/**
 * Durable snapshot store — the reconcile cache, persisted to disk. On boot the
 * app shows the last snapshot instantly (stale-while-revalidate) instead of a
 * blank board while ~2N gitea calls run; if gitea is unreachable, reads fall
 * back to it (offline). It's a rebuildable mirror — the durable truth stays in
 * gitea (the purity split, D4). A plain JSON file: the whole snapshot fits in
 * memory at this scale, so indexed SQL buys nothing yet (see the PR).
 */

import { readFileSync, writeFileSync } from 'node:fs'
import { join } from 'node:path'

import { app } from 'electron'

/** The shape we persist — kept loose so a schema drift degrades to "no cache", not a crash. */
export interface PersistedSnapshot {
  issues: unknown[]
  milestones: unknown[]
  deps: unknown[]
  timelines: Record<number, unknown[]>
  /** ISO time the snapshot was reconciled — shown as "cached since". */
  savedAt: string
}

function snapshotPath(): string {
  return join(app.getPath('userData'), 'commitea-snapshot.json')
}

/**
 * Load the last persisted snapshot, or null if absent/corrupt. Never throws — a
 * deleted or unreadable cache degrades to "no cache" (the purity guarantee, D4:
 * the next reconcile rebuilds it from gitea). `path` is injectable for tests;
 * production always uses the userData file.
 */
export function loadSnapshot(path: string = snapshotPath()): PersistedSnapshot | null {
  try {
    const parsed = JSON.parse(readFileSync(path, 'utf8')) as PersistedSnapshot
    if (parsed && Array.isArray(parsed.issues)) return parsed
    return null
  } catch {
    return null // missing file, bad JSON, or drift — treat as no cache
  }
}

/** Persist a freshly reconciled snapshot. Best-effort — a write failure never breaks a reconcile. */
export function saveSnapshot(snap: Omit<PersistedSnapshot, 'savedAt'>, savedAt: string, path: string = snapshotPath()): void {
  try {
    writeFileSync(path, JSON.stringify({ ...snap, savedAt }), 'utf8')
  } catch {
    // disk full / permissions — the in-memory cache still works this session
  }
}