export const composerDraftStorageKey = "jyotisha.composer-draft";

const composerDraftLimit = 500;
const composerDraftMaxAgeMs = 24 * 60 * 60 * 1000;

const listeners = new Set<() => void>();

let draft = "";
let restored = false;

function draftStorage(): Storage | null {
  if (typeof window === "undefined") return null;
  try {
    return window.sessionStorage;
  } catch {
    // Private-mode browsers without session storage keep the draft in memory only.
    return null;
  }
}

function clearStoredDraft() {
  const storage = draftStorage();
  if (!storage) return;
  try {
    storage.removeItem(composerDraftStorageKey);
  } catch {
    // A rejected write never blocks the composer.
  }
}

export function readStoredComposerDraft(): string {
  const storage = draftStorage();
  if (!storage) return "";
  let stored: string | null = null;
  try {
    stored = storage.getItem(composerDraftStorageKey);
  } catch {
    return "";
  }
  if (!stored) return "";
  try {
    const parsed = JSON.parse(stored) as Record<string, unknown>;
    const text = parsed.text;
    const savedAt = parsed.savedAt;
    if (typeof text !== "string"
      || typeof savedAt !== "number"
      || !Number.isFinite(savedAt)
      || savedAt > Date.now()
      || Date.now() - savedAt > composerDraftMaxAgeMs) {
      clearStoredDraft();
      return "";
    }
    return text.slice(0, composerDraftLimit);
  } catch {
    clearStoredDraft();
    return "";
  }
}

function writeStoredComposerDraft(value: string) {
  if (!value) {
    clearStoredDraft();
    return;
  }
  const storage = draftStorage();
  if (!storage) return;
  try {
    storage.setItem(composerDraftStorageKey, JSON.stringify({
      text: value.slice(0, composerDraftLimit),
      savedAt: Date.now(),
    }));
  } catch {
    // A full or blocked storage never blocks typing.
  }
}

function restoreComposerDraft() {
  if (restored) return;
  restored = true;
  draft = readStoredComposerDraft();
}

export function composerDraftSnapshot(): string {
  restoreComposerDraft();
  return draft;
}

export function serverComposerDraftSnapshot(): string {
  return "";
}

export function setComposerDraft(value: string) {
  restoreComposerDraft();
  if (draft === value) return;
  draft = value;
  writeStoredComposerDraft(value);
  for (const listener of [...listeners]) listener();
}

export function subscribeComposerDraft(listener: () => void): () => void {
  listeners.add(listener);
  return () => {
    listeners.delete(listener);
  };
}