/** Minimal PNG writer, just enough for test previews. */
import { deflateSync } from 'node:zlib';

function crc32(buf) {
  let c, crc = 0xffffffff;
  for (let n = 0; n < buf.length; n++) {
    c = (crc ^ buf[n]) & 0xff;
    for (let k = 0; k < 8; k++) c = c & 1 ? 0xedb88320 ^ (c >>> 1) : c >>> 1;
    crc = c ^ (crc >>> 8);
  }
  return (crc ^ 0xffffffff) >>> 0;
}

function chunk(type, data) {
  const len = Buffer.alloc(4);
  len.writeUInt32BE(data.length);
  const body = Buffer.concat([Buffer.from(type, 'latin1'), data]);
  const crc = Buffer.alloc(4);
  crc.writeUInt32BE(crc32(body));
  return Buffer.concat([len, body, crc]);
}

/** rgb: (x,y) => [r,g,b] */
export function writePNG(width, height, rgb) {
  const raw = Buffer.alloc((width * 3 + 1) * height);
  let p = 0;
  for (let y = 0; y < height; y++) {
    raw[p++] = 0;
    for (let x = 0; x < width; x++) {
      const [r, g, b] = rgb(x, y);
      raw[p++] = r; raw[p++] = g; raw[p++] = b;
    }
  }
  const ihdr = Buffer.alloc(13);
  ihdr.writeUInt32BE(width, 0);
  ihdr.writeUInt32BE(height, 4);
  ihdr[8] = 8; ihdr[9] = 2; ihdr[10] = 0; ihdr[11] = 0; ihdr[12] = 0;
  return Buffer.concat([
    Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]),
    chunk('IHDR', ihdr),
    chunk('IDAT', deflateSync(raw)),
    chunk('IEND', Buffer.alloc(0)),
  ]);
}