// SPDX-License-Identifier: GPL-3.0-or-later

// Package trash implements the freedesktop.org Trash specification well
// enough for krino's (delete) action to be recoverable: Put moves a file
// into $XDG_DATA_HOME/Trash and records where it came from, and Restore
// undoes that. See docs/design.md §7.2.
package trash

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"syscall"
	"time"
	"unicode/utf8"

	"git.labunix.xyz/krino/internal/xdg"
)

// ErrOtherFilesystem is returned by Put when path is not on the same
// filesystem as the Trash: the file is left exactly where it was.
var ErrOtherFilesystem = errors.New("not on the same filesystem as the trash")

// maxSuffixAttempts bounds the collision loop in claimName. internal/plan
// has its own suffixed() with the same cap (internal/plan/conflict.go), but
// the two solve different problems and are free to diverge independently:
// plan's avoids collisions with other planned destinations, this one avoids
// collisions among entries already inside the Trash. They are not shared
// because internal/trash's dependencies are stdlib plus internal/xdg only —
// importing internal/plan for its three-line suffix logic would pull in
// config, scan and dup transitively for that.
const maxSuffixAttempts = 10000

// Dir is $XDG_DATA_HOME/Trash, with its files/ and info/ subdirectories.
func Dir() string { return filepath.Join(xdg.DataHome(), "Trash") }

func filesDir() string { return filepath.Join(Dir(), "files") }
func infoDir() string  { return filepath.Join(Dir(), "info") }

// Put moves path into the Trash and writes its .trashinfo. It returns the
// entry name (the base name inside files/), which the log records so undo
// can find it again.
//
// The name is claimed first: info/<entry>.trashinfo is created with
// O_CREATE|O_EXCL before anything is moved, so two trash clients racing for
// the same name cannot collide. If the subsequent move fails, the info file
// is removed so no orphan is left.
func Put(path string) (entry string, err error) {
	abs, err := filepath.Abs(path)
	if err != nil {
		return "", fmt.Errorf("trash: %w", err)
	}
	if err := os.MkdirAll(filesDir(), 0o700); err != nil {
		return "", fmt.Errorf("trash: %w", err)
	}
	if err := os.MkdirAll(infoDir(), 0o700); err != nil {
		return "", fmt.Errorf("trash: %w", err)
	}

	entry, infoPath, f, err := claimName(fitName(filepath.Base(abs), maxEntryName))
	if err != nil {
		return "", fmt.Errorf("trash: %w", err)
	}

	info := "[Trash Info]\n" +
		"Path=" + percentEncode(abs) + "\n" +
		"DeletionDate=" + time.Now().Format("2006-01-02T15:04:05") + "\n"
	if _, err := f.WriteString(info); err != nil {
		f.Close()
		os.Remove(infoPath)
		return "", fmt.Errorf("trash: %w", err)
	}
	if err := f.Close(); err != nil {
		os.Remove(infoPath)
		return "", fmt.Errorf("trash: %w", err)
	}

	dst := filepath.Join(filesDir(), entry)
	if err := os.Rename(abs, dst); err != nil {
		os.Remove(infoPath)
		if errors.Is(err, syscall.EXDEV) {
			return "", ErrOtherFilesystem
		}
		return "", fmt.Errorf("trash: %w", err)
	}

	return entry, nil
}

// claimName finds a free entry name derived from base and atomically creates
// its .trashinfo, so the name is reserved before anything is moved. On
// collision it tries stem_1.ext, stem_2.ext, ... — the same shape as
// internal/plan's suffixing, but resolving a different, unrelated set of
// collisions; see maxSuffixAttempts for why the two are not shared code.
func claimName(base string) (entry, infoPath string, f *os.File, err error) {
	stem, ext := splitExt(base)
	for n := 0; n <= maxSuffixAttempts; n++ {
		candidate := base
		if n > 0 {
			candidate = stem + "_" + strconv.Itoa(n) + ext
		}
		// A name already used in files/ - a file left there without its
		// trashinfo - is taken too: renaming onto it would destroy it.
		if _, err := os.Lstat(filepath.Join(filesDir(), candidate)); err == nil {
			continue
		}
		path := filepath.Join(infoDir(), candidate+".trashinfo")
		f, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0o600)
		if err == nil {
			return candidate, path, f, nil
		}
		if !os.IsExist(err) {
			return "", "", nil, err
		}
	}
	return "", "", nil, errors.New("too many conflicting names")
}

// maxEntryName is the longest entry name Put uses: 255 bytes, the usual
// limit of a file name, less ".trashinfo" and the longest suffix claimName
// can add ("_10000").
const maxEntryName = 255 - len(".trashinfo") - len("_10000")

// fitName shortens name to at most max bytes for use as a trash entry: the
// stem is cut at a character boundary and an extension shorter than 16 bytes
// is kept. The trashinfo still records the full original path, so Restore
// puts the file back under its own name.
func fitName(name string, max int) string {
	if len(name) <= max {
		return name
	}
	stem, ext := splitExt(name)
	if len(ext) >= 16 || len(ext) >= max {
		stem, ext = name, ""
	}
	cut := max - len(ext)
	for cut > 0 && !utf8.RuneStart(stem[cut]) {
		cut--
	}
	return stem[:cut] + ext
}

// splitExt splits name on its last dot, which does not count when it is the
// first character: "a.tar.gz" -> "a.tar", ".gz"; ".bashrc" -> ".bashrc", "".
func splitExt(name string) (stem, ext string) {
	i := strings.LastIndexByte(name, '.')
	if i <= 0 {
		return name, ""
	}
	return name[:i], name[i:]
}

// percentEncode RFC-2396-encodes s, leaving unreserved characters and '/'
// literal. A byte loop is used rather than url.PathEscape, which also
// escapes '/' and would produce a Path no other trash implementation can
// read.
func percentEncode(s string) string {
	const hex = "0123456789ABCDEF"
	var b strings.Builder
	b.Grow(len(s))
	for i := 0; i < len(s); i++ {
		c := s[i]
		if isUnreserved(c) || c == '/' {
			b.WriteByte(c)
			continue
		}
		b.WriteByte('%')
		b.WriteByte(hex[c>>4])
		b.WriteByte(hex[c&0xf])
	}
	return b.String()
}

// isUnreserved reports whether c is unreserved under RFC 2396: letters,
// digits, and -_.~, which percentEncode passes through unchanged.
func isUnreserved(c byte) bool {
	switch {
	case c >= 'A' && c <= 'Z', c >= 'a' && c <= 'z', c >= '0' && c <= '9':
		return true
	case c == '-' || c == '_' || c == '.' || c == '~':
		return true
	}
	return false
}

// percentDecode reverses percentEncode.
func percentDecode(s string) string {
	var b strings.Builder
	b.Grow(len(s))
	for i := 0; i < len(s); i++ {
		if s[i] == '%' && i+2 < len(s) {
			if v, err := strconv.ParseUint(s[i+1:i+3], 16, 8); err == nil {
				b.WriteByte(byte(v))
				i += 2
				continue
			}
		}
		b.WriteByte(s[i])
	}
	return b.String()
}

// Restore moves an entry back to the Path recorded in its .trashinfo and
// removes the .trashinfo. It refuses when that path already exists. It also
// refuses an entry that is not a plain name inside the Trash, and a
// trashinfo whose Path is not absolute.
//
// Once the rename back to the original path has succeeded, removing the
// .trashinfo is best-effort: that file back in place is the substantive
// result, and a caller must be able to trust a non-error return means the
// restore happened. So a failure to remove the .trashinfo is not reported
// as an error — Restore returns (path, nil) regardless — and the
// .trashinfo may survive as a stale, otherwise-harmless record.
func Restore(entry string) (restored string, err error) {
	path, err := InfoPath(entry)
	if err != nil {
		return "", err
	}
	infoPath := filepath.Join(infoDir(), entry+".trashinfo")
	if _, err := os.Lstat(path); err == nil {
		return "", fmt.Errorf("trash: %s: already exists", path)
	} else if !os.IsNotExist(err) {
		return "", fmt.Errorf("trash: %w", err)
	}

	src := filepath.Join(filesDir(), entry)
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return "", fmt.Errorf("trash: %w", err)
	}
	if err := os.Rename(src, path); err != nil {
		return "", fmt.Errorf("trash: %w", err)
	}
	// The file is back; removing its bookkeeping is best-effort from here
	// (see the doc comment above).
	_ = os.Remove(infoPath)
	return path, nil
}

// InfoPath returns the absolute original path the trash entry's trashinfo
// records. An entry comes from the log; it must name something inside the
// Trash (spec §15.1), never a path that climbs out of it.
func InfoPath(entry string) (string, error) {
	if entry == "" || entry == "." || entry == ".." || strings.ContainsRune(entry, '/') {
		return "", fmt.Errorf("trash: bad entry name %q", entry)
	}
	b, err := os.ReadFile(filepath.Join(infoDir(), entry+".trashinfo"))
	if err != nil {
		return "", fmt.Errorf("trash: %w", err)
	}
	path, err := parsePath(string(b))
	if err != nil {
		return "", fmt.Errorf("trash: %s: %w", entry, err)
	}
	return path, nil
}

// parsePath extracts and decodes the Path= line of a .trashinfo file. The
// path must be absolute: Restore must never resolve one against the
// working directory.
func parsePath(info string) (string, error) {
	for _, line := range strings.Split(info, "\n") {
		if v, ok := strings.CutPrefix(line, "Path="); ok {
			p := percentDecode(v)
			if !filepath.IsAbs(p) {
				return "", fmt.Errorf("trashinfo path %q is not absolute", p)
			}
			return p, nil
		}
	}
	return "", errors.New("trashinfo has no path")
}