puzzle-trainer/lib/generators/zip.ts

import { createRng, shuffle } from "../rng";

export interface ZipPuzzle {
  size: number;
  path: [number, number][];    // solution path (all cells in order)
  numberedCells: Record<string, number>; // "r,c" → waypoint number (1-based)
  // walls[r][c] bitmask: bit0=right wall, bit1=bottom wall, bit2=left wall, bit3=top wall
  walls: number[][];
}

export function canMove(walls: number[][], r1: number, c1: number, r2: number, c2: number): boolean {
  const dr = r2 - r1, dc = c2 - c1;
  if (dr === 0 && dc === 1)  return !(walls[r1][c1] & 1); // right
  if (dr === 1 && dc === 0)  return !(walls[r1][c1] & 2); // down
  if (dr === 0 && dc === -1) return !(walls[r1][c1] & 4); // left
  if (dr === -1 && dc === 0) return !(walls[r1][c1] & 8); // up
  return false;
}

function generateHamiltonianPath(size: number, rng: () => number): [number, number][] | null {
  const total = size * size;
  const visited = Array.from({ length: size }, () => Array(size).fill(false));
  const path: [number, number][] = [];
  const dirs: [number, number][] = [[-1, 0], [1, 0], [0, -1], [0, 1]];

  const startR = Math.floor(rng() * size);
  const startC = Math.floor(rng() * size);
  visited[startR][startC] = true;
  path.push([startR, startC]);

  // Warnsdorff heuristic: prefer neighbors with fewer onward moves
  function warnsdorffScore(r: number, c: number): number {
    let n = 0;
    for (const [dr, dc] of dirs) {
      const nr = r + dr, nc = c + dc;
      if (nr >= 0 && nr < size && nc >= 0 && nc < size && !visited[nr][nc]) n++;
    }
    return n;
  }

  function bt(): boolean {
    if (path.length === total) return true;
    const [r, c] = path[path.length - 1];
    const nbrs = shuffle([...dirs], rng)
      .map(([dr, dc]) => [r + dr, c + dc] as [number, number])
      .filter(([nr, nc]) => nr >= 0 && nr < size && nc >= 0 && nc < size && !visited[nr][nc])
      .sort((a, b) => warnsdorffScore(a[0], a[1]) - warnsdorffScore(b[0], b[1]));

    for (const [nr, nc] of nbrs) {
      visited[nr][nc] = true;
      path.push([nr, nc]);
      if (bt()) return true;
      path.pop();
      visited[nr][nc] = false;
    }
    return false;
  }

  return bt() ? path : null;
}

// Build walls on ALL non-path-adjacent edges
function buildAllWalls(size: number, path: [number, number][]): number[][] {
  const openEdges = new Set<string>();
  for (let i = 0; i < path.length - 1; i++) {
    const [r1, c1] = path[i], [r2, c2] = path[i + 1];
    const key = r1 === r2
      ? `h:${r1},${Math.min(c1, c2)}`
      : `v:${Math.min(r1, r2)},${c1}`;
    openEdges.add(key);
  }

  const walls: number[][] = Array.from({ length: size }, () => Array(size).fill(0));
  for (let r = 0; r < size; r++) {
    for (let c = 0; c < size; c++) {
      if (c < size - 1 && !openEdges.has(`h:${r},${c}`)) {
        walls[r][c] |= 1;
        walls[r][c + 1] |= 4;
      }
      if (r < size - 1 && !openEdges.has(`v:${r},${c}`)) {
        walls[r][c] |= 2;
        walls[r + 1][c] |= 8;
      }
    }
  }
  return walls;
}

// Count Hamiltonian paths visiting waypoints in order — stops at limit
function countZipSolutions(
  size: number,
  walls: number[][],
  numberedCells: Record<string, number>,
  limit = 2
): number {
  const waypointOrder = Object.entries(numberedCells)
    .sort(([, a], [, b]) => a - b)
    .map(([k]) => k);

  if (!waypointOrder.length) return 0;
  const [startR, startC] = waypointOrder[0].split(",").map(Number);
  const total = size * size;

  let count = 0;
  const visited = Array.from({ length: size }, () => Array(size).fill(false));
  visited[startR][startC] = true;
  let visitedCount = 1;
  const dirs: [number, number][] = [[-1, 0], [1, 0], [0, -1], [0, 1]];

  // Island pruning: BFS from (r,c) through unvisited cells — if unreachable cells exist, prune
  const bfsQueue = new Int32Array(total);
  const bfsSeen = new Uint8Array(total);
  function allUnvisitedReachable(r: number, c: number): boolean {
    const remaining = total - visitedCount;
    if (remaining === 0) return true;
    bfsSeen.fill(0);
    let head = 0, tail = 0, found = 0;
    bfsQueue[tail++] = r * size + c;
    bfsSeen[r * size + c] = 1;
    while (head < tail) {
      const idx = bfsQueue[head++];
      const cr = (idx / size) | 0, cc = idx % size;
      for (const [dr, dc] of dirs) {
        const nr = cr + dr, nc = cc + dc;
        if (nr < 0 || nr >= size || nc < 0 || nc >= size) continue;
        if (visited[nr][nc]) continue;
        if (!canMove(walls, cr, cc, nr, nc)) continue;
        const nk = nr * size + nc;
        if (bfsSeen[nk]) continue;
        bfsSeen[nk] = 1;
        bfsQueue[tail++] = nk;
        found++;
        if (found === remaining) return true;
      }
    }
    return found === remaining;
  }

  function dfs(r: number, c: number, nextWpIdx: number): void {
    if (count >= limit) return;

    if (visitedCount === total) {
      if (nextWpIdx === waypointOrder.length) count++;
      return;
    }

    for (const [dr, dc] of dirs) {
      const nr = r + dr, nc = c + dc;
      if (nr < 0 || nr >= size || nc < 0 || nc >= size) continue;
      if (visited[nr][nc]) continue;
      if (!canMove(walls, r, c, nr, nc)) continue;

      const nextKey = `${nr},${nc}`;
      const wpNum = numberedCells[nextKey];
      // If the next cell is a waypoint, it must be the expected one
      if (wpNum !== undefined) {
        if (nextWpIdx >= waypointOrder.length || waypointOrder[nextWpIdx] !== nextKey) continue;
      }

      visited[nr][nc] = true;
      visitedCount++;
      if (allUnvisitedReachable(nr, nc)) {
        dfs(nr, nc, wpNum !== undefined ? nextWpIdx + 1 : nextWpIdx);
      }
      visited[nr][nc] = false;
      visitedCount--;
    }
  }

  dfs(startR, startC, 1); // already at waypoint[0], so next expected is index 1
  return count;
}

// Remove walls while uniqueness holds → sparse, interesting puzzle
function buildMinimalWalls(
  size: number,
  path: [number, number][],
  numberedCells: Record<string, number>,
  rng: () => number
): number[][] {
  const walls = buildAllWalls(size, path);

  // Collect all removable walls (non-path-adjacent edges)
  const candidates: Array<{ r1: number; c1: number; r2: number; c2: number }> = [];
  for (let r = 0; r < size; r++) {
    for (let c = 0; c < size; c++) {
      if (c < size - 1 && walls[r][c] & 1) candidates.push({ r1: r, c1: c, r2: r, c2: c + 1 });
      if (r < size - 1 && walls[r][c] & 2) candidates.push({ r1: r, c1: c, r2: r + 1, c2: c });
    }
  }

  for (const { r1, c1, r2, c2 } of shuffle(candidates, rng)) {
    const dc = c2 - c1;
    // Remove wall
    if (dc === 1) { walls[r1][c1] &= ~1; walls[r2][c2] &= ~4; }
    else          { walls[r1][c1] &= ~2; walls[r2][c2] &= ~8; }

    if (countZipSolutions(size, walls, numberedCells) !== 1) {
      // Restore wall — this wall is necessary
      if (dc === 1) { walls[r1][c1] |= 1; walls[r2][c2] |= 4; }
      else          { walls[r1][c1] |= 2; walls[r2][c2] |= 8; }
    }
  }

  return walls;
}

export function zipSizeForDate(date: string): number {
  const start = new Date("2024-01-01").getTime();
  const days = Math.max(0, Math.floor((new Date(date).getTime() - start) / 86400000));
  if (days < 200) return 6;
  if (days < 500) return 7;
  return 8;
}

export function generateZip(date: string, size?: number): ZipPuzzle {
  const resolvedSize = size ?? zipSizeForDate(date);
  const seed = date.split("-").reduce((a, n) => a * 1000 + parseInt(n), 0) + 777;
  const rng = createRng(seed);

  let path: [number, number][] | null = null;
  let attempts = 0;
  while (!path && attempts < 30) {
    path = generateHamiltonianPath(resolvedSize, rng);
    attempts++;
  }
  if (!path) path = snakePath(resolvedSize);

  // LinkedIn Zip always has exactly 10 waypoints
  const total = path.length;
  const numWaypoints = 10;
  const step = Math.floor((total - 1) / (numWaypoints - 1));
  const waypointIndices = Array.from({ length: numWaypoints }, (_, i) =>
    i === numWaypoints - 1 ? total - 1 : i * step
  );

  const numberedCells: Record<string, number> = {};
  waypointIndices.forEach((idx, i) => {
    const [r, c] = path![idx];
    numberedCells[`${r},${c}`] = i + 1;
  });

  const walls = buildMinimalWalls(resolvedSize, path, numberedCells, rng);

  return { size: resolvedSize, path, numberedCells, walls };
}

function snakePath(size: number): [number, number][] {
  const path: [number, number][] = [];
  for (let r = 0; r < size; r++)
    for (let c = r % 2 === 0 ? 0 : size - 1; r % 2 === 0 ? c < size : c >= 0; r % 2 === 0 ? c++ : c--)
      path.push([r, c]);
  return path;
}