package library

import (
	"context"
	"errors"
	"fmt"
	"sync"
	"time"

	"github.com/jackc/pgx/v5/pgxpool"

	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
)

// Fingerprinting settings (M400 #3913).
//
// Rule 25: what an operator might tune lives in the database and changes without
// a restart. One service instance is shared by the scanner, the backfill, the
// duplicate sweep and the admin API, so a save reaches all of them at once.
//
// The length is the dangerous knob: chromaprints taken at two lengths cannot be
// compared. Rather than bumping fingerprintVersion when it changes, every row
// records the length it was taken at (migration 0061) and every reader filters
// on the current one. A row at another length is stale to the backfill, pending
// to the gauge and invisible to the sweep, which is the effect a version bump
// would have, with one difference worth having: setting the length back makes
// rows not yet re-derived current again, instead of redoing the library twice.

// Bounds for each setting, mirrored by migration 0061's CHECKs.
const (
	// Below 30s a print has too few items to align at the matcher's larger
	// offsets and still overlap by minOverlapItems. Above 600s each print passes
	// 20 KB and a candidate page of them grows past what one sweep should hold.
	minChromaprintLengthSec = 30
	maxChromaprintLengthSec = 600
	// Unrelated audio sits near 0.5; a ceiling of 0.35 keeps a loosened
	// threshold well clear of proposing noise.
	minAcousticMaxBitErrorRate = 0.01
	maxAcousticMaxBitErrorRate = 0.35
	minBackfillConcurrency     = 1
	maxBackfillConcurrency     = 8
	minSweepIntervalHours      = 1
	maxSweepIntervalHours      = 168

	defaultSweepIntervalHours = 1
)

// FingerprintSettings mirrors the fingerprint_settings row.
type FingerprintSettings struct {
	// Enabled off stops every decode: the scan takes only the stream hash (a
	// demux, and what recognises a moved file) and stores nothing, and the
	// backfill idles.
	Enabled                 bool
	ChromaprintLengthSec    int32
	AcousticMaxBitErrorRate float64
	BackfillConcurrency     int32
	SweepIntervalHours      int32
	// UpdatedAt is when the settings were last saved. Set by the database;
	// ignored by Set.
	UpdatedAt time.Time
}

// DefaultFingerprintSettings mirrors migration 0061's column defaults, so a
// database that cannot be read still fingerprints the way a fresh install does.
var DefaultFingerprintSettings = FingerprintSettings{
	Enabled:                 true,
	ChromaprintLengthSec:    defaultChromaprintLengthSec,
	AcousticMaxBitErrorRate: defaultAcousticMaxBitErrorRate,
	BackfillConcurrency:     fingerprintBackfillConcurrency,
	SweepIntervalHours:      defaultSweepIntervalHours,
}

// ErrFingerprintSettingOutOfRange is returned by Set for a value migration
// 0061's CHECKs would reject, so the API answers 400 naming the field rather
// than surfacing a constraint violation.
var ErrFingerprintSettingOutOfRange = errors.New("fingerprint setting out of range")

// FingerprintSettingsService caches the settings and owns their persistence.
// Cached because the scanner reads them for every file it fingerprints.
type FingerprintSettingsService struct {
	pool *pgxpool.Pool

	mu  sync.RWMutex
	cur FingerprintSettings
}

// NewFingerprintSettingsService loads once and caches. It always returns a
// usable service, holding the defaults when the load fails; the error says so.
func NewFingerprintSettingsService(ctx context.Context, pool *pgxpool.Pool) (*FingerprintSettingsService, error) {
	s := &FingerprintSettingsService{pool: pool, cur: DefaultFingerprintSettings}
	row, err := dbq.New(pool).GetFingerprintSettings(ctx)
	if err != nil {
		return s, fmt.Errorf("fingerprint settings: load: %w", err)
	}
	s.cur = fingerprintSettingsFromRow(row)
	return s, nil
}

// Get returns the cached settings. A nil service answers with the defaults, so
// a Scanner or worker built without one fingerprints as a fresh install would.
func (s *FingerprintSettingsService) Get() FingerprintSettings {
	if s == nil {
		return DefaultFingerprintSettings
	}
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.cur
}

// Set validates, persists and re-caches.
func (s *FingerprintSettingsService) Set(ctx context.Context, in FingerprintSettings) (FingerprintSettings, error) {
	if err := validateFingerprintSettings(in); err != nil {
		return FingerprintSettings{}, err
	}
	if s == nil {
		return FingerprintSettings{}, errors.New("fingerprint settings: no settings service")
	}
	row, err := dbq.New(s.pool).UpdateFingerprintSettings(ctx, dbq.UpdateFingerprintSettingsParams{
		Enabled:                 in.Enabled,
		ChromaprintLengthSec:    in.ChromaprintLengthSec,
		AcousticMaxBitErrorRate: in.AcousticMaxBitErrorRate,
		BackfillConcurrency:     in.BackfillConcurrency,
		SweepIntervalHours:      in.SweepIntervalHours,
	})
	if err != nil {
		return FingerprintSettings{}, fmt.Errorf("fingerprint settings: save: %w", err)
	}
	out := fingerprintSettingsFromRow(row)
	s.mu.Lock()
	s.cur = out
	s.mu.Unlock()
	return out, nil
}

func validateFingerprintSettings(in FingerprintSettings) error {
	switch {
	case in.ChromaprintLengthSec < minChromaprintLengthSec || in.ChromaprintLengthSec > maxChromaprintLengthSec:
		return fmt.Errorf("%w: chromaprint_length_sec must be %d-%d",
			ErrFingerprintSettingOutOfRange, minChromaprintLengthSec, maxChromaprintLengthSec)
	// Written as a negated range so NaN, which fails every comparison, is refused.
	case !(in.AcousticMaxBitErrorRate >= minAcousticMaxBitErrorRate && in.AcousticMaxBitErrorRate <= maxAcousticMaxBitErrorRate):
		return fmt.Errorf("%w: acoustic_max_bit_error_rate must be %.2f-%.2f",
			ErrFingerprintSettingOutOfRange, minAcousticMaxBitErrorRate, maxAcousticMaxBitErrorRate)
	case in.BackfillConcurrency < minBackfillConcurrency || in.BackfillConcurrency > maxBackfillConcurrency:
		return fmt.Errorf("%w: backfill_concurrency must be %d-%d",
			ErrFingerprintSettingOutOfRange, minBackfillConcurrency, maxBackfillConcurrency)
	case in.SweepIntervalHours < minSweepIntervalHours || in.SweepIntervalHours > maxSweepIntervalHours:
		return fmt.Errorf("%w: sweep_interval_hours must be %d-%d",
			ErrFingerprintSettingOutOfRange, minSweepIntervalHours, maxSweepIntervalHours)
	}
	return nil
}

func fingerprintSettingsFromRow(row dbq.FingerprintSetting) FingerprintSettings {
	return FingerprintSettings{
		Enabled:                 row.Enabled,
		ChromaprintLengthSec:    row.ChromaprintLengthSec,
		AcousticMaxBitErrorRate: row.AcousticMaxBitErrorRate,
		BackfillConcurrency:     row.BackfillConcurrency,
		SweepIntervalHours:      row.SweepIntervalHours,
		UpdatedAt:               row.UpdatedAt.Time,
	}
}