package main

import (
	"bufio"
	"errors"
	"fmt"
	"os"
	"os/user"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"syscall"
	"time"
)

const (
	mostCopy = 1 << 20
	// majorVersion is clipmenu's store layout: <dir>/clipmenu.<major>.<user>/.
	majorVersion = 6
)

// account is the user clipmenu's store is named for.
func account() string {
	for _, k := range []string{"USER", "MESH_OPERATOR_ACCOUNT", "LOGNAME"} {
		if v := strings.TrimSpace(os.Getenv(k)); v != "" {
			return v
		}
	}
	if u, err := user.Current(); err == nil {
		return u.Username
	}
	return ""
}

// storeDir is where clipmenud keeps the history: CM_DIR, else the account's runtime directory.
func storeDir(s Session) (string, error) {
	base := os.Getenv("CM_DIR")
	if base == "" {
		base = s.RuntimeDir
	}
	if base == "" {
		return "", fmt.Errorf("%w: the account's runtime directory, where the clipboard history lives, is missing (the account is not logged in)", ErrNoBus)
	}
	return filepath.Join(base, fmt.Sprintf("clipmenu.%d.%s", majorVersion, account())), nil
}

// cksum is POSIX cksum(1) of data: clipmenu names each entry's file by the cksum of its first line
// followed by a newline, as "<crc> <length>".
func cksum(data []byte) string {
	var crc uint32
	step := func(b byte) {
		crc ^= uint32(b) << 24
		for i := 0; i < 8; i++ {
			if crc&0x80000000 != 0 {
				crc = crc<<1 ^ 0x04C11DB7
			} else {
				crc <<= 1
			}
		}
	}
	for _, b := range data {
		step(b)
	}
	for n := len(data); n != 0; n >>= 8 {
		step(byte(n))
	}
	return fmt.Sprintf("%d %d", ^crc, len(data))
}

func entryID(line string) string { return cksum([]byte(line + "\n")) }

// Entry is one clip in the history as a caller sees it: when and how long, never what. The text of a
// clip, its first line and its id (a checksum of that line) stay in clipmenu's store, readable on the
// operator's own screen through clipmenu (issue 750).
type Entry struct {
	At    string `json:"at"`
	Bytes int    `json:"bytes"`
	id    string
	line  string
	at    int64
}

// HistoryResult is what node-clipboard.history answers.
type HistoryResult struct {
	Collecting bool    `json:"collecting"`
	Store      string  `json:"store"`
	Total      int     `json:"total"`
	Entries    []Entry `json:"entries"`
}

// readStore reads clipmenu's line cache: one "<nanoseconds> <first line>" per copy, oldest first, a
// line repeated when the same thing was copied again. The newest copy of each line wins.
func readStore(dir string) ([]Entry, error) {
	f, err := os.Open(filepath.Join(dir, "line_cache"))
	if errors.Is(err, os.ErrNotExist) {
		return []Entry{}, nil
	}
	if err != nil {
		return nil, err
	}
	defer f.Close()
	latest := map[string]int64{}
	scan := bufio.NewScanner(f)
	scan.Buffer(make([]byte, 64<<10), 1<<20)
	for scan.Scan() {
		stamp, line, ok := strings.Cut(scan.Text(), " ")
		if !ok {
			continue
		}
		ns, err := strconv.ParseInt(stamp, 10, 64)
		if err != nil {
			continue
		}
		if ns >= latest[line] {
			latest[line] = ns
		}
	}
	out := make([]Entry, 0, len(latest))
	for line, ns := range latest {
		out = append(out, Entry{id: entryID(line), line: line, at: ns, At: time.Unix(0, ns).Format(time.RFC3339)})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].at > out[j].at })
	return out, scan.Err()
}

// History is the clipboard's history, newest first: how many entries, when each was copied and how
// long each is, and whether the daemon is collecting. It never returns a copied text.
func History(limit int) (HistoryResult, error) {
	dir, err := storeDir(findEnvironment())
	if err != nil {
		return HistoryResult{}, err
	}
	entries, err := readStore(dir)
	if err != nil {
		return HistoryResult{}, err
	}
	out := HistoryResult{Collecting: len(processesOf("clipmenud")) > 0, Store: dir, Total: len(entries), Entries: []Entry{}}
	for i, e := range entries {
		if i == limit {
			break
		}
		if info, err := os.Stat(filepath.Join(dir, e.id)); err == nil {
			e.Bytes = int(info.Size())
		}
		out.Entries = append(out.Entries, e)
	}
	return out, nil
}

// CopyResult is what node-clipboard.copy answers.
type CopyResult struct {
	Bytes int    `json:"bytes"`
	Unit  string `json:"unit"`
	// Verified says the clipboard was read back and held exactly the text.
	Verified bool `json:"verified"`
}

// readBackWait is the pause between reads of the clipboard while the owner starts; read-back tries 15 times.
var readBackWait = 200 * time.Millisecond

// readBack waits for the clipboard to hold text and answers why when it does not.
func readBack(s Session, unit, text string) error {
	var last string
	for i := 0; i < 15; i++ {
		time.Sleep(readBackWait)
		r, err := s.run(3*time.Second, "", "xsel", "--output", "--clipboard")
		switch {
		case err != nil:
			last = err.Error()
		case r.Code != 0:
			last = "xsel: " + strings.TrimSpace(r.Stderr)
		case r.Stdout == text:
			return nil
		default:
			last = fmt.Sprintf("it holds %d bytes of something else", len(r.Stdout))
		}
	}
	why := ""
	if s.Display == "" {
		why = "; the session has no DISPLAY, so the owner could not reach the screen"
	}
	if j, err := s.run(5*time.Second, "", "journalctl", "--user", "-u", unit+".service", "-n", "5", "--no-pager", "-o", "cat"); err == nil {
		if tail := strings.TrimSpace(j.Stdout); tail != "" {
			why += "; the owner said: " + tail
		}
	}
	return fmt.Errorf("the clipboard does not hold the text (%d bytes): %s%s", len(text), last, why)
}

// Copy puts text on the clipboard. The clipboard is owned by a process until another copies, so the
// owner (xsel) runs under the account's service manager, not as a child of this tool.
func Copy(text string) (CopyResult, error) {
	if len(text) == 0 {
		return CopyResult{}, errors.New("text is empty; to empty the clipboard's history, clipmenu_clear")
	}
	if len(text) > mostCopy {
		return CopyResult{}, fmt.Errorf("%d bytes; the clipboard takes at most %d here", len(text), mostCopy)
	}
	s, err := findSession()
	if err != nil {
		return CopyResult{}, err
	}
	if s.RuntimeDir == "" {
		return CopyResult{}, fmt.Errorf("%w: no runtime directory to hand the text over in", ErrNoBus)
	}
	// Handed over in a file only the account can read, which the owner reads and removes.
	f, err := os.CreateTemp(s.RuntimeDir, "clipmenu-copy-")
	if err != nil {
		return CopyResult{}, err
	}
	if _, err := f.WriteString(text); err != nil {
		f.Close()
		os.Remove(f.Name())
		return CopyResult{}, err
	}
	f.Close()
	unit := uniqueUnit("clipmenu-copy")
	script := `xsel --nodetach --input --clipboard < "$0" & sleep 1; rm -f "$0"; wait`
	if err := s.detach(unit, "/bin/sh", "-c", script, f.Name()); err != nil {
		os.Remove(f.Name())
		return CopyResult{}, err
	}
	if err := readBack(s, unit, text); err != nil {
		return CopyResult{}, err
	}
	return CopyResult{Bytes: len(text), Unit: unit + ".service", Verified: true}, nil
}

// PasteResult is what clipmenu_paste answers: how much text the clipboard holds, never the text. The
// clipboard is the newest history entry, and a copied secret must not reach a caller (issue 750).
type PasteResult struct {
	Bytes int  `json:"bytes"`
	Text  bool `json:"holds_text"`
}

// Paste reads the clipboard now and says how long it is.
func Paste() (PasteResult, error) {
	s, err := findSession()
	if err != nil {
		return PasteResult{}, err
	}
	r, err := s.run(5*time.Second, "", "xsel", "--output", "--clipboard")
	if err != nil {
		return PasteResult{}, err
	}
	if r.Code != 0 {
		return PasteResult{}, fmt.Errorf("xsel: %s", strings.TrimSpace(r.Stderr))
	}
	return PasteResult{Bytes: len(r.Stdout), Text: len(r.Stdout) > 0}, nil
}

// ChangeResult is what clear and delete answer.
type ChangeResult struct {
	Removed   int `json:"removed"`
	Remaining int `json:"remaining"`
}

// withStoreLock holds clipmenu's own lock on its store, the one clipmenud and clipdel take, while
// change runs, so a copy arriving meanwhile is neither lost nor half-written.
func withStoreLock(dir string, change func() error) error {
	lock, err := os.OpenFile(filepath.Join(dir, "lock"), os.O_CREATE|os.O_WRONLY, 0o600)
	if err != nil {
		return err
	}
	defer lock.Close()
	deadline := time.Now().Add(2 * time.Second)
	for {
		err := syscall.Flock(int(lock.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
		if err == nil {
			break
		}
		if time.Now().After(deadline) {
			return fmt.Errorf("the clipboard store is locked by clipmenud and did not come free within 2s")
		}
		time.Sleep(50 * time.Millisecond)
	}
	defer syscall.Flock(int(lock.Fd()), syscall.LOCK_UN)
	return change()
}

func storeOf() (string, error) {
	dir, err := storeDir(findEnvironment())
	if err != nil {
		return "", err
	}
	if _, err := os.Stat(dir); errors.Is(err, os.ErrNotExist) {
		return "", fmt.Errorf("there is no clipboard history at %s: clipmenud has not run in this login", dir)
	}
	return dir, nil
}

// Clear forgets every entry and its text, keeping the store and its locks (clipdel's own clear
// removes the directory, the daemon's lock with it).
func Clear() (ChangeResult, error) {
	dir, err := storeOf()
	if err != nil {
		return ChangeResult{}, err
	}
	var out ChangeResult
	err = withStoreLock(dir, func() error {
		entries, err := readStore(dir)
		if err != nil {
			return err
		}
		out.Removed = len(entries)
		files, err := os.ReadDir(dir)
		if err != nil {
			return err
		}
		for _, f := range files {
			switch f.Name() {
			case "lock", "session_lock", "line_cache":
				continue
			}
			if f.Type().IsRegular() {
				if err := os.Remove(filepath.Join(dir, f.Name())); err != nil {
					return err
				}
			}
		}
		return os.WriteFile(filepath.Join(dir, "line_cache"), nil, 0o600)
	})
	return out, err
}

// Delete forgets one entry, by its place in the history as node-clipboard.history lists it (1 is the
// newest) together with the time it reports, so a copy arriving since the listing cannot shift the
// place onto another entry. Not by its text or id: either would let a caller confirm a guess of a copied line.
func Delete(place int, at string) (ChangeResult, error) {
	if place < 1 {
		return ChangeResult{}, errors.New("give the entry's place in the history, 1 for the newest")
	}
	dir, err := storeOf()
	if err != nil {
		return ChangeResult{}, err
	}
	var out ChangeResult
	err = withStoreLock(dir, func() error {
		entries, err := readStore(dir)
		if err != nil {
			return err
		}
		if place > len(entries) {
			return fmt.Errorf("no entry %d: the clipboard history holds %d", place, len(entries))
		}
		if entries[place-1].At != at {
			return fmt.Errorf("entry %d was copied at %s, not at %s: the history changed since it was listed; list it again", place, entries[place-1].At, at)
		}
		line := entries[place-1].line
		raw, err := os.ReadFile(filepath.Join(dir, "line_cache"))
		if err != nil && !errors.Is(err, os.ErrNotExist) {
			return err
		}
		var kept []string
		for _, l := range strings.Split(strings.TrimRight(string(raw), "\n"), "\n") {
			if l == "" {
				continue
			}
			_, text, _ := strings.Cut(l, " ")
			if text == line {
				out.Removed++
				_ = os.Remove(filepath.Join(dir, entryID(text)))
				continue
			}
			kept = append(kept, l)
		}
		if out.Removed == 0 {
			return fmt.Errorf("no entry %d in the clipboard history", place)
		}
		content := strings.Join(kept, "\n")
		if content != "" {
			content += "\n"
		}
		tmp := filepath.Join(dir, ".line_cache.mesh")
		if err := os.WriteFile(tmp, []byte(content), 0o600); err != nil {
			return err
		}
		return os.Rename(tmp, filepath.Join(dir, "line_cache"))
	})
	if err != nil {
		return ChangeResult{}, err
	}
	entries, _ := readStore(dir)
	out.Remaining = len(entries)
	return out, nil
}