lmexif-sniffer

import * as fs from "node:fs";
import { inflateSync } from "node:zlib";

/** PNG magic; exported for PNG writers. */
export const PNG_SIGNATURE = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]);

/** zTXt inflate output cap (zip-bomb mitigation). */
export const PNG_ZTXT_MAX_OUTPUT_BYTES = 50 * 1024 * 1024;

function decodePngTextChunk(data: Buffer): { keyword: string; text: string } | null {
  const z = data.indexOf(0);
  if (z < 0) return null;
  const keyword = data.subarray(0, z).toString("latin1");
  const text = data.subarray(z + 1).toString("latin1");
  return { keyword, text };
}

function decodePngZtxtChunk(data: Buffer): { keyword: string; text: string } | null {
  const z = data.indexOf(0);
  if (z < 0 || data.length < z + 2) return null;
  const keyword = data.subarray(0, z).toString("latin1");
  const compMethod = data[z + 1];
  const compressed = data.subarray(z + 2);
  if (compMethod !== 0) {
    return { keyword, text: `<unsupported zTXt compression method ${compMethod}>` };
  }
  try {
    const raw = inflateSync(compressed, { maxOutputLength: PNG_ZTXT_MAX_OUTPUT_BYTES });
    return { keyword, text: raw.toString("latin1") };
  } catch (e) {
    return { keyword, text: `<zlib decompress failed: ${String(e)}>` };
  }
}

export function parsePngTextAndZtxt(filePath: string): {
  tEXt: Array<{ keyword: string; text: string }>;
  zTXt: Array<{ keyword: string; text: string }>;
} {
  const out = { tEXt: [] as Array<{ keyword: string; text: string }>, zTXt: [] as Array<{ keyword: string; text: string }> };
  let data: Buffer;
  try {
    data = fs.readFileSync(filePath);
  } catch {
    return out;
  }
  if (data.length < 8 || !data.subarray(0, 8).equals(PNG_SIGNATURE)) return out;

  let pos = 8;
  while (pos + 8 <= data.length) {
    const length = data.readUInt32BE(pos);
    const ctype = data.subarray(pos + 4, pos + 8).toString("ascii");
    const chunkStart = pos + 8;
    const chunkEnd = chunkStart + length;
    if (chunkEnd + 4 > data.length) break;
    const chunkData = data.subarray(chunkStart, chunkEnd);
    pos = chunkEnd + 4;
    if (ctype === "IEND") break;
    if (ctype === "tEXt") {
      const entry = decodePngTextChunk(chunkData);
      if (entry) out.tEXt.push(entry);
    } else if (ctype === "zTXt") {
      const entry = decodePngZtxtChunk(chunkData);
      if (entry) out.zTXt.push(entry);
    }
  }
  return out;
}