bellpilot/main.go

package main

import (
	"bytes"
	"embed"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
	"os/exec"
	"os/signal"
	"sync"
	"syscall"
	"time"

	"github.com/creack/pty"
	"github.com/ebitengine/oto/v3"
	"golang.org/x/term"
)

//go:embed bells.wav
var bellFile embed.FS

// --- Audio engine ---

type audioEngine struct {
	once    sync.Once
	ctx     *oto.Context
	pcm     []byte
	initErr error
	bellCh  chan struct{}
}

var engine = &audioEngine{
	bellCh: make(chan struct{}, 1),
}

func (e *audioEngine) init() {
	e.once.Do(func() {
		data, err := bellFile.ReadFile("bells.wav")
		if err != nil {
			e.initErr = fmt.Errorf("read bell file: %w", err)
			return
		}

		pcm, sampleRate, channels, format, err := parseWAV(data)
		if err != nil {
			e.initErr = fmt.Errorf("parse WAV: %w", err)
			return
		}

		// oto's FormatUnsignedInt8 path has an integer underflow bug: it computes
		// float32(v8-(1<<7)) with uint8 arithmetic, wrapping values below 128 into
		// large positive numbers. Convert to signed 16-bit to use the correct path.
		if format == oto.FormatUnsignedInt8 {
			pcm16 := make([]byte, len(pcm)*2)
			for i, b := range pcm {
				s := (int16(b) - 128) * 256
				pcm16[2*i] = byte(s)
				pcm16[2*i+1] = byte(s >> 8)
			}
			pcm = pcm16
			format = oto.FormatSignedInt16LE
		}

		ctx, ready, err := oto.NewContext(&oto.NewContextOptions{
			SampleRate:   sampleRate,
			ChannelCount: channels,
			Format:       format,
			BufferSize:   time.Millisecond * 20,
		})
		if err != nil {
			e.initErr = fmt.Errorf("create audio context: %w", err)
			return
		}
		<-ready

		e.ctx = ctx
		e.pcm = pcm
	})
}

// ring queues at most one pending bell, dropping extras while one is already queued.
func (e *audioEngine) ring() {
	select {
	case e.bellCh <- struct{}{}:
	default:
	}
}

// run is the single bell-playing goroutine. It must be started once before ring is called.
func (e *audioEngine) run() {
	e.init()
	if e.initErr != nil {
		log.Printf("audio init failed (%v); falling back to terminal bell", e.initErr)
	}
	for range e.bellCh {
		if e.initErr != nil {
			os.Stdout.Write([]byte{'\x07'})
			continue
		}
		player := e.ctx.NewPlayer(bytes.NewReader(e.pcm))
		player.Play()
		for player.IsPlaying() {
			time.Sleep(time.Millisecond)
		}
	}
}

// --- WAV parser ---

func parseWAV(data []byte) (pcm []byte, sampleRate, channels int, format oto.Format, err error) {
	r := bytes.NewReader(data)

	var id [4]byte
	if err = binary.Read(r, binary.LittleEndian, &id); err != nil {
		return nil, 0, 0, 0, fmt.Errorf("read RIFF id: %w", err)
	}
	if string(id[:]) != "RIFF" {
		return nil, 0, 0, 0, fmt.Errorf("not a RIFF file")
	}
	var fileSize uint32
	if err = binary.Read(r, binary.LittleEndian, &fileSize); err != nil {
		return nil, 0, 0, 0, fmt.Errorf("read file size: %w", err)
	}
	if err = binary.Read(r, binary.LittleEndian, &id); err != nil {
		return nil, 0, 0, 0, fmt.Errorf("read WAVE id: %w", err)
	}
	if string(id[:]) != "WAVE" {
		return nil, 0, 0, 0, fmt.Errorf("not a WAVE file")
	}

	var hdr struct {
		AudioFormat   uint16
		NumChannels   uint16
		SampleRate    uint32
		ByteRate      uint32
		BlockAlign    uint16
		BitsPerSample uint16
	}
	var fmtFound bool

	for {
		var chunkID [4]byte
		if err = binary.Read(r, binary.LittleEndian, &chunkID); err != nil {
			if errors.Is(err, io.EOF) || errors.Is(err, io.ErrUnexpectedEOF) {
				break
			}
			return nil, 0, 0, 0, fmt.Errorf("read chunk id: %w", err)
		}
		var chunkSize uint32
		if err = binary.Read(r, binary.LittleEndian, &chunkSize); err != nil {
			return nil, 0, 0, 0, fmt.Errorf("read chunk size: %w", err)
		}

		switch string(chunkID[:]) {
		case "fmt ":
			if chunkSize < 16 {
				return nil, 0, 0, 0, fmt.Errorf("fmt chunk too small (%d bytes)", chunkSize)
			}
			if err = binary.Read(r, binary.LittleEndian, &hdr); err != nil {
				return nil, 0, 0, 0, fmt.Errorf("read fmt chunk: %w", err)
			}
			if chunkSize > 16 {
				skip := int64(chunkSize - 16)
				if chunkSize%2 != 0 {
					skip++
				}
				if _, err = r.Seek(skip, io.SeekCurrent); err != nil {
					return nil, 0, 0, 0, fmt.Errorf("skip fmt extra: %w", err)
				}
			}
			fmtFound = true
		case "data":
			if !fmtFound {
				return nil, 0, 0, 0, fmt.Errorf("data chunk before fmt chunk")
			}
			sz := chunkSize
			if sz > uint32(r.Len()) {
				sz = uint32(r.Len())
			}
			pcm = make([]byte, sz)
			if _, err = io.ReadFull(r, pcm); err != nil {
				return nil, 0, 0, 0, fmt.Errorf("read data chunk: %w", err)
			}
			if chunkSize%2 != 0 {
				r.Seek(1, io.SeekCurrent)
			}
		default:
			skip := int64(chunkSize)
			if chunkSize%2 != 0 {
				skip++
			}
			if _, err = r.Seek(skip, io.SeekCurrent); err != nil {
				return nil, 0, 0, 0, fmt.Errorf("skip chunk: %w", err)
			}
		}
	}

	if !fmtFound {
		return nil, 0, 0, 0, fmt.Errorf("no fmt chunk in WAV file")
	}
	if pcm == nil {
		return nil, 0, 0, 0, fmt.Errorf("no data chunk in WAV file")
	}

	switch {
	case hdr.AudioFormat == 1 && hdr.BitsPerSample == 8:
		format = oto.FormatUnsignedInt8
	case hdr.AudioFormat == 1 && hdr.BitsPerSample == 16:
		format = oto.FormatSignedInt16LE
	case hdr.AudioFormat == 3 && hdr.BitsPerSample == 32:
		format = oto.FormatFloat32LE
	default:
		return nil, 0, 0, 0, fmt.Errorf("unsupported WAV format: audio_format=%d, bits=%d", hdr.AudioFormat, hdr.BitsPerSample)
	}

	return pcm, int(hdr.SampleRate), int(hdr.NumChannels), format, nil
}

// --- PTY filter loop ---

// filterLoop copies src to dst, stripping standalone BEL (0x07) bytes and calling bell()
// for each one. BEL bytes that appear as OSC string terminators are passed through unchanged
// so that the outer terminal does not get stuck mid-sequence.
//
// State machine covers the subset of ANSI/VT escape sequences that can contain 0x07:
//
//	Normal → ESC → CSI  (ESC [  ...  final)
//	         ESC → OSC  (ESC ]  ...  BEL or ST)
//	         ESC → Str  (ESC P/X/^/_  ...  ST)   (DCS, SOS, PM, APC)
//	         ESC → <two-char>
//
// ST = String Terminator = ESC \
func filterLoop(dst io.Writer, src io.Reader, bell func()) error {
	const (
		stNormal = iota
		stEsc    // saw 0x1b
		stCSI    // saw 0x1b [  — terminated by 0x40–0x7E
		stOSC    // saw 0x1b ]  — terminated by BEL or ST
		stStr    // saw 0x1b P/X/^/_  — terminated by ST only
		stST     // saw 0x1b inside stOSC or stStr, checking for '\' (ST)
	)

	buf := make([]byte, 32*1024)
	state := stNormal
	parent := stNormal // parent of stST (stOSC or stStr)
	esc := make([]byte, 0, 256)

	writeAll := func(p []byte) error {
		_, err := dst.Write(p)
		return err
	}
	flushEsc := func() error {
		err := writeAll(esc)
		esc = esc[:0]
		return err
	}

	for {
		n, readErr := src.Read(buf)
		p := buf[:n]
		i := 0
		for i < len(p) {
			b := p[i]

			// Fast path: bulk-copy normal bytes until a special byte is hit.
			if state == stNormal {
				j := i
				for i < len(p) && p[i] != 0x07 && p[i] != 0x1b {
					i++
				}
				if i > j {
					if err := writeAll(p[j:i]); err != nil {
						return err
					}
				}
				if i >= len(p) {
					break
				}
				b = p[i]
			}
			i++

			switch state {
			case stNormal:
				if b == 0x07 {
					bell()
				} else { // 0x1b
					esc = append(esc[:0], b)
					state = stEsc
				}

			case stEsc:
				switch b {
				case '[':
					esc = append(esc, b)
					state = stCSI
				case ']':
					esc = append(esc, b)
					state = stOSC
				case 'P', 'X', '^', '_': // DCS, SOS, PM, APC
					esc = append(esc, b)
					state = stStr
				case 0x1b:
					// Another ESC: flush previous incomplete sequence, start fresh.
					if err := writeAll(esc); err != nil {
						return err
					}
					esc = esc[:0]
					esc = append(esc, b)
					// stay in stEsc
				case 0x07:
					// BEL right after bare ESC: flush ESC, ring bell.
					if err := flushEsc(); err != nil {
						return err
					}
					state = stNormal
					bell()
				default:
					// Two-character escape sequence complete.
					esc = append(esc, b)
					if err := flushEsc(); err != nil {
						return err
					}
					state = stNormal
				}

			case stCSI:
				if b == 0x07 {
					// Unexpected BEL mid-CSI: flush incomplete sequence, ring bell.
					if err := flushEsc(); err != nil {
						return err
					}
					state = stNormal
					bell()
				} else {
					esc = append(esc, b)
					if b >= 0x40 && b <= 0x7e { // final byte
						if err := flushEsc(); err != nil {
							return err
						}
						state = stNormal
					}
				}

			case stOSC:
				esc = append(esc, b)
				if b == 0x07 {
					// BEL terminates OSC — pass the whole sequence through unchanged.
					if err := flushEsc(); err != nil {
						return err
					}
					state = stNormal
				} else if b == 0x1b {
					parent = stOSC
					state = stST
				}

			case stStr: // DCS/SOS/PM/APC — only ST terminates
				esc = append(esc, b)
				if b == 0x1b {
					parent = stStr
					state = stST
				}

			case stST:
				esc = append(esc, b)
				switch b {
				case '\\': // String Terminator complete
					if err := flushEsc(); err != nil {
						return err
					}
					state = stNormal
				case 0x07:
					if parent == stOSC {
						// BEL terminates OSC even after an intermediate ESC.
						if err := flushEsc(); err != nil {
							return err
						}
						state = stNormal
					} else {
						// Not a terminator for DCS/SOS/PM/APC; back to parent.
						state = parent
					}
				case 0x1b:
					// Another ESC inside string; stay in stST.
				default:
					// Not ST; back to parent string state.
					state = parent
				}
			}
		}

		if readErr != nil {
			_ = flushEsc() // flush any buffered incomplete sequence
			if isExpectedPTYErr(readErr) {
				return nil
			}
			return readErr
		}
	}
}

// isExpectedPTYErr returns true for errors that indicate normal PTY shutdown.
func isExpectedPTYErr(err error) bool {
	if errors.Is(err, io.EOF) {
		return true
	}
	var errno syscall.Errno
	if errors.As(err, &errno) {
		return errno == syscall.EIO
	}
	return false
}

// --- Main ---

func main() {
	if len(os.Args) < 2 {
		fmt.Fprintf(os.Stderr, "Usage: bellpilot <command> [args...]\n")
		os.Exit(1)
	}
	os.Exit(run(os.Args[1], os.Args[2:]))
}

func run(name string, args []string) int {
	go engine.run()

	c := exec.Command(name, args...)

	ptmx, err := pty.Start(c)
	if err != nil {
		log.Printf("start: %v", err)
		return 1
	}
	defer ptmx.Close()

	// Propagate terminal resize to PTY.
	winchCh := make(chan os.Signal, 1)
	signal.Notify(winchCh, syscall.SIGWINCH)
	go func() {
		for range winchCh {
			if err := pty.InheritSize(os.Stdin, ptmx); err != nil {
				log.Printf("resize: %v", err)
			}
		}
	}()
	winchCh <- syscall.SIGWINCH

	// Forward termination signals to the child.
	sigCh := make(chan os.Signal, 1)
	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP)
	go func() {
		for sig := range sigCh {
			c.Process.Signal(sig)
		}
	}()

	// Put stdin in raw mode if it is a terminal.
	var oldState *term.State
	if term.IsTerminal(int(os.Stdin.Fd())) {
		oldState, err = term.MakeRaw(int(os.Stdin.Fd()))
		if err != nil {
			log.Printf("make raw: %v", err)
			return 1
		}
		defer term.Restore(int(os.Stdin.Fd()), oldState)
	}

	childPid := c.Process.Pid
	termState := oldState // updated on each MakeRaw; only touched by suspend goroutine

	var suspendMu sync.Mutex // ensures only one suspend/resume cycle runs at a time

	// Copy stdin → PTY, intercepting Ctrl+Z (0x1A) for job control.
	go func() {
		buf := make([]byte, 4096)
		for {
			n, err := os.Stdin.Read(buf)
			if n > 0 {
				i := 0
				for j := 0; j < n; j++ {
					if buf[j] == 0x1A { // Ctrl+Z
						if j > i {
							ptmx.Write(buf[i:j])
						}
						// Run the full suspend/resume cycle in one goroutine so
						// there is no cross-goroutine state to synchronize.
						go func() {
							if !suspendMu.TryLock() {
								return // already suspending
							}
							defer suspendMu.Unlock()

							// Stop the child immediately without forwarding 0x1A.
							// Forwarding 0x1A causes copilot to write partial teardown
							// sequences that glitch the display before we can stop it.
							// We send to both the direct PID and the process group so
							// that subprocesses are also stopped regardless of whether
							// pty.Start created a new process group.
							// SIGSTOP is synchronous — the child is stopped before
							// Kill() returns, so no sleep is needed.
							syscall.Kill(childPid, syscall.SIGSTOP)
							syscall.Kill(-childPid, syscall.SIGSTOP)

							// Exit alternate screen and show cursor in case copilot had
							// them active, so the shell prompt appears on a clean screen.
							os.Stdout.Write([]byte("\x1b[?1049l\x1b[?25h\r\n"))
							// Hand the terminal back to the shell.
							if termState != nil {
								term.Restore(int(os.Stdin.Fd()), termState)
							}
							// Stop ourselves; the shell takes over.
							syscall.Kill(os.Getpid(), syscall.SIGSTOP)

							// ── Execution resumes here after "fg" ──────────
							// Re-apply raw mode.
							if termState != nil {
								if s, err := term.MakeRaw(int(os.Stdin.Fd())); err == nil {
									termState = s
								}
							}
							// Continue the child (direct PID + process group).
							syscall.Kill(childPid, syscall.SIGCONT)
							syscall.Kill(-childPid, syscall.SIGCONT)
							// Sync terminal size and ask copilot to redraw its UI.
							pty.InheritSize(os.Stdin, ptmx)
							syscall.Kill(childPid, syscall.SIGWINCH)
						}()
						i = j + 1
					}
				}
				if i < n {
					ptmx.Write(buf[i:n])
				}
			}
			if err != nil {
				return
			}
		}
	}()

	// exitCodeCh receives the child's exit code when it terminates.
	exitCodeCh := make(chan int, 1)

	// waitChild monitors the child for exit/signal; stop events are handled by
	// the suspend goroutine above so we just loop past them here.
	go func() {
		for {
			var ws syscall.WaitStatus
			_, err := syscall.Wait4(childPid, &ws, syscall.WUNTRACED, nil)
			if err != nil {
				if err == syscall.EINTR {
					continue
				}
				exitCodeCh <- 1
				return
			}

			if ws.Exited() {
				exitCodeCh <- ws.ExitStatus()
				return
			}
			if ws.Signaled() {
				exitCodeCh <- 128 + int(ws.Signal())
				return
			}
			// ws.Stopped() — suspend goroutine handles this; loop for next event.
		}
	}()

	if err := filterLoop(os.Stdout, ptmx, engine.ring); err != nil {
		log.Printf("filter: %v", err)
	}

	return <-exitCodeCh
}