binary-search.ts

import type { SourceMapSegment, ReverseSegment } from './sourcemap-segment';
import { COLUMN } from './sourcemap-segment';

export type MemoState = {
  lastKey: number;
  lastNeedle: number;
  lastIndex: number;
};

export let found = false;

/**
 * A binary search implementation that returns the index if a match is found.
 * If no match is found, then the left-index (the index associated with the item that comes just
 * before the desired index) is returned. To maintain proper sort order, a splice would happen at
 * the next index:
 *
 * ```js
 * const array = [1, 3];
 * const needle = 2;
 * const index = binarySearch(array, needle, (item, needle) => item - needle);
 *
 * assert.equal(index, 0);
 * array.splice(index + 1, 0, needle);
 * assert.deepEqual(array, [1, 2, 3]);
 * ```
 */
export function binarySearch(
  haystack: SourceMapSegment[] | ReverseSegment[],
  needle: number,
  low: number,
  high: number,
): number {
  while (low <= high) {
    const mid = low + ((high - low) >> 1);
    const cmp = haystack[mid][COLUMN] - needle;

    if (cmp === 0) {
      found = true;
      return mid;
    }

    if (cmp < 0) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
  }

  found = false;
  return low - 1;
}

export function upperBound(
  haystack: SourceMapSegment[] | ReverseSegment[],
  needle: number,
  index: number,
): number {
  for (let i = index + 1; i < haystack.length; i++, index++) {
    if (haystack[i][COLUMN] !== needle) break;
  }
  return index;
}

export function lowerBound(
  haystack: SourceMapSegment[] | ReverseSegment[],
  needle: number,
  index: number,
): number {
  for (let i = index - 1; i >= 0; i--, index--) {
    if (haystack[i][COLUMN] !== needle) break;
  }
  return index;
}

export function memoizedState(): MemoState {
  return {
    lastKey: -1,
    lastNeedle: -1,
    lastIndex: -1,
  };
}

/**
 * This overly complicated beast is just to record the last tested line/column and the resulting
 * index, allowing us to skip a few tests if mappings are monotonically increasing.
 */
export function memoizedBinarySearch(
  haystack: SourceMapSegment[] | ReverseSegment[],
  needle: number,
  state: MemoState,
  key: number,
): number {
  const { lastKey, lastNeedle, lastIndex } = state;

  let low = 0;
  let high = haystack.length - 1;
  if (key === lastKey) {
    if (needle === lastNeedle) {
      return lastIndex;
    }

    if (needle >= lastNeedle) {
      // lastIndex may be -1 if the previous needle was not found.
      low = lastIndex === -1 ? 0 : lastIndex;
    } else {
      high = lastIndex;
    }
  }
  state.lastKey = key;
  state.lastNeedle = needle;

  return (state.lastIndex = binarySearch(haystack, needle, low, high));
}