// Package dbsnap makes the hub's nightly database snapshot (R-173, hub v0.136.0, `05` §16.3,
// runbooks/RUNBOOK-hub-db-offsite-backup.md Step 3).
//
// Every night the hub writes `<data>/snapshots/hub-<UTC stamp>.db` with SQLite's VACUUM INTO — one consistent point
// in time, WAL-aware — then keeps the newest Keep (2). DooPlex picks up the newest one, checks it, encrypts it and
// pushes it to ep0 (Step 4). The hub never ships anything itself: it has no ep0 credential.
//
// Written as `<name>.tmp` and renamed, so a reader never sees half a file; a run never overlaps another (a second call
// returns ErrBusy). Pinned by dbsnap_test.go.
package dbsnap

import (
	"context"
	"errors"
	"fmt"
	"log"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"time"
)

// Keep is how many snapshots stay on the volume (the newest). Decided by CC — operator may reverse (the volume was
// grown to 2 GiB for it, operator choice 2026-10-05).
const Keep = 2

// Prefix and Suffix make a snapshot's file name: hub-20261005T020000Z.db.
const (
	Prefix = "hub-"
	Suffix = ".db"
)

// ErrBusy is returned when a snapshot is already being written.
var ErrBusy = errors.New("dbsnap: a snapshot is already being written")

// Snapshotter is the store's VACUUM INTO.
type Snapshotter interface {
	SnapshotInto(ctx context.Context, path string) error
}

// Maker writes and prunes snapshots in Dir.
type Maker struct {
	Store  Snapshotter
	Dir    string
	Logger *log.Logger
	Now    func() time.Time

	mu      sync.Mutex
	running bool
}

// Result is one snapshot written.
type Result struct {
	Name     string        `json:"name"`
	Bytes    int64         `json:"bytes"`
	Duration time.Duration `json:"duration_ns"`
	Pruned   []string      `json:"pruned,omitempty"`
}

// Make writes one snapshot and prunes to Keep. Safe to call from the nightly job and the operator button at once.
func (m *Maker) Make(ctx context.Context) (Result, error) {
	m.mu.Lock()
	if m.running {
		m.mu.Unlock()
		return Result{}, ErrBusy
	}
	m.running = true
	m.mu.Unlock()
	defer func() { m.mu.Lock(); m.running = false; m.mu.Unlock() }()

	now := time.Now
	if m.Now != nil {
		now = m.Now
	}
	if err := os.MkdirAll(m.Dir, 0o700); err != nil {
		return Result{}, fmt.Errorf("dbsnap: %w", err)
	}
	name := Prefix + now().UTC().Format("20060102T150405Z") + Suffix
	final := filepath.Join(m.Dir, name)
	tmp := final + ".tmp"
	_ = os.Remove(tmp) // a leftover from a crash mid-write
	start := time.Now()
	if err := m.Store.SnapshotInto(ctx, tmp); err != nil {
		_ = os.Remove(tmp)
		return Result{}, err
	}
	if err := os.Chmod(tmp, 0o600); err != nil {
		_ = os.Remove(tmp)
		return Result{}, fmt.Errorf("dbsnap: chmod: %w", err)
	}
	if err := os.Rename(tmp, final); err != nil {
		_ = os.Remove(tmp)
		return Result{}, fmt.Errorf("dbsnap: rename: %w", err)
	}
	fi, err := os.Stat(final)
	if err != nil {
		return Result{}, fmt.Errorf("dbsnap: stat: %w", err)
	}
	res := Result{Name: name, Bytes: fi.Size(), Duration: time.Since(start)}
	res.Pruned = m.prune()
	if m.Logger != nil {
		m.Logger.Printf("[INFO] db snapshot written: %s (%d bytes, %s); pruned %d, keeping %d",
			name, res.Bytes, res.Duration.Round(time.Millisecond), len(res.Pruned), Keep)
	}
	return res, nil
}

// List returns the snapshot names in Dir, oldest first (the stamp sorts as text).
func List(dir string) []string {
	entries, _ := os.ReadDir(dir)
	var out []string
	for _, e := range entries {
		n := e.Name()
		if !e.IsDir() && strings.HasPrefix(n, Prefix) && strings.HasSuffix(n, Suffix) {
			out = append(out, n)
		}
	}
	sort.Strings(out)
	return out
}

func (m *Maker) prune() []string {
	names := List(m.Dir)
	var pruned []string
	for len(names) > Keep {
		if err := os.Remove(filepath.Join(m.Dir, names[0])); err != nil && m.Logger != nil {
			m.Logger.Printf("[WARN] db snapshot prune %s: %v", names[0], err)
		}
		pruned = append(pruned, names[0])
		names = names[1:]
	}
	return pruned
}

// NeedsCatchUp reports whether the newest snapshot in dir is missing or older than maxAge — the hub runs one at
// start-up then, so a pod that was down at 02:00 does not leave DooPlex pushing a two-day-old copy.
func NeedsCatchUp(dir string, now time.Time, maxAge time.Duration) bool {
	names := List(dir)
	if len(names) == 0 {
		return true
	}
	stamp := strings.TrimSuffix(strings.TrimPrefix(names[len(names)-1], Prefix), Suffix)
	t, err := time.Parse("20060102T150405Z", stamp)
	if err != nil {
		return true
	}
	return now.Sub(t) > maxAge
}