package storage

// Scheduled snapshots with a retention period: take a snapshot of a dataset on a schedule,
// and drop the ones that have aged out.
//
// **The naming is zfSnap's, deliberately.** grid has run zfSnap v1.11.1 from root cron since
// before this repo existed and it works well, so rather than invent a second convention and
// have two tools disagree about what a snapshot is, this writes and reads exactly what
// zfSnap does: `<dataset>@2026-09-24_02.30.01--6w`, with the retention encoded in the name.
// That has three consequences worth knowing. Ham Lab can show and manage the snapshots
// zfSnap already took. The two can run side by side without fighting, because both only
// ever destroy snapshots that have passed the expiry written into their own name. And
// changing a policy's retention does not retroactively re-date old snapshots — each one
// carries the rule it was taken under, which is the behaviour you want from a backup.
//
// Two rules make this safe to run unattended. Only a snapshot whose name parses as a stamp
// and a TTL is ever a candidate — anything taken by hand or named anything else is invisible
// to it. And every policy can be asked what it would delete, before it deletes anything.

import (
	"context"
	"errors"
	"fmt"
	"sort"
	"strconv"
	"strings"
	"time"
)

// stampLayout is zfSnap's timestamp: 2026-09-24_02.30.01.
const stampLayout = "2006-01-02_15.04.05"

// ttlSep separates the stamp from the retention in a snapshot name, as zfSnap writes it.
const ttlSep = "--"

// Policy is a standing instruction: snapshot this dataset on this schedule, keep the
// snapshots this long.
type Policy struct {
	ID      string `json:"id"`
	Name    string `json:"name"`
	Dataset string `json:"dataset"`
	// Recursive takes one atomic snapshot of the dataset and everything under it (zfs -r).
	Recursive bool `json:"recursive"`
	// Every is hourly | daily | weekly | monthly. At is "HH:MM" (the minute alone is used
	// for hourly). Weekday is 0=Sunday, Day is 1–28.
	Every   string `json:"every"`
	At      string `json:"at"`
	Weekday int    `json:"weekday"`
	Day     int    `json:"day"`
	// KeepFor is how long a snapshot lives: "6w", "30d", "12h". It is written into the
	// snapshot's own name, zfSnap-style, so each one carries the rule it was taken under.
	// KeepMin is a floor — that many newest snapshots survive however old they are, so a
	// dataset is never left with none.
	KeepFor string `json:"keep_for"`
	KeepMin int    `json:"keep_min"`
	Prefix  string `json:"prefix"`
	Enabled bool   `json:"enabled"`

	LastRun   time.Time `json:"last_run,omitempty"`
	LastError string    `json:"last_error,omitempty"`
	LastTaken string    `json:"last_taken,omitempty"` // the snapshot the last run made
	Pruned    int       `json:"pruned,omitempty"`     // how many the last run destroyed
}

func (p *Policy) prefix() string { return strings.TrimSpace(p.Prefix) }

// Validate reports what is wrong with a policy, or nil.
func (p *Policy) Validate() error {
	if !datasetRe.MatchString(p.Dataset) {
		return errors.New("dataset: a pool or pool/dataset name")
	}
	if pre := p.prefix(); pre != "" && (!snapRe.MatchString(pre) || strings.Contains(pre, ttlSep)) {
		return errors.New("prefix: letters, digits, . _ : and - only, and no --")
	}
	switch p.Every {
	case "hourly", "daily", "weekly", "monthly":
	default:
		return errors.New("every: hourly, daily, weekly or monthly")
	}
	if _, _, err := p.clock(); err != nil {
		return err
	}
	if p.Every == "weekly" && (p.Weekday < 0 || p.Weekday > 6) {
		return errors.New("weekday: 0 (Sunday) to 6")
	}
	if p.Every == "monthly" && (p.Day < 1 || p.Day > 28) {
		return errors.New("day: 1 to 28, so every month has one")
	}
	if p.KeepFor != "" {
		if _, err := ParseAge(p.KeepFor); err != nil {
			return err
		}
	}
	if p.KeepMin < 0 {
		return errors.New("keep at least: 0 or more")
	}
	if p.KeepFor == "" {
		return errors.New("a retention period is required — it is written into each snapshot's name, and is what lets it be cleaned up later")
	}
	return nil
}

func (p *Policy) clock() (hour, min int, err error) {
	at := strings.TrimSpace(p.At)
	if at == "" {
		return 0, 0, nil
	}
	h, m, ok := strings.Cut(at, ":")
	if !ok {
		return 0, 0, errors.New("at: HH:MM")
	}
	hour, err = strconv.Atoi(h)
	if err == nil {
		min, err = strconv.Atoi(m)
	}
	if err != nil || hour < 0 || hour > 23 || min < 0 || min > 59 {
		return 0, 0, errors.New("at: HH:MM, 00:00 to 23:59")
	}
	return hour, min, nil
}

// ParseAge reads "6w", "30d", "12h", "90m" as a duration. Weeks and days are what a
// retention period is actually written in, and time.ParseDuration knows neither.
func ParseAge(s string) (time.Duration, error) {
	s = strings.TrimSpace(strings.ToLower(s))
	if s == "" {
		return 0, errors.New("empty period")
	}
	unit := s[len(s)-1]
	n, err := strconv.Atoi(s[:len(s)-1])
	if err != nil || n <= 0 {
		return 0, fmt.Errorf("%q: a number and a unit, like 6w, 30d or 12h", s)
	}
	switch unit {
	case 'm':
		return time.Duration(n) * time.Minute, nil
	case 'h':
		return time.Duration(n) * time.Hour, nil
	case 'd':
		return time.Duration(n) * 24 * time.Hour, nil
	case 'w':
		return time.Duration(n) * 7 * 24 * time.Hour, nil
	}
	return 0, fmt.Errorf("%q: the unit is m, h, d or w", s)
}

// lastDue is the most recent moment this policy was supposed to run, at or before now.
// A policy whose LastRun is older than that is due — so a machine that was off through
// its window takes one snapshot when it comes back, rather than skipping silently.
func (p *Policy) lastDue(now time.Time) time.Time {
	hour, min, _ := p.clock()
	y, mo, d := now.Date()
	loc := now.Location()
	switch p.Every {
	case "hourly":
		t := time.Date(y, mo, d, now.Hour(), min, 0, 0, loc)
		if t.After(now) {
			t = t.Add(-time.Hour)
		}
		return t
	case "daily":
		t := time.Date(y, mo, d, hour, min, 0, 0, loc)
		if t.After(now) {
			t = t.AddDate(0, 0, -1)
		}
		return t
	case "weekly":
		t := time.Date(y, mo, d, hour, min, 0, 0, loc)
		back := (int(t.Weekday()) - p.Weekday + 7) % 7
		t = t.AddDate(0, 0, -back)
		if t.After(now) {
			t = t.AddDate(0, 0, -7)
		}
		return t
	case "monthly":
		t := time.Date(y, mo, p.Day, hour, min, 0, 0, loc)
		if t.After(now) {
			t = t.AddDate(0, -1, 0)
		}
		return t
	}
	return time.Time{}
}

// NextRun is when this policy runs next, for the UI to show.
func (p *Policy) NextRun(now time.Time) time.Time {
	last := p.lastDue(now)
	switch p.Every {
	case "hourly":
		return last.Add(time.Hour)
	case "daily":
		return last.AddDate(0, 0, 1)
	case "weekly":
		return last.AddDate(0, 0, 7)
	case "monthly":
		return last.AddDate(0, 1, 0)
	}
	return time.Time{}
}

// Due reports whether the policy should run now.
func (p *Policy) Due(now time.Time) bool {
	if !p.Enabled {
		return false
	}
	d := p.lastDue(now)
	return !d.IsZero() && p.LastRun.Before(d)
}

// SnapTime reads a snapshot's name in zfSnap's convention and returns when it was taken,
// when it expires, and whether the name parsed at all. A name without a TTL never expires.
// An optional prefix in front of the stamp is allowed, so a policy can label its own.
func SnapTime(name string) (taken time.Time, expires time.Time, ok bool) {
	_, snap, cut := strings.Cut(name, "@")
	if !cut {
		return
	}
	body, ttl, hasTTL := strings.Cut(snap, ttlSep)
	// an optional "<prefix>-" before the stamp
	stamp := body
	if len(body) > len(stampLayout) {
		stamp = body[len(body)-len(stampLayout):]
	}
	t, err := time.ParseInLocation(stampLayout, stamp, time.Local)
	if err != nil {
		return time.Time{}, time.Time{}, false
	}
	taken, ok = t, true
	if hasTTL {
		if d, err := ParseAge(ttl); err == nil {
			expires = t.Add(d)
		}
	}
	return
}

// Doomed is which of this dataset's snapshots have passed the expiry written into their own
// names, oldest first — exactly what zfSnap -d would remove. This is what the UI shows
// before anything is deleted, and what Run then acts on. KeepMin holds back that many of the
// newest whatever their names say, so a dataset is never left with none.
func (m *Manager) Doomed(ctx context.Context, p *Policy) ([]Snapshot, error) {
	if err := p.Validate(); err != nil {
		return nil, err
	}
	all, err := m.Snapshots(ctx, p.Dataset)
	if err != nil {
		return nil, err
	}
	var dated []Snapshot
	expiry := map[string]time.Time{}
	for _, s := range all {
		taken, exp, ok := SnapTime(s.Name)
		if !ok || exp.IsZero() {
			continue // hand-made, or kept forever: not ours to remove
		}
		_ = taken
		dated = append(dated, s)
		expiry[s.Name] = exp
	}
	sort.Slice(dated, func(i, j int) bool { return dated[i].Created > dated[j].Created }) // newest first
	now := time.Now()
	var doomed []Snapshot
	for i, s := range dated {
		if i < p.KeepMin {
			continue // the floor: these survive however old they are
		}
		if expiry[s.Name].Before(now) {
			doomed = append(doomed, s)
		}
	}
	sort.Slice(doomed, func(i, j int) bool { return doomed[i].Created < doomed[j].Created }) // oldest first
	return doomed, nil
}

// DestroySnapshot removes one snapshot. recursive also removes the same-named snapshot on
// every dataset beneath it, which is what a recursive policy created.
func (m *Manager) DestroySnapshot(ctx context.Context, full string, recursive bool) error {
	dataset, snap, ok := strings.Cut(full, "@")
	if !ok || !datasetRe.MatchString(dataset) || !snapRe.MatchString(snap) {
		return errors.New("bad snapshot name")
	}
	args := []string{"zfs", "destroy"}
	if recursive {
		args = append(args, "-r")
	}
	out, code, err := m.run(ctx, append(args, full)...)
	if err != nil {
		return err
	}
	if code != 0 {
		return errors.New(strings.TrimSpace(string(out)))
	}
	return nil
}

// SnapshotRecursive takes dataset@name, optionally of everything beneath it too.
func (m *Manager) SnapshotRecursive(ctx context.Context, dataset, name string, recursive bool) error {
	if !datasetRe.MatchString(dataset) || !snapRe.MatchString(name) {
		return errors.New("bad snapshot name")
	}
	args := []string{"zfs", "snapshot"}
	if recursive {
		args = append(args, "-r")
	}
	out, code, err := m.run(ctx, append(args, dataset+"@"+name)...)
	if err != nil {
		return err
	}
	if code != 0 {
		return errors.New(strings.TrimSpace(string(out)))
	}
	return nil
}

// RunResult is what one pass of a policy did.
type RunResult struct {
	Taken   string   `json:"taken"`
	Pruned  []string `json:"pruned"`
	Skipped int      `json:"skipped"` // snapshots that are not this policy's, left alone
}

// Run takes the snapshot and prunes what has aged out. It is the same code the scheduler
// calls, so "Run now" and the timer cannot drift apart.
func (m *Manager) Run(ctx context.Context, p *Policy) (RunResult, error) {
	var res RunResult
	if err := p.Validate(); err != nil {
		return res, err
	}
	name := time.Now().Format(stampLayout)
	if pre := strings.TrimSpace(p.Prefix); pre != "" {
		name = pre + "-" + name
	}
	if p.KeepFor != "" {
		name += ttlSep + strings.TrimSpace(strings.ToLower(p.KeepFor)) // zfSnap's convention: the retention lives in the name
	}
	if err := m.SnapshotRecursive(ctx, p.Dataset, name, p.Recursive); err != nil {
		return res, err
	}
	res.Taken = p.Dataset + "@" + name
	doomed, err := m.Doomed(ctx, p)
	if err != nil {
		return res, err // the snapshot was taken; the prune is what failed
	}
	for _, s := range doomed {
		if err := m.DestroySnapshot(ctx, s.Name, p.Recursive); err != nil {
			return res, fmt.Errorf("destroying %s: %w", s.Name, err)
		}
		res.Pruned = append(res.Pruned, s.Name)
	}
	return res, nil
}