// ── canonical identifiers ────────────────────────────────────────────

const CANONICAL_KEY  = "confidence-engine-investigation";
const LEGACY_KEY     = "confidence-engine-session";
const SCHEMA_VERSION = 1;

// ── helpers ──────────────────────────────────────────────────────────

function safeGet(storage, key) {
  if (!storage || typeof storage.getItem !== "function") return null;
  try { return storage.getItem(key); } catch (_) { return null; }
}

function isPlainObject(value) {
  return (
    value !== null &&
    typeof value === "object" &&
    !Array.isArray(value) &&
    Object.prototype.toString.call(value) === "[object Object]"
  );
}

// ── loadInvestigation ────────────────────────────────────────────────

/**
 * Returns the persisted investigation snapshot (normalised to current schema)
 * or null when no usable state exists.
 */
export function loadInvestigation() {
  const storage = _getTargetStorage();
  if (!storage) return null;

  // 1 – canonical key first
  let raw = safeGet(storage, CANONICAL_KEY);
  if (raw !== null) {
    try {
      const parsed = JSON.parse(raw);
      if (isPlainObject(parsed)) {
        if (!("schemaVersion" in parsed)) parsed.schemaVersion = SCHEMA_VERSION;
        return parsed;
      }
    } catch (_) { /* malformed — fall through to legacy */ }
  }

  // 2 – legacy fallback
  const legacyStorage = _getLegacyStorage();
  if (!legacyStorage) return null;

  raw = safeGet(legacyStorage, LEGACY_KEY);
  if (raw === null) return null;
  let parsed;
  try {
    parsed = JSON.parse(raw);
  } catch (_) { return null; }

  if (!isPlainObject(parsed)) return null;

  // Normalise schemaVersion for legacy data
  if (!("schemaVersion" in parsed)) parsed.schemaVersion = SCHEMA_VERSION;

  // Migrate into canonical key
  _persist(storage, CANONICAL_KEY, JSON.stringify(parsed));

  return parsed;
}

// ── saveInvestigation ────────────────────────────────────────────────

/**
 * Persists the supplied snapshot to the canonical localStorage key.
 * The caller's object is never mutated.
 */
export function saveInvestigation(snapshot) {
  const storage = _getTargetStorage();
  if (!storage) return; // silently no-op in non-browser

  try {
    const record = JSON.parse(JSON.stringify(snapshot));
    record.schemaVersion = SCHEMA_VERSION;
    _persist(storage, CANONICAL_KEY, JSON.stringify(record));
  } catch (_) { /* storage errors must not crash caller */ }
}

// ── clearInvestigation ───────────────────────────────────────────────

/** Removes only the canonical investigation key. */
export function clearInvestigation() {
  const storage = _getTargetStorage();
  if (!storage) return;
  try { storage.removeItem(CANONICAL_KEY); } catch (_) {}
}

// ── internals ────────────────────────────────────────────────────────

function _persist(storage, key, value) {
  try { storage.setItem(key, value); } catch (_) {}
}

function _getTargetStorage() {
  if (typeof globalThis.window === "undefined") return null;
  const s = globalThis.window.localStorage;
  if (!s || typeof s.getItem !== "function") return null;
  return s;
}

function _getLegacyStorage() {
  if (typeof globalThis.window === "undefined") return null;
  const s = globalThis.window.sessionStorage;
  if (!s || typeof s.getItem !== "function") return null;
  return s;
}