package archive

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"slices"
	"strings"
	"time"

	"sneak.berlin/go/webhooker/internal/database"
)

// The archive rotations: how often a database target starts a new
// archive file. Every rotation but none puts the period of an event's
// receive time, in UTC, in the name of the file the event goes to,
// written in the layout beside it.
const (
	archiveRotationNone    = "none"
	archiveRotationMonthly = "monthly"
	archiveRotationDaily   = "daily"
	archiveRotationHourly  = "hourly"

	archiveMonthLayout = "2006-01"
	archiveDayLayout   = "2006-01-02"
	archiveHourLayout  = "2006-01-02-15"
)

// errArchiveRotationUnknown is returned for a rotation that is not one
// of the four.
var errArchiveRotationUnknown = errors.New(
	"rotation must be none, monthly, daily or hourly",
)

// archiveFile is one of a database target's archive files, and the
// period in its name: "" for the file named without one.
type archiveFile struct {
	path   string
	period string
}

// ValidateRotation checks a user-supplied archive rotation for
// a database target: empty or none (both meaning one file), monthly,
// daily or hourly.
func ValidateRotation(rotation string) error {
	switch rotation {
	case "", archiveRotationNone, archiveRotationMonthly,
		archiveRotationDaily, archiveRotationHourly:
		return nil
	default:
		return fmt.Errorf("%w: %q", errArchiveRotationUnknown, rotation)
	}
}

// archivePeriod returns the period, in UTC, that a rotation puts an
// event received at receivedAt in: "2026-10" for monthly,
// "2026-10-01" for daily, "2026-10-01-19" for hourly, and "" for none
// or an empty rotation.
func archivePeriod(rotation string, receivedAt time.Time) string {
	switch rotation {
	case archiveRotationMonthly:
		return receivedAt.UTC().Format(archiveMonthLayout)
	case archiveRotationDaily:
		return receivedAt.UTC().Format(archiveDayLayout)
	case archiveRotationHourly:
		return receivedAt.UTC().Format(archiveHourLayout)
	default:
		return ""
	}
}

// archivePeriodPath returns the path of a database target's archive
// file for a period: path, as Path gives it, with "-" and the
// period put before its ".db". The period "" gives path itself.
func archivePeriodPath(path, period string) string {
	if period == "" {
		return path
	}

	return strings.TrimSuffix(path, ".db") + "-" + period + ".db"
}

// PathAt returns the archive file a database target writes an
// event received at receivedAt to: Path's file, with the
// period in its name when the target rotates.
func PathAt(
	dbMgr *database.WebhookDBManager,
	webhook *database.Webhook,
	target *database.Target,
	receivedAt time.Time,
) string {
	return archivePeriodPath(
		Path(dbMgr, webhook, target),
		archivePeriod(target.ArchiveRotation, receivedAt),
	)
}

// archiveFiles lists the database target's archive files that exist,
// given the path Path gives it: the file at path, then each
// file archivePeriodPath names for path and a period, oldest period
// first. Which rotation wrote a file does not matter, so the files of
// an earlier rotation setting are listed too.
func archiveFiles(path string) ([]archiveFile, error) {
	dir := filepath.Dir(path)

	entries, err := os.ReadDir(dir)
	if err != nil {
		return nil, fmt.Errorf("listing archive files: %w", err)
	}

	stem := strings.TrimSuffix(filepath.Base(path), ".db")

	var files []archiveFile

	for _, entry := range entries {
		period, ok := archiveFilePeriod(stem, entry.Name())
		if ok {
			files = append(files, archiveFile{
				path:   filepath.Join(dir, entry.Name()),
				period: period,
			})
		}
	}

	// A month sorts before the days and hours in it.
	slices.SortFunc(files, func(a, b archiveFile) int {
		return strings.Compare(a.period, b.period)
	})

	return files, nil
}

// archiveFilePeriod reports whether name is the name of an archive
// file of the target whose file name without a period is stem+".db",
// and the period in it.
func archiveFilePeriod(stem, name string) (string, bool) {
	rest, ok := strings.CutPrefix(name, stem)
	if !ok {
		return "", false
	}

	rest, ok = strings.CutSuffix(rest, ".db")
	if !ok {
		return "", false
	}

	if rest == "" {
		return "", true
	}

	period, ok := strings.CutPrefix(rest, "-")
	if !ok {
		return "", false
	}

	for _, layout := range []string{
		archiveMonthLayout, archiveDayLayout, archiveHourLayout,
	} {
		_, err := time.Parse(layout, period)
		if err == nil {
			return period, true
		}
	}

	return "", false
}