lodash/internal/mergeData.js

import arrayCopy from './arrayCopy';
import composeArgs from './composeArgs';
import composeArgsRight from './composeArgsRight';
import replaceHolders from './replaceHolders';

/** Used to compose bitmasks for wrapper metadata. */
var BIND_FLAG = 1,
    BIND_KEY_FLAG = 2,
    CURRY_BOUND_FLAG = 4,
    CURRY_RIGHT_FLAG = 16,
    REARG_FLAG = 128,
    ARY_FLAG = 256;

/** Used as the internal argument placeholder. */
var PLACEHOLDER = '__lodash_placeholder__';

/* Native method references for those with the same name as other `lodash` methods. */
var nativeMin = Math.min;

/**
 * Merges the function metadata of `source` into `data`.
 *
 * Merging metadata reduces the number of wrappers required to invoke a function.
 * This is possible because methods like `_.bind`, `_.curry`, and `_.partial`
 * may be applied regardless of execution order. Methods like `_.ary` and `_.rearg`
 * augment function arguments, making the order in which they are executed important,
 * preventing the merging of metadata. However, we make an exception for a safe
 * common case where curried functions have `_.ary` and or `_.rearg` applied.
 *
 * @private
 * @param {Array} data The destination metadata.
 * @param {Array} source The source metadata.
 * @returns {Array} Returns `data`.
 */
function mergeData(data, source) {
  var bitmask = data[1],
      srcBitmask = source[1],
      newBitmask = bitmask | srcBitmask;

  var arityFlags = ARY_FLAG | REARG_FLAG,
      bindFlags = BIND_FLAG | BIND_KEY_FLAG,
      comboFlags = arityFlags | bindFlags | CURRY_BOUND_FLAG | CURRY_RIGHT_FLAG;

  var isAry = bitmask & ARY_FLAG && !(srcBitmask & ARY_FLAG),
      isRearg = bitmask & REARG_FLAG && !(srcBitmask & REARG_FLAG),
      argPos = (isRearg ? data : source)[7],
      ary = (isAry ? data : source)[8];

  var isCommon = !(bitmask >= REARG_FLAG && srcBitmask > bindFlags) &&
    !(bitmask > bindFlags && srcBitmask >= REARG_FLAG);

  var isCombo = (newBitmask >= arityFlags && newBitmask <= comboFlags) &&
    (bitmask < REARG_FLAG || ((isRearg || isAry) && argPos.length <= ary));

  // Exit early if metadata can't be merged.
  if (!(isCommon || isCombo)) {
    return data;
  }
  // Use source `thisArg` if available.
  if (srcBitmask & BIND_FLAG) {
    data[2] = source[2];
    // Set when currying a bound function.
    newBitmask |= (bitmask & BIND_FLAG) ? 0 : CURRY_BOUND_FLAG;
  }
  // Compose partial arguments.
  var value = source[3];
  if (value) {
    var partials = data[3];
    data[3] = partials ? composeArgs(partials, value, source[4]) : arrayCopy(value);
    data[4] = partials ? replaceHolders(data[3], PLACEHOLDER) : arrayCopy(source[4]);
  }
  // Compose partial right arguments.
  value = source[5];
  if (value) {
    partials = data[5];
    data[5] = partials ? composeArgsRight(partials, value, source[6]) : arrayCopy(value);
    data[6] = partials ? replaceHolders(data[5], PLACEHOLDER) : arrayCopy(source[6]);
  }
  // Use source `argPos` if available.
  value = source[7];
  if (value) {
    data[7] = arrayCopy(value);
  }
  // Use source `ary` if it's smaller.
  if (srcBitmask & ARY_FLAG) {
    data[8] = data[8] == null ? source[8] : nativeMin(data[8], source[8]);
  }
  // Use source `arity` if one is not provided.
  if (data[9] == null) {
    data[9] = source[9];
  }
  // Use source `func` and merge bitmasks.
  data[0] = source[0];
  data[1] = newBitmask;

  return data;
}

export default mergeData;