// Erkennung von Buchstaben-Platzhaltern (eine dominante Hintergrundfarbe, Initialen darauf).
// Nextcloud liefert solche Bilder teils als „eigene" Avatare aus; im Kalender sollen aber nur echte Fotos landen.
import zlib from 'node:zlib';
import { imageMime } from './store.js';

const PNG_SIG = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]);

/** Anteil der häufigsten Farbe (0–1) und Zahl unterschiedlicher Farben, per Stichprobe. Nur PNG (8 Bit, RGB/RGBA/Grau). */
export function pngDominantShare(buffer, step = 4) {
  if (!buffer || buffer.length < 33 || !buffer.subarray(0, 8).equals(PNG_SIG)) return null;
  let pos = 8;
  let width = 0; let height = 0; let depth = 0; let colorType = 0;
  const idat = [];
  while (pos + 8 <= buffer.length) {
    const len = buffer.readUInt32BE(pos);
    const type = buffer.toString('ascii', pos + 4, pos + 8);
    const body = buffer.subarray(pos + 8, pos + 8 + len);
    if (type === 'IHDR') { width = body.readUInt32BE(0); height = body.readUInt32BE(4); depth = body[8]; colorType = body[9]; }
    else if (type === 'IDAT') idat.push(body);
    else if (type === 'IEND') break;
    pos += 12 + len;
  }
  const channels = { 0: 1, 2: 3, 4: 2, 6: 4 }[colorType];
  if (!channels || depth !== 8 || !width || !height) return null;
  let raw;
  try { raw = zlib.inflateSync(Buffer.concat(idat)); } catch { return null; }
  const bpp = channels;
  const stride = width * bpp;
  let prev = Buffer.alloc(stride);
  let offset = 0;
  const counts = new Map();
  let total = 0;
  for (let y = 0; y < height; y += 1) {
    const filter = raw[offset];
    const line = Buffer.from(raw.subarray(offset + 1, offset + 1 + stride));
    offset += 1 + stride;
    for (let x = 0; x < stride; x += 1) {
      const a = x >= bpp ? line[x - bpp] : 0;
      const b = prev[x];
      const c = x >= bpp ? prev[x - bpp] : 0;
      if (filter === 1) line[x] = (line[x] + a) & 255;
      else if (filter === 2) line[x] = (line[x] + b) & 255;
      else if (filter === 3) line[x] = (line[x] + ((a + b) >> 1)) & 255;
      else if (filter === 4) {
        const p = a + b - c;
        const pa = Math.abs(p - a); const pb = Math.abs(p - b); const pc = Math.abs(p - c);
        line[x] = (line[x] + (pa <= pb && pa <= pc ? a : pb <= pc ? b : c)) & 255;
      }
    }
    if (y % step === 0) {
      for (let x = 0; x < stride; x += bpp * step) {
        const key = channels >= 3 ? (line[x] << 16) | (line[x + 1] << 8) | line[x + 2] : line[x] * 0x010101;
        counts.set(key, (counts.get(key) || 0) + 1);
        total += 1;
      }
    }
    prev = line;
  }
  if (total === 0) return null;
  let max = 0;
  for (const n of counts.values()) if (n > max) max = n;
  return { share: max / total, colors: counts.size, width, height };
}

/** true, wenn das Bild wie ein generierter Initialen-Avatar aussieht: eine Farbe füllt den Großteil der Fläche. */
export function looksLikePlaceholder(buffer, threshold = 0.75) {
  if (imageMime(buffer) !== 'image/png') return false; // JPEG/WebP sind praktisch immer echte Fotos
  const info = pngDominantShare(buffer);
  return Boolean(info && info.share >= threshold);
}