuffer(b) {
  return b instanceof Buffer;
};

Buffer.compare = function compare(buf1, buf2) {
  if (!isUint8Array(buf1)) {
    throw new ERR_INVALID_ARG_TYPE('buf1', ['Buffer', 'Uint8Array'], buf1);
  }

  if (!isUint8Array(buf2)) {
    throw new ERR_INVALID_ARG_TYPE('buf2', ['Buffer', 'Uint8Array'], buf2);
  }

  if (buf1 === buf2) {
    return 0;
  }

  return _compare(buf1, buf2);
};

Buffer.isEncoding = function isEncoding(encoding) {
  return typeof encoding === 'string' && encoding.length !== 0 &&
         normalizeEncoding(encoding) !== undefined;
};
Buffer[kIsEncodingSymbol] = Buffer.isEncoding;

Buffer.concat = function concat(list, length) {
  if (!ArrayIsArray(list)) {
    throw new ERR_INVALID_ARG_TYPE('list', 'Array', list);
  }

  if (list.length === 0)
    return new FastBuffer();

  if (length === undefined) {
    length = 0;
    for (let i = 0; i < list.length; i++) {
      if (list[i].length) {
        length += list[i].length;
      }
    }
  } else {
    validateOffset(length, 'length');
  }

  const buffer = Buffer.allocUnsafe(length);
  let pos = 0;
  for (let i = 0; i < list.length; i++) {
    const buf = list[i];
    if (!isUint8Array(buf)) {
      // TODO(BridgeAR): This should not be of type ERR_INVALID_ARG_TYPE.
      // Instead, find the proper error code for this.
      throw new ERR_INVALID_ARG_TYPE(
        `list[${i}]`, ['Buffer', 'Uint8Array'], list[i]);
    }
    pos += _copyActual(buf, buffer, pos, 0, buf.length);
  }

  // Note: `length` is always equal to `buffer.length` at this point
  if (pos < length) {
    // Zero-fill the remaining bytes if the specified `length` was more than
    // the actual total length, i.e. if we have some remaining allocated bytes
    // there were not initialized.
    TypedArrayFill.call(buffer, 0, pos, length);
  }

  return buffer;
};

function base64ByteLength(str, bytes) {
  // Handle padding
  if (str.charCodeAt(bytes - 1) === 0x3D)
    bytes--;
  if (bytes > 1 && str.charCodeAt(bytes - 1) === 0x3D)
    bytes--;

  // Base64 ratio: 3/4
  return (bytes * 3) >>> 2;
}

const encodingOps = {
  utf8: {
    encoding: 'utf8',
    encodingVal: encodingsMap.utf8,
    byteLength: byteLengthUtf8,
    write: (buf, string, offset, len) => buf.utf8Write(string, offset, len),
    slice: (buf, start, end) => buf.utf8Slice(start, end),
    indexOf: (buf, val, byteOffset, dir) =>
      indexOfString(buf, val, byteOffset, encodingsMap.utf8, dir)
  },
  ucs2: {
    encoding: 'ucs2',
    encodingVal: encodingsMap.utf16le,
    byteLength: (string) => string.length * 2,
    write: (buf, string, offset, len) => buf.ucs2Write(string, offset, len),
    slice: (buf, start, end) => buf.ucs2Slice(start, end),
    indexOf: (buf, val, byteOffset, dir) =>
      indexOfString(buf, val, byteOffset, encodingsMap.utf16le, dir)
  },
  utf16le: {
    encoding: 'utf16le',
    encodingVal: encodingsMap.utf16le,
    byteLength: (string) => string.length * 2,
    write: (buf, string, offset, len) => buf.ucs2Write(string, offset, len),
    slice: (buf, start, end) => buf.ucs2Slice(start, end),
    indexOf: (buf, val, byteOffset, dir) =>
      indexOfString(buf, val, byteOffset, encodingsMap.utf16le, dir)
  },
  latin1: {
    encoding: 'latin1',
    encodingVal: encodingsMap.latin1,
    byteLength: (string) => string.length,
    write: (buf, string, offset, len) => buf.latin1Write(string, offset, len),
    slice: (buf, start, end) => buf.latin1Slice(start, end),
    indexOf: (buf, val, byteOffset, dir) =>
      indexOfString(buf, val, byteOffset, encodingsMap.latin1, dir)
  },
  ascii: {
    encoding: 'ascii',
    encodingVal: encodingsMap.ascii,
    byteLength: (string) => string.length,
    write: (buf, string, offset, len) => buf.asciiWrite(string, offset, len),
    slice: (buf, start, end) => buf.asciiSlice(start, end),
    indexOf: (buf, val, byteOffset, dir) =>
      indexOfBuffer(buf,
                    fromStringFast(val, encodingOps.ascii),
                    byteOffset,
                    encodingsMap.ascii,
                    dir)
  },
  base64: {
    encoding: 'base64',
    encodingVal: encodingsMap.base64,
    byteLength: (string) => base64ByteLength(string, string.length),
    write: (buf, string, offset, len) => buf.base64Write(string, offset, len),
    slice: (buf, start, end) => buf.base64Slice(start, end),
    indexOf: (buf, val, byteOffset, dir) =>
      indexOfBuffer(buf,
                    fromStringFast(val, encodingOps.base64),
                    byteOffset,
                    encodingsMap.base64,
                    dir)
  },
  hex: {
    encoding: 'hex',
    encodingVal: encodingsMap.hex,
    byteLength: (string) => string.length >>> 1,
    write: (buf, string, offset, len) => buf.hexWrite(string, offset, len),
    slice: (buf, start, end) => buf.hexSlice(start, end),
    indexOf: (buf, val, byteOffset, dir) =>
      indexOfBuffer(buf,
                    fromStringFast(val, encodingOps.hex),
                    byteOffset,
                    encodingsMap.hex,
                    dir)
  }
};
function getEncodingOps(encoding) {
  encoding += '';
  switch (encoding.length) {
    case 4:
      if (encoding === 'utf8') return encodingOps.utf8;
      if (encoding === 'ucs2') return encodingOps.ucs2;
      encoding = encoding.toLowerCase();
      if (encoding === 'utf8') return encodingOps.utf8;
      if (encoding === 'ucs2') return encodingOps.ucs2;
      break;
    case 5:
      if (encoding === 'utf-8') return encodingOps.utf8;
      if (encoding === 'ascii') return encodingOps.ascii;
      if (encoding === 'ucs-2') return encodingOps.ucs2;
      encoding = encoding.toLowerCase();
      if (encoding === 'utf-8') return encodingOps.utf8;
      if (encoding === 'ascii') return encodingOps.ascii;
      if (encoding === 'ucs-2') return encodingOps.ucs2;
      break;
    case 7:
      if (encoding === 'utf16le' || encoding.toLowerCase() === 'utf16le')
        return encodingOps.utf16le;
      break;
    case 8:
      if (encoding === 'utf-16le' || encoding.toLowerCase() === 'utf-16le')
        return encodingOps.utf16le;
      break;
    case 6:
      if (encoding === 'latin1' || encoding === 'binary')
        return encodingOps.latin1;
      if (encoding === 'base64') return encodingOps.base64;
      encoding = encoding.toLowerCase();
      if (encoding === 'latin1' || encoding === 'binary')
        return encodingOps.latin1;
      if (encoding === 'base64') return encodingOps.base64;
      break;
    case 3:
      if (encoding === 'hex' || encoding.toLowerCase() === 'hex')
        return encodingOps.hex;
      break;
  }
}

function byteLength(string, encoding) {
  if (typeof string !== 'string') {
    if (isArrayBufferView(string) || isAnyArrayBuffer(string)) {
      return string.byteLength;
    }

    throw new ERR_INVALID_ARG_TYPE(
      'string', ['string', 'Buffer', 'ArrayBuffer'], string
    );
  }

  const len = string.length;
  const mustMatch = (arguments.length > 2 && arguments[2] === true);
  if (!mustMatch && len === 0)
    return 0;

  if (!encoding)
    return (mustMatch ? -1 : byteLengthUtf8(string));

  const ops = getEncodingOps(encoding);
  if (ops === undefined)
    return (mustMatch ? -1 : byteLengthUtf8(string));
  return ops.byteLength(string);
}

Buffer.byteLength = byteLength;

// For backwards compatibility.
ObjectDefineProperty(Buffer.prototype, 'parent', {
  enumerable: true,
  get() {
    if (!(this instanceof Buffer))
      return undefined;
    return this.buffer;
  }
});
ObjectDefineProperty(Buffer.prototype, 'offset', {
  enumerable: true,
  get() {
    if (!(this instanceof Buffer))
      return undefined;
    return this.byteOffset;
  }
});

Buffer.prototype.copy =
  function copy(target, targetStart, sourceStart, sourceEnd) {
    return _copy(this, target, targetStart, sourceStart, sourceEnd);
  };

// No need to verify that "buf.length <= MAX_UINT32" since it's a read-only
// property of a typed array.
// This behaves neither like String nor Uint8Array in that we set start/end
// to their upper/lower bounds if the value passed is out of range.
Buffer.prototype.toString = function toString(encoding, start, end) {
  if (arguments.length === 0) {
    return this.utf8Slice(0, this.length);
  }

  const len = this.length;

  if (start <= 0)
    start = 0;
  else if (start >= len)
    return '';
  else
    start |= 0;

  if (end === undefined || end > len)
    end = len;
  else
    end |= 0;

  if (end <= start)
    return '';

  if (encoding === undefined)
    return this.utf8Slice(start, end);

  const ops = getEncodingOps(encoding);
  if (ops === undefined)
    throw new ERR_UNKNOWN_ENCODING(encoding);

  return ops.slice(this, start, end);
};

Buffer.prototype.equals = function equals(otherBuffer) {
  if (!isUint8Array(otherBuffer)) {
    throw new ERR_INVALID_ARG_TYPE(
      'otherBuffer', ['Buffer', 'Uint8Array'], otherBuffer);
  }

  if (this === otherBuffer)
    return true;

  if (this.byteLength !== otherBuffer.byteLength)
    return false;

  return this.byteLength === 0 || _compare(this, otherBuffer) === 0;
};

let INSPECT_MAX_BYTES = 50;
// Override how buffers are presented by util.inspect().
Buffer.prototype[customInspectSymbol] = function inspect(recurseTimes, ctx) {
  const max = INSPECT_MAX_BYTES;
  const actualMax = MathMin(max, this.length);
  const remaining = this.length - max;
  let str = this.hexSlice(0, actualMax).replace(/(.{2})/g, '$1 ').trim();
  if (remaining > 0)
    str += ` ... ${remaining} more byte${remaining > 1 ? 's' : ''}`;
  // Inspect special properties as well, if possible.
  if (ctx) {
    let extras = false;
    const filter = ctx.showHidden ? ALL_PROPERTIES : ONLY_ENUMERABLE;
    const obj = getOwnNonIndexProperties(this, filter).reduce((obj, key) => {
      extras = true;
      obj[key] = this[key];
      return obj;
    }, ObjectCreate(null));
    if (extras) {
      if (this.length !== 0)
        str += ', ';
      // '[Object: null prototype] {'.length === 26
      // This is guarded with a test.
      str += utilInspect(obj, {
        ...ctx,
        breakLength: Infinity,
        compact: true
      }).slice(27, -2);
    }
  }
  return `<${this.constructor.name} ${str}>`;
};
Buffer.prototype.inspect = Buffer.prototype[customInspectSymbol];

Buffer.prototype.compare = function compare(target,
                                            targetStart,
                                            targetEnd,
                                            sourceStart,
                                            sourceEnd) {
  if (!isUint8Array(target)) {
    throw new ERR_INVALID_ARG_TYPE('target', ['Buffer', 'Uint8Array'], target);
  }
  if (arguments.length === 1)
    return _compare(this, target);

  if (targetStart === undefined)
    targetStart = 0;
  else
    validateOffset(targetStart, 'targetStart');

  if (targetEnd === undefined)
    targetEnd = target.length;
  else
    validateOffset(targetEnd, 'targetEnd', 0, target.length);

  if (sourceStart === undefined)
    sourceStart = 0;
  else
    validateOffset(sourceStart, 'sourceStart');

  if (sourceEnd === undefined)
    sourceEnd = this.length;
  else
    validateOffset(sourceEnd, 'sourceEnd', 0, this.length);

  if (sourceStart >= sourceEnd)
    return (targetStart >= targetEnd ? 0 : -1);
  if (targetStart >= targetEnd)
    return 1;

  return compareOffset(this, target, targetStart, sourceStart, targetEnd,
                       sourceEnd);
};

// Finds either the first index of `val` in `buffer` at offset >= `byteOffset`,
// OR the last index of `val` in `buffer` at offset <= `byteOffset`.
//
// Arguments:
// - buffer - a Buffer to search
// - val - a string, Buffer, or number
// - byteOffset - an index into `buffer`; will be clamped to an int32
// - encoding - an optional encoding, relevant if val is a string
// - dir - true for indexOf, false for lastIndexOf
function bidirectionalIndexOf(buffer, val, byteOffset, encoding, dir) {
  validateBuffer(buffer);

  if (typeof byteOffset === 'string') {
    encoding = byteOffset;
    byteOffset = undefined;
  } else if (byteOffset > 0x7fffffff) {
    byteOffset = 0x7fffffff;
  } else if (byteOffset < -0x80000000) {
    byteOffset = -0x80000000;
  }
  // Coerce to Number. Values like null and [] become 0.
  byteOffset = +byteOffset;
  // If the offset is undefined, "foo", {}, coerces to NaN, search whole buffer.
  if (NumberIsNaN(byteOffset)) {
    byteOffset = dir ? 0 : (buffer.length || buffer.byteLength);
  }
  dir = !!dir;  // Cast to bool.

  if (typeof val === 'number')
    return indexOfNumber(buffer, val >>> 0, byteOffset, dir);

  let ops;
  if (encoding === undefined)
    ops = encodingOps.utf8;
  else
    ops = getEncodingOps(encoding);

  if (typeof val === 'string') {
    if (ops === undefined)
      throw new ERR_UNKNOWN_ENCODING(encoding);
    return ops.indexOf(buffer, val, byteOffset, dir);
  }

  if (isUint8Array(val)) {
    const encodingVal =
      (ops === undefined ? encodingsMap.utf8 : ops.encodingVal);
    return indexOfBuffer(buffer, val, byteOffset, encodingVal, dir);
  }

  throw new ERR_INVALID_ARG_TYPE(
    'value', ['number', 'string', 'Buffer', 'Uint8Array'], val
  );
}

Buffer.prototype.indexOf = function indexOf(val, byteOffset, encoding) {
  return bidirectionalIndexOf(this, val, byteOffset, encoding, true);
};

Buffer.prototype.lastIndexOf = function lastIndexOf(val, byteOffset, encoding) {
  return bidirectionalIndexOf(this, val, byteOffset, encoding, false);
};

Buffer.prototype.includes = function includes(val, byteOffset, encoding) {
  return this.indexOf(val, byteOffset, encoding) !== -1;
};

// Usage:
//    buffer.fill(number[, offset[, end]])
//    buffer.fill(buffer[, offset[, end]])
//    buffer.fill(string[, offset[, end]][, encoding])
Buffer.prototype.fill = function fill(value, offset, end, encoding) {
  return _fill(this, value, offset, end, encoding);
};

function _fill(buf, value, offset, end, encoding) {
  if (typeof value === 'string') {
    if (offset === undefined || typeof offset === 'string') {
      encoding = offset;
      offset = 0;
      end = buf.length;
    } else if (typeof end === 'string') {
      encoding = end;
      end = buf.length;
    }

    const normalizedEncoding = normalizeEncoding(encoding);
    if (normalizedEncoding === undefined) {
      validateString(encoding, 'encoding');
      throw new ERR_UNKNOWN_ENCODING(encoding);
    }

    if (value.length === 0) {
      // If value === '' default to zero.
      value = 0;
    } else if (value.length === 1) {
      // Fast path: If `value` fits into a single byte, use that numeric value.
      if (normalizedEncoding === 'utf8') {
        const code = value.charCodeAt(0);
        if (code < 128) {
          value = code;
        }
      } else if (normalizedEncoding === 'latin1') {
        value = value.charCodeAt(0);
      }
    }
  } else {
    encoding = undefined;
  }

  if (offset === undefined) {
    offset = 0;
    end = buf.length;
  } else {
    validateOffset(offset, 'offset');
    // Invalid ranges are not set to a default, so can range check early.
    if (end === undefined) {
      end = buf.length;
    } else {
      validateOffset(end, 'end', 0, buf.length);
    }
    if (offset >= end)
      return buf;
  }


  if (typeof value === 'number') {
    // OOB check
    const byteLen = TypedArrayProto_byteLength.call(buf);
    const fillLength = end - offset;
    if (offset > end || fillLength + offset > byteLen)
      throw new ERR_BUFFER_OUT_OF_BOUNDS();

    TypedArrayFill.call(buf, value, offset, end);
  } else {
    const res = bindingFill(buf, value, offset, end, encoding);
    if (res < 0) {
      if (res === -1)
        throw new ERR_INVALID_ARG_VALUE('value', value);
      throw new ERR_BUFFER_OUT_OF_BOUNDS();
    }
  }

  return buf;
}

Buffer.prototype.write = function write(string, offset, length, encoding) {
  // Buffer#write(string);
  if (offset === undefined) {
    return this.utf8Write(string, 0, this.length);
  }
  // Buffer#write(string, encoding)
  if (length === undefined && typeof offset === 'string') {
    encoding = offset;
    length = this.length;
    offset = 0;

  // Buffer#write(string, offset[, length][, encoding])
  } else {
    validateOffset(offset, 'offset', 0, this.length);

    const remaining = this.length - offset;

    if (length === undefined) {
      length = remaining;
    } else if (typeof length === 'string') {
      encoding = length;
      length = remaining;
    } else {
      validateOffset(length, 'length', 0, this.length);
      if (length > remaining)
        length = remaining;
    }
  }

  if (!encoding)
    return this.utf8Write(string, offset, length);

  const ops = getEncodingOps(encoding);
  if (ops === undefined)
    throw new ERR_UNKNOWN_ENCODING(encoding);
  return ops.write(this, string, offset, length);
};

Buffer.prototype.toJSON = function toJSON() {
  if (this.length > 0) {
    const data = new Array(this.length);
    for (let i = 0; i < this.length; ++i)
      data[i] = this[i];
    return { type: 'Buffer', data };
  }
  return { type: 'Buffer', data: [] };
};

function adjustOffset(offset, length) {
  // Use Math.trunc() to convert offset to an integer value that can be larger
  // than an Int32. Hence, don't use offset | 0 or similar techniques.
  offset = MathTrunc(offset);
  if (offset === 0) {
    return 0;
  }
  if (offset < 0) {
    offset += length;
    return offset > 0 ? offset : 0;
  }
  if (offset < length) {
    return offset;
  }
  return NumberIsNaN(offset) ? 0 : length;
}

Buffer.prototype.slice = function slice(start, end) {
  const srcLength = this.length;
  start = adjustOffset(start, srcLength);
  end = end !== undefined ? adjustOffset(end, srcLength) : srcLength;
  const newLength = end > start ? end - start : 0;
  return new FastBuffer(this.buffer, this.byteOffset + start, newLength);
};

function swap(b, n, m) {
  const i = b[n];
  b[n] = b[m];
  b[m] = i;
}

Buffer.prototype.swap16 = function swap16() {
  // For Buffer.length < 128, it's generally faster to
  // do the swap in javascript. For larger buffers,
  // dropping down to the native code is faster.
  const len = this.length;
  if (len % 2 !== 0)
    throw new ERR_INVALID_BUFFER_SIZE('16-bits');
  if (len < 128) {
    for (let i = 0; i < len; i += 2)
      swap(this, i, i + 1);
    return this;
  }
  return _swap16(this);
};

Buffer.prototype.swap32 = function swap32() {
  // For Buffer.length < 192, it's generally faster to
  // do the swap in javascript. For larger buffers,
  // dropping down to the native code is faster.
  const len = this.length;
  if (len % 4 !== 0)
    throw new ERR_INVALID_BUFFER_SIZE('32-bits');
  if (len < 192) {
    for (let i = 0; i < len; i += 4) {
      swap(this, i, i + 3);
      swap(this, i + 1, i + 2);
    }
    return this;
  }
  return _swap32(this);
};

Buffer.prototype.swap64 = function swap64() {
  // For Buffer.length < 192, it's generally faster to
  // do the swap in javascript. For larger buffers,
  // dropping down to the native code is faster.
  const len = this.length;
  if (len % 8 !== 0)
    throw new ERR_INVALID_BUFFER_SIZE('64-bits');
  if (len < 192) {
    for (let i = 0; i < len; i += 8) {
      swap(this, i, i + 7);
      swap(this, i + 1, i + 6);
      swap(this, i + 2, i + 5);
      swap(this, i + 3, i + 4);
    }
    return this;
  }
  return _swap64(this);
};

Buffer.prototype.toLocaleString = Buffer.prototype.toString;

let transcode;
if (internalBinding('config').hasIntl) {
  const {
    icuErrName,
    transcode: _transcode
  } = internalBinding('icu');

  // Transcodes the Buffer from one encoding to another, returning a new
  // Buffer instance.
  transcode = function transcode(source, fromEncoding, toEncoding) {
    if (!isUint8Array(source)) {
      throw new ERR_INVALID_ARG_TYPE('source',
                                     ['Buffer', 'Uint8Array'], source);
    }
    if (source.length === 0) return Buffer.alloc(0);

    fromEncoding = normalizeEncoding(fromEncoding) || fromEncoding;
    toEncoding = normalizeEncoding(toEncoding) || toEncoding;
    const result = _transcode(source, fromEncoding, toEncoding);
    if (typeof result !== 'number')
      return result;

    const code = icuErrName(result);
    // eslint-disable-next-line no-restricted-syntax
    const err = new Error(`Unable to transcode Buffer [${code}]`);
    err.code = code;
    err.errno = result;
    throw err;
  };
}

module.exports = {
  Buffer,
  SlowBuffer,
  transcode,
  // Legacy
  kMaxLength,
  kStringMaxLength
};

ObjectDefineProperties(module.exports, {
  constants: {
    configurable: false,
    enumerable: true,
    value: constants
  },
  INSPECT_MAX_BYTES: {
    configurable: true,
    enumerable: true,
    get() { return INSPECT_MAX_BYTES; },
    set(val) { INSPECT_MAX_BYTES = val; }
  }
});
  • Buffer对象用于表示固定长度的字节序列
  • Buffer类是JavaScript的Uint8Array类的子类，且继承时带上了涵盖外用例的方法
  • Buffer类在全局作用域中，因此无需使用require（‘buffer’）.Buffer
// 创建一个长度为 10 的 Buffer，
// 其中填充了全部值为 `1` 的字节。
const buf1 = Buffer.alloc(10);

// 创建一个长度为 10、且用 0x1 填充的 Buffer。 
const buf2 = Buffer.alloc(10, 1);

// 创建一个长度为 10、且未初始化的 Buffer。
// 这个方法比调用 Buffer.alloc() 更快，
// 但返回的 Buffer 实例可能包含旧数据，
// 因此需要使用 fill()、write() 或其他能填充 Buffer 的内容的函数进行重写。
const buf3 = Buffer.allocUnsafe(10);

// 创建一个包含字节 [1, 2, 3] 的 Buffer。
const buf4 = Buffer.from([1, 2, 3]);

// 创建一个包含字节 [1, 1, 1, 1] 的 Buffer，
// 其中所有条目均使用 `(value & 255)` 进行截断以符合 0-255 的范围。
const buf5 = Buffer.from([257, 257.5, -255, '1']);

// 创建一个 Buffer，其中包含字符串 'tést' 的 UTF-8 编码字节：
// [0x74, 0xc3, 0xa9, 0x73, 0x74]（以十六进制表示）
// [116, 195, 169, 115, 116]（以十进制表示）
const buf6 = Buffer.from('tést');

// 创建一个包含 Latin-1 字节 [0x74, 0xe9, 0x73, 0x74] 的 Buffer。
const buf7 = Buffer.from('tést', 'latin1');

Buffer与字符编码

  • 当在Buffer和字符串之间转换时，可以指定字符编码，如果未指定字符编码，则使用UTF-8作为默认值
const buf = Buffer.from('hello world', 'utf8');

console.log(buf.toString('hex'));
// 打印: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
// 打印: aGVsbG8gd29ybGQ=

console.log(Buffer.from('fhqwhgads', 'utf8'));
// 打印: <Buffer 66 68 71 77 68 67 61 64 73>
console.log(Buffer.from('fhqwhgads', 'utf16le'));
// 打印: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>

在这里插入图片描述

  • ‘utf8’: 多字节编码的 Unicode 字符。 许多网页和其他文档格式都使用 UTF-8。 这是默认的字符编码。 当将 Buffer 解码为不专门包含有效 UTF-8 数据的字符串时，则会使用 Unicode 替换字符 U+FFFD来表示这些错误。
  • ‘utf16le’: 多字节编码的 Unicode 字符。 与 ‘utf8’ 不同，字符串中的每个字符都会使用 2 个或 4 个字节进行编码。 Node.js 仅支持 UTF-16 的小端序变体。
  • 'latin1: Latin-1 代表 ISO-8859-1。 此字符编码仅支持从U+0000到 U+00FF的 Unicode 字符。 每个字符使用单个字节进行编码。 超出该范围的字符会被截断，并映射成该范围内的字符。

使用以上方法之一将 Buffer 转换为字符串，称为解码；将字符串转换为Buffer，称为编码。

Buffer.from('1ag', 'hex');
// 打印 <Buffer 1a>，当遇到第一个非十六进制的值（'g'）时，则数据会被截断。

Buffer.from('1a7g', 'hex');
// 打印 <Buffer 1a>，当数据以一个数字（'7'）结尾时，则数据会被截断。

Buffer.from('1634', 'hex');
// 打印 <Buffer 16 34>，所有数据均可用。

Buffer与TypedArry

Buffer实例也是在JavaScript的Uint8Arry和TypeArray实例，所有的TypeArray方法在Buffer上也可用，但是，Buffer的API和TypedArray的API之间存在细微的不兼容

主要表现

  • TypedArray#slice（）会创建TypeArray的片段的拷贝，而Buffer#slice（）是在现有的Buffer上创建视图而不进行拷贝，此行为可能产生意外，并且仅用旧版的兼容性，TypedArray#subarray（）可用于在Buffer和其它TypedArray上实现Buffer#slice（）的行为
  • buf.tpString（）与它在TypedArray上的等价物并不兼容

两种方法可以从一个Buffer创建新的TypeArray实例

  • 将Buffer传给TypeArray的构造函数，则会拷贝Buffer的内容（会被解析为整数数组，而不是目标类型的字节序列）

    const buf = Buffer.from([1, 2, 3, 4]);
const uint32array = new Uint32Array(buf);

console.log(uint32array);

  • 传入Buffer底层的ArrayBuffer，则会创建与Buffer共享其内存的TypedArray

    const buf = Buffer.from('hello', 'utf16le');
const uint16array = new Uint16Array(
  buf.buffer,
  buf.byteOffset,
  buf.length / Uint16Array.BYTES_PER_ELEMENT
  );

console.log(uint16array);

  • 通过使用TypedArray对象的.buffer属性，以相同的方式来创建一个与TypedArray实例共享相同分配内存的新Buffer，在这种情况下，Buffer.from（）的行为类似于new Uint8Array（）

    const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// 拷贝 `arr` 的内容。
const buf1 = Buffer.from(arr);

// 与 `arr` 共享内存。
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// 打印: <Buffer 88 a0>
console.log(buf2);
// 打印: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// 打印: <Buffer 88 a0>
console.log(buf2);

当使用TypedArray的.buffer创建Buffer时，也可以通过传入byteOffset和length参数只使用底层ArrayBuffer的一部分

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);

注意：Buffer.from()与TypedArray.from()有着不同的实现，具体来说，TypedArray可以接受第二个函数作为映射函数，在类型数组的每个元素上调用

  • TypedArray.from(source[, mapFn[, thisArg]])

Buffer与迭代器

Buffer实例可以使用for…of语法进行迭代

const buf = Buffer.from([1, 2, 3]);

for (const b of buf) {
  console.log(b);
}

  const buf = Buffer.from('hello', 'utf16le');
  const uint16array = new Uint16Array(
    buf.buffer,
    buf.byteOffset,
    buf.length / Uint16Array.BYTES_PER_ELEMENT
    );
  
  console.log(uint16array);

在这里插入图片描述

  • 通过使用TypedArray对象的.buffer属性，以相同的方式来创建一个与TypedArray实例共享相同分配内存的新Buffer，在这种情况下，Buffer.from（）的行为类似于new Uint8Array（）

    const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// 拷贝 `arr` 的内容。
const buf1 = Buffer.from(arr);

// 与 `arr` 共享内存。
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// 打印: <Buffer 88 a0>
console.log(buf2);
// 打印: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// 打印: <Buffer 88 a0>
console.log(buf2);

在这里插入图片描述

当使用TypedArray的.buffer创建Buffer时，也可以通过传入byteOffset和length参数只使用底层ArrayBuffer的一部分

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);

在这里插入图片描述

注意：Buffer.from()与TypedArray.from()有着不同的实现，具体来说，TypedArray可以接受第二个函数作为映射函数，在类型数组的每个元素上调用

  • TypedArray.from(source[, mapFn[, thisArg]])

Buffer与迭代器

Buffer实例可以使用for…of语法进行迭代

const buf = Buffer.from([1, 2, 3]);

for (const b of buf) {
  console.log(b);
}
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