package backend

import (
	"context"
	"os"
	"strconv"
	"strings"
)

// File resolves refs from env files on disk (KEY=VALUE lines, parsed in
// Go, never sourced). House rule: secret files are 0600. A file whose
// permission bits reach beyond the allowed mask (default 0600; stricter
// masks such as 0400 always pass) is refused before it is read. Material
// is read on demand per resolve and kept in memory only.
type File struct{}

// NewFile returns the file backend.
func NewFile() *File { return &File{} }

// Name implements Backend.
func (f *File) Name() string { return "file" }

// Resolve implements Backend.
func (f *File) Resolve(ctx context.Context, ref Ref) (string, error) {
	path := expandHome(ref.Source)
	info, err := os.Stat(path)
	if err != nil || info.IsDir() {
		return "", Err(ref, f.Name(), ReasonUnreadableSource)
	}
	if info.Mode().Perm()&^allowedMode(ref.Mode) != 0 {
		return "", Err(ref, f.Name(), ReasonInsecureMode)
	}
	data, err := os.ReadFile(path)
	if err != nil {
		return "", Err(ref, f.Name(), ReasonUnreadableSource)
	}
	kv, err := parseEnvFile(data)
	if err != nil {
		return "", Err(ref, f.Name(), ReasonMalformedSource)
	}
	v, ok := kv[ref.Key]
	if !ok {
		return "", Err(ref, f.Name(), ReasonMissingKey)
	}
	if v == "" {
		return "", Err(ref, f.Name(), ReasonEmptyValue)
	}
	return v, nil
}

// allowedMode parses ref.Mode ("0600"-style octal, default 0600) into a
// permission mask; unparsable masks fail closed to 0 (nothing allowed).
func allowedMode(mode string) os.FileMode {
	if mode == "" {
		return 0o600
	}
	m, err := strconv.ParseUint(strings.TrimPrefix(mode, "0o"), 8, 32)
	if err != nil {
		return 0
	}
	return os.FileMode(m) & os.ModePerm
}

// expandHome resolves a leading ~ / ~/ to the user's home directory;
// every other path is returned unchanged.
func expandHome(path string) string {
	if path == "~" || strings.HasPrefix(path, "~/") {
		home, err := os.UserHomeDir()
		if err == nil {
			if path == "~" {
				return home
			}
			return home + path[1:]
		}
	}
	return path
}