package config

import (
	"fmt"
	"log/slog"
	"math"
	"os"
	"path/filepath"
	"syscall"
)

// DefaultCacheMaxBytesFloor is the minimum computed default for the
// cache_max_bytes setting: 500 MiB. The floor applies only to the
// computed default (when the key is omitted from the configuration),
// never to explicitly configured values.
const DefaultCacheMaxBytesFloor int64 = 524288000

// cacheDirPerms is the permission mode for the cache directory created
// before probing free space, matching the state directory permissions.
const cacheDirPerms = 0o750

// freeSpaceFractionNumerator and freeSpaceFractionDenominator express
// the 75% share of free space used for the computed default limit as
// integer arithmetic (dividing before multiplying avoids overflow).
const (
	freeSpaceFractionNumerator   uint64 = 3
	freeSpaceFractionDenominator uint64 = 4
)

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

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

	if stat.Bsize < 0 {
		return 0, fmt.Errorf("statfs reported negative block size %d for %q", stat.Bsize, path)
	}

	blockSize := uint64(stat.Bsize) //nolint:gosec // G115: negative Bsize rejected above

	return stat.Bavail * blockSize, nil
}

// ComputeDefaultCacheMaxBytes returns the default cache size limit for
// the filesystem containing cacheDir: 75% of the free bytes reported
// by probe, with a floor of DefaultCacheMaxBytesFloor.
func ComputeDefaultCacheMaxBytes(cacheDir string, probe FreeSpaceProbeFunc) (int64, error) {
	freeBytes, err := probe(cacheDir)
	if err != nil {
		return 0, fmt.Errorf("config key %q: cannot determine free space for %q: %w",
			"cache_max_bytes", cacheDir, err)
	}

	computed := freeBytes / freeSpaceFractionDenominator * freeSpaceFractionNumerator
	if computed > math.MaxInt64 {
		computed = math.MaxInt64
	}

	limit := int64(computed) //nolint:gosec // G115: clamped to MaxInt64 above

	if limit < DefaultCacheMaxBytesFloor {
		limit = DefaultCacheMaxBytesFloor
	}

	return limit, nil
}

// resolveCacheMaxBytes finalizes CacheMaxBytes after state_dir
// validation: an explicitly configured value is kept as-is (no floor
// applies), while an omitted key receives the computed default based
// on free space in <state_dir>/cache/. The cache directory is created
// first so statfs measures the filesystem that will actually hold the
// cache. The effective limit is logged either way.
func (c *Config) resolveCacheMaxBytes(log *slog.Logger, probe FreeSpaceProbeFunc) error {
	if !c.cacheMaxBytesExplicit {
		cacheDir := filepath.Join(c.StateDir, "cache")

		if err := os.MkdirAll(cacheDir, cacheDirPerms); err != nil {
			return fmt.Errorf("config key %q: cannot create cache directory %q: %w",
				"cache_max_bytes", cacheDir, err)
		}

		limit, err := ComputeDefaultCacheMaxBytes(cacheDir, probe)
		if err != nil {
			return err
		}

		c.CacheMaxBytes = limit

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

	log.Info("effective cache size limit",
		"cache_max_bytes", c.CacheMaxBytes,
		"cache_disabled", c.CacheMaxBytes == 0,
	)

	return nil
}