// Package providers builds from the config the parts pixad serves images
// and logins with, and provides them to the fx graph:
//   - the image cache, with its default limit
//   - the fetcher
//   - the processor
//   - the image service
//   - the session manager
//   - the encrypted URL generator
package providers

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"syscall"

	"github.com/prometheus/client_golang/prometheus"
	"go.uber.org/fx"
	"sneak.berlin/go/pixa/internal/config"
	"sneak.berlin/go/pixa/internal/database"
	"sneak.berlin/go/pixa/internal/encurl"
	"sneak.berlin/go/pixa/internal/httpfetcher"
	"sneak.berlin/go/pixa/internal/imageprocessor"
	"sneak.berlin/go/pixa/internal/imgcache"
	"sneak.berlin/go/pixa/internal/logger"
	"sneak.berlin/go/pixa/internal/session"
)

// Constructors returns the constructors of the parts this package builds,
// for fx.Provide. The fx graph must also provide the prometheus.Registerer
// the cache registers its counters on.
func Constructors() []any {
	return []any{
		newCache,
		newFetcherConfig,
		httpfetcher.New,
		newProcessor,
		newImageService,
		newSessionManager,
		newEncryptedURLGenerator,
	}
}

// cacheParams defines dependencies for newCache.
type cacheParams struct {
	fx.In

	Config          *config.Config
	Database        *database.Database
	Logger          *logger.Logger
	MetricsRegistry prometheus.Registerer
}

// newCache builds the image cache with the disk cache limit cacheMaxBytes
// returns. Its start hook starts the cache's background goroutines. Its
// stop hook stops them, which writes the pending access times. The
// database is open for both: it opens before the cache is built and
// closes after this stop hook.
func newCache(lc fx.Lifecycle, params cacheParams) (*imgcache.Cache, error) {
	cfg := params.Config
	db := params.Database.DB()

	maxBytes, err := cacheMaxBytes(
		context.Background(), cfg, db, params.Logger.Get(), defaultFreeSpaceProbe)
	if err != nil {
		return nil, err
	}

	cache, err := imgcache.NewCache(db, imgcache.CacheConfig{
		StateDir:    cfg.StateDir,
		NegativeTTL: imgcache.DefaultNegativeTTL,
		MaxBytes:    maxBytes,
		Logger:      params.Logger.Get(),
	}, params.MetricsRegistry)
	if err != nil {
		return nil, err
	}

	// The start hook takes no context: the goroutines it starts outlive it,
	// and the stop hook stops them.
	lc.Append(fx.StartStopHook(
		func() {
			// Background eviction: startup reconciliation, then periodic
			// and write-pressure passes, and the removal of expired
			// negative cache rows, which is all it does when the disk
			// cache is off.
			cache.StartEviction(imgcache.DefaultEvictionInterval)

			// Writes the access times that requests hand over, so that no
			// request waits for the database to record them
			cache.StartPendingAccessTimeWrites()
		},
		func(ctx context.Context) error {
			return errors.Join(
				cache.StopEviction(ctx),
				cache.StopPendingAccessTimeWrites(ctx),
			)
		},
	))

	return cache, nil
}

// freeSpaceProbeFunc reports the number of free bytes available on the
// filesystem containing path. It is a function type so tests can pass a
// fake probe instead of depending on the host disk.
type freeSpaceProbeFunc func(path string) (uint64, error)

var errNegativeBlockSize = errors.New("statfs reported negative block size")

// defaultFreeSpaceProbe reports free filesystem bytes via statfs on the
// given path, as available to unprivileged processes.
func defaultFreeSpaceProbe(path string) (uint64, error) {
	var stat syscall.Statfs_t

	err := syscall.Statfs(path, &stat)
	if err != nil {
		return 0, err
	}

	if stat.Bsize < 0 {
		return 0, fmt.Errorf("%w %d for %q", errNegativeBlockSize, stat.Bsize, path)
	}

	blockSize := uint64(stat.Bsize)

	return stat.Bavail * blockSize, nil
}

// cacheMaxBytes returns cache_max_bytes as set. When the key is omitted,
// it returns config.DefaultCacheMaxBytes of the free bytes probe reports
// for <state_dir>/cache/ and the bytes the cache holds, as db records
// them. It creates that directory first, so the probe measures the
// filesystem that holds the cache.
func cacheMaxBytes(
	ctx context.Context,
	cfg *config.Config,
	db *sql.DB,
	log *slog.Logger,
	probe freeSpaceProbeFunc,
) (int64, error) {
	if cfg.CacheMaxBytesExplicit {
		return cfg.CacheMaxBytes, nil
	}

	cacheDir := filepath.Join(cfg.StateDir, "cache")

	err := os.MkdirAll(cacheDir, imgcache.StorageDirPerm)
	if err != nil {
		return 0, fmt.Errorf("failed to create cache directory: %w", err)
	}

	freeBytes, err := probe(cacheDir)
	if err != nil {
		return 0, fmt.Errorf(
			"default cache_max_bytes: cannot determine free space for %q: %w",
			cacheDir, err)
	}

	heldBytes, err := imgcache.UsageBytes(ctx, db)
	if err != nil {
		return 0, err
	}

	limit := config.DefaultCacheMaxBytes(freeBytes, heldBytes)

	log.Info("computed default cache size limit from free space and cache contents",
		"cache_max_bytes", limit,
		"cache_dir", cacheDir,
	)

	return limit, nil
}

// newFetcherConfig builds the upstream fetcher's settings from cfg, and
// httpfetcher.New builds the fetcher from them. The settings are a value
// of their own in the fx graph, so that a test can replace the dial
// function alone.
func newFetcherConfig(cfg *config.Config) *httpfetcher.Config {
	return &httpfetcher.Config{
		Timeout:               cfg.UpstreamFetchTimeout,
		MaxResponseSize:       cfg.UpstreamMaxResponseSize,
		AllowHTTP:             cfg.AllowHTTP,
		MaxConnectionsPerHost: cfg.UpstreamConnectionsPerHost,
		MaxConnections:        cfg.UpstreamConnections,
		BlockedNetworks:       cfg.BlockedNetworks,
	}
}

// newProcessor builds the image processor from cfg. Its input limit is
// the largest upstream response the fetcher accepts.
func newProcessor(cfg *config.Config) *imageprocessor.ImageProcessor {
	return imageprocessor.New(imageprocessor.Params{
		MaxInputBytes:           cfg.UpstreamMaxResponseSize,
		MaxConcurrentProcessing: cfg.MaxConcurrentProcessing,
	})
}

// imageServiceParams defines dependencies for newImageService.
type imageServiceParams struct {
	fx.In

	Config    *config.Config
	Cache     *imgcache.Cache
	Fetcher   *httpfetcher.HTTPFetcher
	Processor *imageprocessor.ImageProcessor
	Logger    *logger.Logger
}

// newImageService builds the image service from the cache, the fetcher
// and the processor, with the signing key, the allowlist and the upstream
// settings of the config.
func newImageService(
	_ fx.Lifecycle, params imageServiceParams,
) (*imgcache.Service, error) {
	cfg := params.Config

	return imgcache.NewService(&imgcache.ServiceConfig{
		Cache:           params.Cache,
		Fetcher:         params.Fetcher,
		Processor:       params.Processor,
		SigningKey:      cfg.SigningKey,
		Allowlist:       cfg.AllowlistHosts,
		AllowHTTP:       cfg.AllowHTTP,
		MaxResponseSize: cfg.UpstreamMaxResponseSize,
		Logger:          params.Logger.Get(),
	})
}

// sessionManagerParams defines dependencies for newSessionManager.
type sessionManagerParams struct {
	fx.In

	Config   *config.Config
	Database *database.Database
}

// newSessionManager builds the session manager. Session cookies are
// always Secure, HttpOnly and SameSite=Strict. The random value that each
// logout replaces is saved in the database, so it outlasts a restart.
func newSessionManager(
	_ fx.Lifecycle, params sessionManagerParams,
) (*session.Manager, error) {
	return session.NewManagerWithDatabase(
		context.Background(), params.Config.SigningKey, params.Database.DB())
}

// newEncryptedURLGenerator builds the encrypted URL generator, with a key
// derived from the signing key.
func newEncryptedURLGenerator(cfg *config.Config) (*encurl.Generator, error) {
	return encurl.NewGenerator(cfg.SigningKey)
}