// package cas implements Content-Addressable Storage for immutable files.
//
// Files are stored by SHA-1 hash under <cas_dir>/<first_2_chars>/<full_hash>.
// Because files are immutable (changing content changes the hash), long cache
// headers are safe and desirable.
package cas

import (
	"crypto/sha1"
	"crypto/subtle"
	"encoding/hex"
	"net/http"
	"os"
	"path/filepath"
	"strings"

	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/config"
	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/database"
)

// Handler serves CAS endpoints.
type Handler struct {
	db  *database.DB
	cfg *config.Config
}

// NewHandler creates a new CAS handler.
func NewHandler(db *database.DB, cfg *config.Config) *Handler {
	return &Handler{db: db, cfg: cfg}
}

// RegisterRoutes mounts the CAS endpoints.
func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
	// Public file serving — immutable, long cache.
	mux.HandleFunc("GET /files/{hash}", h.serveFile)

	// Launcher binary downloads — also served from CAS-like paths.
	mux.HandleFunc("GET /files/launcher/{version}/{os}/{arch}/{filename}", h.serveLauncherAsset)
}

// serveFile serves a file from CAS by its SHA-1 hash.
// Files are immutable, so we set Cache-Control: public, max-age=31536000 (1 year).
func (h *Handler) serveFile(w http.ResponseWriter, r *http.Request) {
	hash := r.PathValue("hash")
	if !isValidHash(hash) {
		http.NotFound(w, r)
		return
	}

	path := filepath.Join(h.cfg.CASDir, hash[:2], hash)
	data, err := os.ReadFile(path)
	if err != nil {
		http.NotFound(w, r)
		return
	}

	w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
	w.Write(data)
}

// serveLauncherAsset serves a released launcher binary.
// Path format: /files/launcher/{version}/{os}/{arch}/{filename}
func (h *Handler) serveLauncherAsset(w http.ResponseWriter, r *http.Request) {
	version := r.PathValue("version")
	osParam := r.PathValue("os")
	arch := r.PathValue("arch")
	filename := r.PathValue("filename")

	// Basic validation against path traversal.
	for _, v := range []string{version, osParam, arch, filename} {
		if strings.ContainsAny(v, "/\\") {
			http.NotFound(w, r)
			return
		}
	}

	path := filepath.Join(h.cfg.CASDir, "..", "launcher", version, osParam, arch, filename)
	// Verify the resolved path is under the expected launcher storage dir.
	clean := filepath.Clean(path)
	expected := filepath.Clean(filepath.Join(h.cfg.CASDir, "..", "launcher"))
	if !strings.HasPrefix(clean, expected+string(os.PathSeparator)) {
		http.NotFound(w, r)
		return
	}

	data, err := os.ReadFile(clean)
	if err != nil {
		http.NotFound(w, r)
		return
	}

	w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
	w.Write(data)
}

// isValidHash checks that a hash string is exactly 40 hex characters (SHA-1 length).
func isValidHash(hash string) bool {
	if len(hash) != 40 {
		return false
	}
	for _, c := range hash {
		if !((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f')) {
			return false
		}
	}
	return true
}

// StoreFile writes data to the CAS directory structure.
// Returns the SHA-1 hash of the stored data.
func StoreFile(casDir string, data []byte) (string, error) {
	hash := sha1Hex(data)
	destDir := filepath.Join(casDir, hash[:2])
	if err := os.MkdirAll(destDir, 0o755); err != nil {
		return "", err
	}
	dest := filepath.Join(destDir, hash)
	if err := os.WriteFile(dest, data, 0o644); err != nil {
		return "", err
	}
	return hash, nil
}

// FileExists checks if a file with the given SHA-1 hash exists in CAS.
func FileExists(casDir, hash string) bool {
	path := filepath.Join(casDir, hash[:2], hash)
	_, err := os.Stat(path)
	return err == nil
}

// VerifyAndStore writes data to CAS only if the SHA-1 matches the expected hash.
// This prevents corrupt or tampered uploads from being stored.
func VerifyAndStore(casDir string, data []byte, expectedHash string) (string, error) {
	got := sha1Hex(data)
	if subtle.ConstantTimeCompare([]byte(got), []byte(expectedHash)) != 1 {
		return "", ErrHashMismatch
	}
	if FileExists(casDir, expectedHash) {
		return expectedHash, nil // Already stored; idempotent.
	}
	return StoreFile(casDir, data)
}

var errHashMismatch = &hashMismatchError{}

// ErrHashMismatch is returned when computed hash doesn't match expected.
var ErrHashMismatch = errHashMismatch

type hashMismatchError struct{}

func (e *hashMismatchError) Error() string { return "SHA-1 hash mismatch" }

func sha1Hex(data []byte) string {
	h := sha1.Sum(data)
	return hex.EncodeToString(h[:])
}

func writeJSON(w http.ResponseWriter, status int, v any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
}