SnapOtter/apps/api/src/routes/tools/find-duplicates.ts

import { basename } from "node:path";
import type { FastifyInstance } from "fastify";
import sharp from "sharp";

/**
 * Compute a dHash (difference hash) for perceptual duplicate detection.
 * Resize to 9x8 grayscale, compare adjacent pixels to create 64-bit hash.
 */
async function computeDHash(buffer: Buffer): Promise<string> {
  const pixels = await sharp(buffer).resize(9, 8, { fit: "fill" }).grayscale().raw().toBuffer();

  let hash = "";
  for (let y = 0; y < 8; y++) {
    for (let x = 0; x < 8; x++) {
      const left = pixels[y * 9 + x];
      const right = pixels[y * 9 + x + 1];
      hash += left > right ? "1" : "0";
    }
  }
  return hash;
}

/**
 * Compute hamming distance between two 64-bit hash strings.
 */
function hammingDistance(a: string, b: string): number {
  let distance = 0;
  for (let i = 0; i < a.length; i++) {
    if (a[i] !== b[i]) distance++;
  }
  return distance;
}

export function registerFindDuplicates(app: FastifyInstance) {
  app.post("/api/v1/tools/find-duplicates", async (request, reply) => {
    const files: Array<{ buffer: Buffer; filename: string }> = [];

    try {
      const parts = request.parts();
      for await (const part of parts) {
        if (part.type === "file") {
          const chunks: Buffer[] = [];
          for await (const chunk of part.file) {
            chunks.push(chunk);
          }
          const buf = Buffer.concat(chunks);
          if (buf.length > 0) {
            files.push({
              buffer: buf,
              filename: basename(part.filename ?? `image-${files.length}`),
            });
          }
        }
      }
    } catch (err) {
      return reply.status(400).send({
        error: "Failed to parse multipart request",
        details: err instanceof Error ? err.message : String(err),
      });
    }

    if (files.length < 2) {
      return reply
        .status(400)
        .send({ error: "At least 2 images are required for duplicate detection" });
    }

    try {
      // Compute hashes for all images
      const hashes: Array<{ filename: string; hash: string }> = [];
      for (const file of files) {
        const hash = await computeDHash(file.buffer);
        hashes.push({ filename: file.filename, hash });
      }

      // Compare all pairs, group duplicates
      const threshold = 10; // Hamming distance threshold for "similar"
      const groups: Array<{
        files: Array<{ filename: string; similarity: number }>;
      }> = [];
      const assigned = new Set<number>();

      for (let i = 0; i < hashes.length; i++) {
        if (assigned.has(i)) continue;

        const group: Array<{ filename: string; similarity: number }> = [
          { filename: hashes[i].filename, similarity: 100 },
        ];

        for (let j = i + 1; j < hashes.length; j++) {
          if (assigned.has(j)) continue;
          const dist = hammingDistance(hashes[i].hash, hashes[j].hash);
          if (dist <= threshold) {
            const similarity = Math.round((1 - dist / 64) * 10000) / 100;
            group.push({ filename: hashes[j].filename, similarity });
            assigned.add(j);
          }
        }

        if (group.length > 1) {
          assigned.add(i);
          groups.push({ files: group });
        }
      }

      return reply.send({
        totalImages: files.length,
        duplicateGroups: groups,
        uniqueImages: files.length - assigned.size,
      });
    } catch (err) {
      return reply.status(422).send({
        error: "Duplicate detection failed",
        details: err instanceof Error ? err.message : "Unknown error",
      });
    }
  });
}