@mjackson/multipart-parser/src/lib/buffer-search.ts

A fast, efficient parser for multipart streams in any JavaScript environment

export interface SearchFunction {
  (haystack: Uint8Array, start?: number): number;
}

export function createSearch(pattern: string): SearchFunction {
  let needle = new TextEncoder().encode(pattern);

  let search: SearchFunction;
  if ('Buffer' in globalThis && !('Bun' in globalThis || 'Deno' in globalThis)) {
    // Use the built-in Buffer.indexOf method on Node.js for better perf.
    search = (haystack, start = 0) => Buffer.prototype.indexOf.call(haystack, needle, start);
  } else {
    let needleEnd = needle.length - 1;
    let skipTable = new Uint8Array(256).fill(needle.length);
    for (let i = 0; i < needleEnd; ++i) {
      skipTable[needle[i]] = needleEnd - i;
    }

    search = (haystack, start = 0) => {
      let haystackLength = haystack.length;
      let i = start + needleEnd;

      while (i < haystackLength) {
        for (let j = needleEnd, k = i; j >= 0 && haystack[k] === needle[j]; --j, --k) {
          if (j === 0) return k;
        }

        i += skipTable[haystack[i]];
      }

      return -1;
    };
  }

  return search;
}

export interface PartialTailSearchFunction {
  (haystack: Uint8Array): number;
}

export function createPartialTailSearch(pattern: string): PartialTailSearchFunction {
  let needle = new TextEncoder().encode(pattern);

  let byteIndexes: Record<number, number[]> = {};
  for (let i = 0; i < needle.length; ++i) {
    let byte = needle[i];
    if (byteIndexes[byte] === undefined) byteIndexes[byte] = [];
    byteIndexes[byte].push(i);
  }

  return function (haystack: Uint8Array): number {
    let haystackEnd = haystack.length - 1;

    if (haystack[haystackEnd] in byteIndexes) {
      let indexes = byteIndexes[haystack[haystackEnd]];

      for (let i = indexes.length - 1; i >= 0; --i) {
        for (let j = indexes[i], k = haystackEnd; j >= 0 && haystack[k] === needle[j]; --j, --k) {
          if (j === 0) return k;
        }
      }
    }

    return -1;
  };
}