index.js

import crc from 'crc';

// https://stackoverflow.com/a/39244362/2715716
// TODO: Refactor it so that it allocates one buffer at the start
// which is sized approximately so that it fits the signature, IHDR from the 1st
// frame, ACTL, the entire total of the buffers' sizes and IEND, then gets
// written to: the signature, the IHDR from the initial frame, the FCTL and FDAT
// of all the buffers in a for-of loop and then an IEND after the loop and after
// this, slice the buffer from zero to the actual length it got written to to
// get rid of the excess (which wouldn't break the PNG, there can be anything
// after IEND, but would make it larger than necessary, albeit not by very much
// at all)
export default function makeApng(/** @type {Buffer[]} */ buffers, /** @type {number} */ delay) {
  function findChunk(buffer, type, offset = 8) {
    while (offset < buffer.length) {
      const chunkLength = buffer.readUInt32BE(offset);
      const chunkType = buffer.slice(offset + 4, offset + 8).toString('ascii');

      if (chunkType === type) {
        return buffer.slice(offset, offset + chunkLength + 12);
      }

      offset += 4 + 4 + chunkLength + 4;
    }

    return null;
  }

  const actl = Buffer.alloc(20);
  actl.writeUInt32BE(8, 0); // Length of chunk
  actl.write('acTL', 4); // Type of chunk
  actl.writeUInt32BE(buffers.length, 8); // Number of frames
  actl.writeUInt32BE(0, 12); // Number of times to loop (0 - infinite)
  actl.writeUInt32BE(crc.crc32(actl.slice(4, 16)), 16); // CRC

  let sequenceNumber = 0;
  const frames = buffers.map((data, index) => {
    const ihdr = findChunk(data, 'IHDR');
    if (ihdr === null) {
      throw new Error('IHDR chunk not found!');
    }

    const fctl = Buffer.alloc(38);
    fctl.writeUInt32BE(26, 0); // Length of chunk
    fctl.write('fcTL', 4); // Type of chunk
    fctl.writeUInt32BE(sequenceNumber++, 8); // Sequence number
    fctl.writeUInt32BE(ihdr.readUInt32BE(8), 12); // Width
    fctl.writeUInt32BE(ihdr.readUInt32BE(12), 16); // Height
    fctl.writeUInt32BE(0, 20); // X offset
    fctl.writeUInt32BE(0, 24); // Y offset
    const { numerator, denominator } = delay(index);
    fctl.writeUInt16BE(numerator, 28); // Frame delay - fraction numerator
    fctl.writeUInt16BE(denominator, 30); // Frame delay - fraction denominator
    fctl.writeUInt8(0, 32); // Dispose mode
    fctl.writeUInt8(0, 33); // Blend mode
    fctl.writeUInt32BE(crc.crc32(fctl.slice(4, 34)), 34); // CRC

    let offset = 8;
    const fdats = [];
    while (true) {
      const idat = findChunk(data, 'IDAT', offset);
      if (idat === null) {
        if (offset === 8) {
          throw new Error('No IDAT chunks found!');
        }
        else {
          break;
        }
      }

      offset = idat.byteOffset + idat.length;

      // All IDAT chunks except first one are converted to fdAT chunks
      if (index === 0) {
        fdats.push(idat);
      } else {
        const length = idat.length + 4;
        const fdat = Buffer.alloc(length);
        fdat.writeUInt32BE(length - 12, 0); // Length of chunk
        fdat.write('fdAT', 4); // Type of chunk
        fdat.writeUInt32BE(sequenceNumber++, 8); // Sequence number
        idat.copy(fdat, 12, 8); // Image data
        fdat.writeUInt32BE(crc.crc32(fdat.slice(4, length - 4)), length - 4); // CRC
        fdats.push(fdat);
      }
    }

    return Buffer.concat([fctl, ...fdats]);
  });

  const signature = Buffer.from('89504e470d0a1a0a', 'hex');
  const ihdr = findChunk(buffers[0], 'IHDR');
  if (ihdr === null) {
    throw new Error('IHDR chunk not found!');
  }

  const iend = Buffer.from('0000000049454e44ae426082', 'hex');
  return Buffer.concat([signature, ihdr, actl, ...frames, iend]);
}