lmexif-sniffer

const fs = require("node:fs");
const os = require("node:os");
const path = require("node:path");
const zlib = require("node:zlib");
const assert = require("node:assert/strict");
const { describe, it } = require("node:test");

const { parsePngTextAndZtxt } = require("../dist/pngText.js");

/** PNG CRC32 over *buf* (chunk type + chunk data). */
function crc32(buf) {
  let c = -1 >>> 0;
  for (let i = 0; i < buf.length; i++) {
    c ^= buf[i];
    for (let k = 0; k < 8; k++) c = (c >>> 1) ^ (0xedb88320 & -(c & 1));
  }
  return (c ^ -1) >>> 0;
}

/** Minimal 1x1 RGB PNG (IHDR + IDAT + IEND); insert ancillary chunks after IHDR (byte offset 33). */
function minimalRgbPngWithChunk(insertAfterIhdr) {
  const minimal = Buffer.from(
    "89504e470d0a1a0a0000000d4948445200000001000000010802000000907753de0000000c4944415408d763f8cfc000000301010018dd8db40000000049454e44ae426082",
    "hex",
  );
  return Buffer.concat([minimal.subarray(0, 33), insertAfterIhdr, minimal.subarray(33)]);
}

describe("parsePngTextAndZtxt", () => {
  it("returns empty for non-png", () => {
    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "lmexif-png-"));
    const p = path.join(dir, "x.txt");
    fs.writeFileSync(p, "not a png");
    assert.deepEqual(parsePngTextAndZtxt(p), { tEXt: [], zTXt: [] });
  });

  it("decodes zTXt with zlib (not gzip)", () => {
    const text = "hello-zTXt";
    const comp = zlib.deflateSync(Buffer.from(text, "utf8"));
    const chunkData = Buffer.concat([Buffer.from("Test\x00\x00", "binary"), comp]);
    const type = Buffer.from("zTXt");
    const crc = crc32(Buffer.concat([type, chunkData]));
    const chunk = Buffer.alloc(4 + 4 + chunkData.length + 4);
    chunk.writeUInt32BE(chunkData.length, 0);
    type.copy(chunk, 4);
    chunkData.copy(chunk, 8);
    chunk.writeUInt32BE(crc, 8 + chunkData.length);

    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "lmexif-png-"));
    const p = path.join(dir, "ztxt.png");
    fs.writeFileSync(p, minimalRgbPngWithChunk(chunk));

    const got = parsePngTextAndZtxt(p);
    assert.equal(got.tEXt.length, 0);
    assert.equal(got.zTXt.length, 1);
    assert.equal(got.zTXt[0].keyword, "Test");
    assert.equal(got.zTXt[0].text, text);
  });
});