// Kiitos engine: dictionary word-set + prefix trie, validation, and AI move choice.
// Pure logic — no Express. Initialized once at server start from the ENABLE list.
//
// Game shape (see public/src/games/kiitos/KiitosLogic.js for the full state model):
//   A shared centre sequence is built one letter at a time toward an owner's
//   declared target word. On your turn you MUST play the next required letter if
//   you hold it; otherwise you may "deviate" — play a letter that repurposes the
//   sequence toward a new word you declare. This engine answers the dictionary
//   questions (is this a word? does this prefix lead anywhere? what should the AI
//   play?) that the pure logic layer deliberately does not embed.

const ABS_MIN = 4;   // shortest legal Kiitos word (round 1 minimum)
const MAX_LEN = 12;  // cap word length so play stays snappy

let root = null;     // trie root: { word: boolean, kids: Map<char, node> }
let wordSet = null;  // Set<string> of every legal Kiitos word

const completionCache = new Map(); // `${prefix}|${minLen}` -> boolean
const wordCache = new Map();       // `${prefix}|${minLen}` -> string|null

function makeNode() { return { word: false, kids: new Map() }; }

export function initKiitosDictionary(words) {
  root = makeNode();
  wordSet = new Set();
  completionCache.clear();
  wordCache.clear();
  for (const raw of words) {
    const w = String(raw).toUpperCase();
    if (w.length < ABS_MIN || w.length > MAX_LEN || !/^[A-Z]+$/.test(w)) continue;
    wordSet.add(w);
    let node = root;
    for (const ch of w) {
      let next = node.kids.get(ch);
      if (!next) { next = makeNode(); node.kids.set(ch, next); }
      node = next;
    }
    node.word = true;
  }
  return { words: wordSet.size };
}

export const MIN_WORD_LEN = ABS_MIN;

// ── Lookups ─────────────────────────────────────────────────────────────────────

function nodeFor(prefix) {
  let n = root;
  for (const ch of prefix) {
    if (!n) return null;
    n = n.kids.get(ch);
  }
  return n ?? null;
}

// Does the subtree rooted at n contain any word marker (n itself counts)?
function subtreeHasWord(n) {
  if (!n) return false;
  if (n.word) return true;
  for (const c of n.kids.values()) if (subtreeHasWord(c)) return true;
  return false;
}

// Does the subtree contain a word whose total length is >= prefixLen + depth?
function subtreeHasWordAtDepth(n, depth) {
  if (!n) return false;
  if (depth <= 0) return subtreeHasWord(n);
  for (const c of n.kids.values()) if (subtreeHasWordAtDepth(c, depth - 1)) return true;
  return false;
}

export function isValidKiitosWord(word, minLen = ABS_MIN) {
  if (!wordSet) return false;
  const s = String(word).toUpperCase();
  return s.length >= minLen && wordSet.has(s);
}

// Is `prefix` the start of at least one legal word of length >= minLen?
export function hasCompletion(prefix, minLen = ABS_MIN) {
  const key = `${prefix}|${minLen}`;
  const cached = completionCache.get(key);
  if (cached !== undefined) return cached;
  const n = nodeFor(prefix);
  const ok = n ? subtreeHasWordAtDepth(n, minLen - prefix.length) : false;
  completionCache.set(key, ok);
  return ok;
}

// Shallowest legal word (length >= minLen) that starts with `prefix`, or null.
export function findWord(prefix, minLen = ABS_MIN) {
  const key = `${prefix}|${minLen}`;
  const cached = wordCache.get(key);
  if (cached !== undefined) return cached;
  const start = nodeFor(prefix);
  let result = null;
  if (start) {
    let frontier = [[start, prefix]];
    while (frontier.length && !result) {
      const next = [];
      for (const [n, s] of frontier) {
        if (n.word && s.length >= minLen) { result = s; break; }
        if (s.length < MAX_LEN) for (const [ch, c] of n.kids) next.push([c, s + ch]);
      }
      frontier = next;
    }
  }
  wordCache.set(key, result);
  return result;
}

// ── AI move selection ─────────────────────────────────────────────────────────
// Returns one of:
//   { type: 'play', letter, forced: true }            — mandatory next letter
//   { type: 'deviate', letter, prefix, word }          — repurpose / start a word
//   { type: 'pass' }                                   — no legal move
//
// params: { hand:[letters], built, targetWord, minLen, skill, allowPrepend, superKiitos }

function letterCounts(letters) {
  const m = Object.create(null);
  for (const c of letters) m[c] = (m[c] || 0) + 1;
  return m;
}

function weightedPick(items, weights) {
  let total = 0;
  for (const w of weights) total += w;
  if (total <= 0) return items[Math.floor(Math.random() * items.length)];
  let roll = Math.random() * total;
  for (let i = 0; i < items.length; i++) {
    roll -= weights[i];
    if (roll <= 0) return items[i];
  }
  return items[items.length - 1];
}

export function chooseMove({
  hand, built = '', targetWord = null, minLen = ABS_MIN,
  skill = 3, allowPrepend = false, superKiitos = false,
}) {
  const H = hand.map((c) => String(c).toUpperCase());

  // 1. Forced play: holding the next required letter is mandatory.
  if (targetWord && built.length < targetWord.length) {
    const need = targetWord[built.length];
    if (H.includes(need)) return { type: 'play', letter: need, forced: true };
  }

  // 2. Deviate / start: every letter-and-placement that leads to a real word.
  const handCounts = letterCounts(H);
  const seen = new Set();
  const cands = [];
  const consider = (prefix, letter) => {
    if (seen.has(prefix)) return;
    seen.add(prefix);
    if (!hasCompletion(prefix, minLen)) return;
    const word = findWord(prefix, minLen);
    if (word) cands.push({ letter, prefix, word });
  };
  for (const L of new Set(H)) {
    consider(built + L, L);
    if (allowPrepend) consider(L + built, L);
    if (superKiitos) {
      for (let i = 0; i <= built.length; i++) consider(built.slice(0, i) + L + built.slice(i), L);
    }
  }
  if (!cands.length) return { type: 'pass' };

  // Score each candidate. The prize move is one that completes a word right now;
  // failing that, favour words that are close to done and that the AI can help
  // finish from its own hand. Higher skill sharpens these preferences and adds a
  // mild taste for longer (higher-scoring) words.
  const k = Math.max(1, Math.min(5, skill | 0));
  const weights = cands.map((c) => {
    const wordCounts = letterCounts(c.word.split(''));
    const prefixCounts = letterCounts(c.prefix.split(''));
    const handAfter = { ...handCounts };
    handAfter[c.letter] = (handAfter[c.letter] || 0) - 1;
    let need = 0, have = 0;
    for (const ch of Object.keys(wordCounts)) {
      const remaining = Math.max(0, wordCounts[ch] - (prefixCounts[ch] || 0));
      need += remaining;
      have += Math.min(remaining, Math.max(0, handAfter[ch] || 0));
    }
    if (need === 0) return 40 + c.word.length;       // immediate completion — grab it
    const selfFrac = have / need;
    const closeness = 1 / (1 + need);                // fewer letters left is better
    const base = closeness * (0.3 + selfFrac);
    return Math.pow(base, 0.6 + k * 0.25) * Math.pow(c.word.length, (k - 1) * 0.04);
  });

  const pick = weightedPick(cands, weights);
  return { type: 'deviate', letter: pick.letter, prefix: pick.prefix, word: pick.word };
}