// Copyright 2017-2023 @polkadot/util authors & contributors
// SPDX-License-Identifier: Apache-2.0

// This is supposed to be a faster concat...
// https://dev.to/uilicious/javascript-array-push-is-945x-faster-than-array-concat-1oki

/**
 * @name arrayFlatten
 * @summary Merge T[][] into T[]
 * @description
 * Returns a new array with all arrays merged into one
 * @example
 * <BR>
 *
 * ```javascript
 * import { arrayFlatten } from '@polkadot/util';
 *
 * arrayFlatten([[1, 2], [3, 4], [5]]); // [1, 2, 3, 4, 5]
 * ```
 */
export function arrayFlatten <T> (arrays: readonly T[][]): T[] {
  const num = arrays.length;

  // shortcuts for the empty & single-entry case
  if (num === 0) {
    return [];
  } else if (num === 1) {
    return arrays[0];
  }

  // pre-allocate based on the combined size
  let size = 0;

  for (let i = 0; i < num; i++) {
    size += arrays[i].length;
  }

  const output = new Array<T>(size);
  let i = -1;

  for (let j = 0; j < num; j++) {
    const a = arrays[j];

    // instead of pushing, we just set the entries
    for (let e = 0, count = a.length; e < count; e++) {
      output[++i] = a[e];
    }
  }

  return output;
}