// Fenrir console screen reader.
// Copyright (c) 2026 Storm Dragon
// SPDX-License-Identifier: GPL-3.0-or-later
//
// This program is free software: you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by the Free
// Software Foundation, either version 3 of the License, or (at your option)
// any later version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
// more details.
//
// You should have received a copy of the GNU General Public License along with
// this program. If not, see <https://www.gnu.org/licenses/gpl-3.0.html>.

//go:build linux && cgo

package terminal

/*
#include "native.h"
*/
import "C"

import (
	"errors"
	"fmt"
	"io"
	"syscall"

	"git.stormux.org/storm/fenrir/internal/screen"
)

func open_pty(rows, cols int) (int, int, error) {
	var master, slave C.int
	err := native_error(C.terminal_pty(&master, &slave, C.int(rows), C.int(cols)))
	return int(master), int(slave), err
}

type waitState struct {
	inputPending, outputPending bool
}

// The app loop replaces stale requests before handing readiness back to watch.
func (s *Session) arm_watch() {
	select {
	case <-s.arm:
	default:
	}
	s.arm <- waitState{inputPending: len(s.pending) > 0, outputPending: len(s.outputPending) > 0}
}

func (s *Session) watch(xfd int) {
	defer s.workers.Done()
	for {
		var state waitState
		select {
		case <-s.stop:
			return
		case state = <-s.arm:
		}
		writeFlag := C.int(0)
		if state.inputPending {
			writeFlag = 1
		}
		outputFlag := C.int(0)
		if state.outputPending {
			outputFlag = 1
		}
		code := C.terminal_wait(C.int(s.master), C.int(s.input.Fd()), C.int(s.output.Fd()), C.int(xfd), C.int(s.wake[0]), writeFlag, outputFlag)
		if code == 1 {
			return
		}
		if code < 0 || code&2 != 0 {
			select {
			case s.events <- native_error(code):
			default:
			}
		}
		if code&4 != 0 {
			select {
			case s.inputEvents <- nil:
			default:
			}
		}
		if code < 0 {
			return
		}
	}
}

func (s *Session) Events() <-chan error { return s.events }

// Read drains bounded PTY output and snapshots the emulator. Terminal responses
// come from the outer xterm through stdin, avoiding duplicate query replies.
func (s *Session) Read() (screen.Snapshot, error) {
	if s.eof && len(s.outputPending) == 0 {
		return screen.Snapshot{}, ErrClosed
	}
	if _, _, _, err := s.State(); err != nil {
		return screen.Snapshot{}, err
	}
	if err := s.flush_output(); err != nil {
		return screen.Snapshot{}, err
	}
	var buffer [32768]byte
	// Stop reading the child while xterm is full. At most one chunk remains
	// queued, and stdout readiness resumes delivery without blocking input.
	for total := 0; total < 131072 && len(s.outputPending) == 0 && !s.eof; {
		n, err := syscall.Read(s.master, buffer[:])
		if errors.Is(err, syscall.EAGAIN) {
			break
		}
		if errors.Is(err, syscall.EINTR) {
			continue
		}
		if errors.Is(err, syscall.EIO) || n == 0 && err == nil {
			s.eof = true
			if total == 0 {
				return screen.Snapshot{}, ErrClosed
			}
			break
		}
		if err != nil {
			return screen.Snapshot{}, err
		}
		total += n
		s.vt.feed(buffer[:n])
		s.outputPending = append(s.outputPending, buffer[:n]...)
		if err := s.flush_output(); err != nil {
			return screen.Snapshot{}, err
		}
	}
	snapshot := s.vt.snapshot(s.active, s.lock)
	s.retire_paste()
	snapshot.CapsLock = s.capsLock
	s.arm_watch()
	return snapshot, nil
}

// flush_output preserves partial writes and retries after stdout becomes writable.
// Raw nonblocking writes keep a slow terminal from holding the app event loop.
func (s *Session) flush_output() error {
	for len(s.outputPending) > 0 {
		n, err := syscall.Write(int(s.output.Fd()), s.outputPending)
		if n > 0 {
			s.outputPending = s.outputPending[n:]
		}
		if errors.Is(err, syscall.EAGAIN) {
			return nil
		}
		if errors.Is(err, syscall.EINTR) {
			continue
		}
		if err != nil {
			return fmt.Errorf("write terminal output: %w", err)
		}
		if n == 0 {
			return io.ErrNoProgress
		}
	}
	s.outputPending = nil
	return nil
}

// relay_input forwards stdin and queued key bytes to the child PTY.
// The caller handles X11 key events before relaying terminal input.
func (s *Session) relay_input() error {
	var bytes [4096]byte
	for total := 0; total < 65536 && len(s.pending) < 1<<20; {
		n, err := syscall.Read(int(s.input.Fd()), bytes[:])
		if errors.Is(err, syscall.EAGAIN) {
			break
		}
		if errors.Is(err, syscall.EINTR) {
			continue
		}
		if err != nil {
			return err
		}
		if n == 0 {
			return ErrClosed
		}
		total += n
		// X key events own interruption. Stdin also carries terminal query
		// responses, which must never cancel speech as if they were typing.
		if !s.router.Captured() {
			s.pending = append(s.pending, bytes[:n]...)
		}
	}
	for len(s.pending) > 0 {
		n, err := syscall.Write(s.master, s.pending)
		if n > 0 {
			s.pending = s.pending[n:]
			s.pasteStart, s.pasteEnd = max(0, s.pasteStart-n), max(0, s.pasteEnd-n)
		}
		if errors.Is(err, syscall.EAGAIN) {
			break
		}
		if errors.Is(err, syscall.EINTR) {
			continue
		}
		if err != nil {
			return err
		}
	}
	s.arm_watch()
	return nil
}