RecipesCam/docker/frontend/shared/utils/jpegDpi.ts

// Pure JPEG DPI metadata patcher.
// Re-encoding a JPEG through Skia drops the original EXIF/JFIF headers, so the
// DPI stamp ("72 dpi") is lost or defaulted. DPI is header metadata only — pixel
// dimensions never change — but print pipelines read it, so this rewrites the
// JFIF density (and EXIF XResolution/YResolution when present) to the requested
// value. All writes are same-length, so no segment offsets move.
//
// If the JPEG carries neither JFIF nor EXIF, a minimal JFIF APP0 segment is
// inserted right after SOI.

function u16(bytes: Uint8Array, p: number): number {
  return (bytes[p] << 8) | bytes[p + 1];
}
function setU16(bytes: Uint8Array, p: number, v: number): void {
  bytes[p] = (v >> 8) & 0xff;
  bytes[p + 1] = v & 0xff;
}

// Patches `bytes` in place for JFIF; returns the (possibly rebuilt) array.
export function patchJpegDpi(input: Uint8Array, dpi: number): Uint8Array {
  if (input.length < 2 || input[0] !== 0xff || input[1] !== 0xd8) return input;
  const bytes = input.slice(); // work on a copy; insertion path rebuilds anyway
  const dpiLo = dpi & 0xff;
  const dpiHi = (dpi >> 8) & 0xff;

  let sawJfif = false;
  let sawExif = false;

  let i = 2;
  while (i + 1 < bytes.length && bytes[i] === 0xff) {
    while (bytes[i] === 0xff) i++;
    const marker = bytes[i];
    i++;
    if (marker === 0xd9 || marker === 0xda) break; // EOI / SOS — headers only
    if (marker === 0x01 || (marker >= 0xd0 && marker <= 0xd7)) continue;
    if (i + 2 > bytes.length) break;
    const len = u16(bytes, i);
    if (len < 2 || i + len > bytes.length) break;
    const dataStart = i + 2;
    const payloadLen = len - 2;

    // APP0 JFIF: [FF E0] len [5x id "JFIF\0"][2 ver][1 units][2 Xd][2 Yd][1 thW][1 thH]...
    if (marker === 0xe0 && payloadLen >= 14 && bytes[dataStart] === 0x4a &&
        bytes[dataStart + 1] === 0x46 && bytes[dataStart + 2] === 0x49 &&
        bytes[dataStart + 3] === 0x46 && bytes[dataStart + 4] === 0x00) {
      sawJfif = true;
      bytes[dataStart + 7] = 1; // units = dots per inch
      bytes[dataStart + 8] = dpiHi;
      bytes[dataStart + 9] = dpiLo;
      bytes[dataStart + 10] = dpiHi;
      bytes[dataStart + 11] = dpiLo;
    }

    // APP1 EXIF: [FF E1] len "Exif\0\0" then TIFF
    if (marker === 0xe1 && payloadLen >= 14 &&
        bytes[dataStart] === 0x45 && bytes[dataStart + 1] === 0x78 &&
        bytes[dataStart + 2] === 0x69 && bytes[dataStart + 3] === 0x66 &&
        bytes[dataStart + 4] === 0x00 && bytes[dataStart + 5] === 0x00) {
      sawExif = true;
      const tiff = dataStart + 6;
      const le = bytes[tiff] === 0x49 && bytes[tiff + 1] === 0x49;
      const be = bytes[tiff] === 0x4d && bytes[tiff + 1] === 0x4d;
      if ((le || be) && tiff + 8 <= bytes.length) {
        const read32 = (p: number): number =>
          le
            ? bytes[p] | (bytes[p + 1] << 8) | (bytes[p + 2] << 16) | (bytes[p + 3] << 24)
            : ((bytes[p] << 24) | (bytes[p + 1] << 16) | (bytes[p + 2] << 8) | bytes[p + 3]) >>> 0;
        const write32 = (p: number, v: number): void => {
          if (le) {
            bytes[p] = v & 0xff; bytes[p + 1] = (v >> 8) & 0xff;
            bytes[p + 2] = (v >> 16) & 0xff; bytes[p + 3] = (v >> 24) & 0xff;
          } else {
            bytes[p] = (v >> 24) & 0xff; bytes[p + 1] = (v >> 16) & 0xff;
            bytes[p + 2] = (v >> 8) & 0xff; bytes[p + 3] = v & 0xff;
          }
        };
        // TIFF integer fields (tag/type/count/values) follow the file byte order
        const u16r = (p: number): number =>
          le ? bytes[p] | (bytes[p + 1] << 8) : (bytes[p] << 8) | bytes[p + 1];
        const u16w = (p: number, v: number): void => {
          if (le) { bytes[p] = v & 0xff; bytes[p + 1] = (v >> 8) & 0xff; }
          else { bytes[p] = (v >> 8) & 0xff; bytes[p + 1] = v & 0xff; }
        };
        const ifdOff = read32(tiff + 4);
        if (ifdOff + 2 <= bytes.length - tiff) {
          const ifd = tiff + ifdOff;
          const count = u16r(ifd);
          for (let e = 0; e < count; e++) {
            const entry = ifd + 2 + e * 12;
            if (entry + 12 > bytes.length) break;
            const tag = u16r(entry);
            const type = u16r(entry + 2);
            const cnt = read32(entry + 4);
            if ((tag === 0x011a || tag === 0x011b) && type === 5 && cnt === 1) {
              // RATIONAL: entry+8 holds offset to (num, den), relative to TIFF start
              const rat = tiff + read32(entry + 8);
              if (rat + 8 <= bytes.length) write32(rat, dpi); // keep denominator
            } else if (tag === 0x0128 && type === 3 && cnt === 1) {
              // ResolutionUnit (2 = inch): normalize to inch so the new density reads correctly
              u16w(entry + 8, 2);
            }
          }
        }
      }
    }

    i += len;
  }

  if (sawJfif || sawExif) return bytes;

  // Neither segment exists — insert a minimal JFIF APP0 (density = dpi inch) after SOI.
  const app0 = new Uint8Array([
    0xff, 0xe0, 0x00, 0x10,
    0x4a, 0x46, 0x49, 0x46, 0x00, // "JFIF\0"
    0x01, 0x01, // version 1.01
    0x01, // units: inch
    dpiHi, dpiLo, dpiHi, dpiLo,
    0x00, 0x00, // no thumbnail
  ]);
  const out = new Uint8Array(bytes.length + app0.length);
  out[0] = 0xff; out[1] = 0xd8;
  out.set(app0, 2);
  out.set(bytes.subarray(2), 2 + app0.length);
  return out;
}