import fs from 'node:fs';
import path from 'node:path';

export function createJsonStateStore({ stateDir, fileName, initialState = {} }) {
  if (!stateDir) throw new Error('Missing stateDir');
  if (!fileName) throw new Error('Missing fileName');

  fs.mkdirSync(stateDir, { recursive: true });
  const filePath = path.join(stateDir, fileName);
  const state = loadState(filePath, initialState);

  return {
    getState() {
      return state;
    },
    setState(nextState) {
      replaceState(state, nextState);
      persistState(filePath, state);
      return state;
    },
    update(mutator) {
      const nextState = mutator(structuredClone(state));
      replaceState(state, nextState);
      persistState(filePath, state);
      return state;
    },
  };
}

function replaceState(target, nextState) {
  const source = nextState === target ? structuredClone(nextState) : (nextState || {});
  for (const key of Object.keys(target)) delete target[key];
  Object.assign(target, source);
}

function loadState(filePath, initialState) {
  if (!fs.existsSync(filePath)) return structuredClone(initialState);

  try {
    return {
      ...structuredClone(initialState),
      ...JSON.parse(fs.readFileSync(filePath, 'utf8')),
    };
  } catch {
    return structuredClone(initialState);
  }
}

function persistState(filePath, state) {
  const tempPath = `${filePath}.tmp`;
  fs.writeFileSync(tempPath, JSON.stringify(state, null, 2));
  fs.renameSync(tempPath, filePath);
}