import sharp from 'sharp';
import { renderLabel } from 'react-canvas-label-editor';

const THUMB_W = 150;
const THUMB_H = 80;

/**
 * Renders a 150×80 thumbnail for a label template.
 *
 * Workflow:
 *  1. Render the label at its native pixel resolution via renderLabel().
 *  2. Resize to fit within 150×80 preserving aspect ratio (fit: 'contain').
 *     Letterbox areas remain transparent.
 *  3. Process raw RGBA pixels:
 *     - Transparent letterbox padding (alpha < 128) → fully transparent (0,0,0,0).
 *     - Label pixels (alpha >= 128) → fully opaque (alpha=255) with 4-bit greyscale
 *       (16 levels: 0, 17, 34, ..., 255) for smooth anti-aliased text and barcodes.
 *  4. Output as palette PNG with transparency support.
 *
 * The result is a "4-bit grey + 1-bit alpha" image:
 *   letterbox area is transparent, and the label area is anti-aliased 4-bit greyscale.
 *
 * @param {object} opts
 * @param {number} opts.width   - Label width in pixels.
 * @param {number} opts.height  - Label height in pixels.
 * @param {Array}  opts.objects - Label editor objects array.
 * @returns {Promise<Buffer>} PNG buffer.
 */
export async function renderThumb({ width, height, objects }) {
  // 1. Render at native resolution → RGBA PNG buffer
  const nativePng = await renderLabel({ width, height, objects });

  // 2. Resize to fit within 150×80, preserving aspect ratio.
  //    Transparent background for letterbox padding.
  const { data, info } = await sharp(nativePng)
    .ensureAlpha()
    .resize(THUMB_W, THUMB_H, {
      fit: 'contain',
      background: { r: 0, g: 0, b: 0, alpha: 0 },
      kernel: sharp.kernel.lanczos3,
    })
    .raw()
    .toBuffer({ resolveWithObject: true });

  const { width: w, height: h } = info;

  // 3. Per-pixel: transparent letterbox remains transparent (alpha=0),
  //    label area is opaque (alpha=255) with 4-bit greyscale (16 levels).
  const rgba = Buffer.alloc(w * h * 4);
  for (let i = 0; i < w * h; i++) {
    const base = i * 4;
    const a = data[base + 3];

    if (a < 128) {
      // Transparent letterbox area — leave as (0,0,0,0)
    } else {
      const factor = a === 255 ? 1 : 255 / a;
      const r = Math.min(255, Math.round(data[base] * factor));
      const g = Math.min(255, Math.round(data[base + 1] * factor));
      const b = Math.min(255, Math.round(data[base + 2] * factor));
      const grey = 0.299 * r + 0.587 * g + 0.114 * b;
      // Quantize to 4-bit grey (16 levels: 0, 17, 34, ..., 255)
      const qGrey = Math.round(grey / 17) * 17;
      rgba[base]     = qGrey;
      rgba[base + 1] = qGrey;
      rgba[base + 2] = qGrey;
      rgba[base + 3] = 255;
    }
  }

  // 4. Encode as palette PNG with transparency support (16 greys + 1 transparent entry).
  const thumb = await sharp(rgba, { raw: { width: w, height: h, channels: 4 } })
    .png({ palette: true, colours: 17, compressionLevel: 9 })
    .toBuffer();

  return thumb;
}