/**
 * codecs.js — Minimal codecs for decoding tree results.
 *
 * Implements: decodeResult (from Research.hs)
 *   - Leaf → "t"
 *   - Numbers: toNumber
 *   - Strings: toString
 *   - Lists: toList
 *   - Fallback: raw tree format
 */

// ── toNumber ────────────────────────────────────────────────────────────────

/**
 * Decode a tree as a binary number (big-endian).
 * Leaf = 0, Fork(Leaf, rest) = 2*n, Fork(Stem Leaf, rest) = 2*n+1.
 */
export function toNumber(t) {
  if (!Array.isArray(t)) return null;
  if (t.length === 0) return 0; // Leaf = 0
  if (t.length !== 2) return null; // must be Fork

  const [right, left] = t;
  // Fork structure: [right, left]
  // left child determines bit: Leaf = 0, Stem(Leaf) = 1
  let bit;
  if (Array.isArray(left) && left.length === 0) {
    bit = 0; // Leaf
  } else if (Array.isArray(left) && left.length === 1) {
    const child = left[0];
    if (Array.isArray(child) && child.length === 0) {
      bit = 1; // Stem(Leaf) = 1
    } else {
      return null; // Stem of something other than Leaf
    }
  } else {
    return null;
  }

  const rest = toNumber(right);
  if (rest === null) return null;

  return bit + 2 * rest;
}

// ── toString ────────────────────────────────────────────────────────────────

/**
 * Decode a tree as a list of numbers (characters).
 * Fork(x, rest) = x : list.
 */
export function toList(t) {
  if (!Array.isArray(t)) return null;
  if (t.length === 0) return []; // Leaf = empty list
  if (t.length !== 2) return null; // must be Fork

  const [right, left] = t;
  const rest = toList(right);
  if (rest === null) return null;

  return [left, ...rest];
}

/**
 * Decode a tree as a string.
 */
export function toString(t) {
  const list = toList(t);
  if (list === null) return null;
  try {
    return list.map((ch) => String.fromCharCode(ch)).join("");
  } catch {
    return null;
  }
}

// ── decodeResult ────────────────────────────────────────────────────────────

/**
 * Decode a tree result using multiple strategies:
 *   1. Leaf → "t"
 *   2. String (if all chars are printable)
 *   3. Number
 *   4. List
 *   5. Raw tree format
 */
export function decodeResult(t) {
  if (!Array.isArray(t)) {
    return String(t);
  }

  // Leaf
  if (t.length === 0) {
    return "t";
  }

  // Try string first (list of char codes)
  const list = toList(t);
  if (list !== null && list.length > 0) {
    const str = list.map((n) => {
      if (n < 32 || n > 126) return null;
      return String.fromCharCode(n);
    }).join("");
    if (str) return `"${str}"`;
  }

  // Try number
  const num = toNumber(t);
  if (num !== null) {
    return String(num);
  }

  // Try list (elements are trees)
  if (t.length === 2) {
    const elements = toList(t);
    if (elements !== null) {
      const decoded = elements.map((e) => decodeResult(e));
      return `[${decoded.join(", ")}]`;
    }
  }

  // Raw tree format
  return formatTree(t);
}

/**
 * Format a tree as a parenthesized expression.
 */
export function formatTree(t) {
  if (!Array.isArray(t)) return String(t);
  if (t.length === 0) return "Leaf";
  if (t.length === 1) return `Stem(${formatTree(t[0])})`;
  if (t.length === 2) return `Fork(${formatTree(t[1])}, ${formatTree(t[0])})`;
  return `[${t.map(formatTree).join(", ")}]`;
}