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• [FILE] bit-converter.js
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• [FILE] index.js

Просмотр файла: bit-converter.js

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.createBitArray = createBitArray;
exports.fromBits = fromBits;
exports.toBits = toBits;

/* eslint-disable no-unused-vars */

/**
 * Virtual type for bit arrays, i.e., arrays in which each element contains
 * an integer in range `[0, 1 << L)`, where `1 <= L <= 8`.
 */

/* eslint-enable no-unused-vars */

/**
 * Performs unchecked conversion from `Uint8Array` to `BitArray`.
 * This function is translated as the indentity operation by Babel; it's needed purely
 * for Flow type checks.
 *
 * @param {Uint8Array} src
 *   array to convert
 * @returns {Uint8Array}
 *   `src` interpreted as a `BitArray` with the specified bitness
 *
 * @api private
 */
function toBitArrayUnchecked(src) {
  return src;
}
/**
 * Creates a new array with specified bitness.
 *
 * @param {number} len
 *   length of the created array
 * @returns {Uint8Array}
 *
 * @api private
 */


function createBitArray(len) {
  return toBitArrayUnchecked(new Uint8Array(len));
}
/**
 * Converts an array from one number of bits per element to another.
 *
 * @api private
 */


function convert(src, srcBits, dst, dstBits, pad) {
  var mask = (1 << dstBits) - 1;
  var acc = 0;
  var bits = 0;
  var pos = 0;
  src.forEach(function (b) {
    // Pull next bits from the input buffer into accumulator.
    acc = (acc << srcBits) + b;
    bits += srcBits; // Push into the output buffer while there are enough bits in the accumulator.

    while (bits >= dstBits) {
      bits -= dstBits;
      dst[pos] = acc >> bits & mask;
      pos += 1;
    }
  });

  if (pad) {
    if (bits > 0) {
      // `dstBits - rem.bits` is the number of trailing zero bits needed to be appended
      // to accumulator bits to get the trailing bit group.
      dst[pos] = acc << dstBits - bits & mask;
    }
  } else {
    // Truncate the remaining padding, but make sure that it is zeroed and not
    // overly long first.
    if (bits >= srcBits) {
      throw new Error("Excessive padding: ".concat(bits, " (max ").concat(srcBits - 1, " allowed)"));
    }

    if (acc % (1 << bits) !== 0) {
      throw new Error('Non-zero padding');
    }
  }
}
/**
 * Encodes a `Uint8Array` buffer as an array with a lesser number of bits per element.
 *
 * @api private
 */


function toBits(src, bits, dst) {
  if (bits > 8 || bits < 1) {
    throw new RangeError('Invalid bits per element; 1 to 8 expected');
  } // `BitArray<8>` is equivalent to `Uint8Array`; unfortunately, Flow
  // has problems expressing this, so the explicit conversion is performed here.


  convert(toBitArrayUnchecked(src), 8, dst, bits, true);
  return dst;
}

function fromBits(src, bits, dst) {
  if (bits > 8 || bits < 1) {
    throw new RangeError('Invalid bits per element; 1 to 8 expected');
  }

  convert(src, bits, toBitArrayUnchecked(dst), 8, false);
  return dst;
}

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