// 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>.

package scripts

import (
	"context"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"strings"
	"sync"
	"syscall"
	"time"
	"unicode/utf8"
)

type Limits struct {
	Workers     int
	Queue       int
	OutputBytes int
	Timeout     time.Duration // Zero preserves Python's unlimited invocation lifetime.
}

type Invocation struct {
	Script     Script
	Terminal   int
	Generation uint64
}

type Result struct {
	Invocation
	Stdout, Stderr string
	Err            error
	Truncated      bool
}

// Runner owns subprocesses only. The application loop owns bindings, session
// state and speech. Its bounded channels never block the input dispatcher.
type Runner struct {
	ctx        context.Context
	cancel     context.CancelFunc
	limits     Limits
	username   func(int) string
	jobs       chan Invocation
	results    chan Result
	workers    sync.WaitGroup
	mu         sync.Mutex
	generation uint64
	active     map[*Invocation]context.CancelFunc
	done       chan struct{}
}

func New_runner(ctx context.Context, limits Limits, username func(int) string) *Runner {
	ctx, cancel := context.WithCancel(ctx)
	r := &Runner{ctx: ctx, cancel: cancel, limits: limits, username: username, jobs: make(chan Invocation, limits.Queue), results: make(chan Result, limits.Workers), active: make(map[*Invocation]context.CancelFunc), done: make(chan struct{})}
	for range limits.Workers {
		r.workers.Go(r.work)
	}
	go func() { r.workers.Wait(); close(r.done) }()
	return r
}

func (r *Runner) Submit(invocation Invocation) bool {
	if r.ctx.Err() != nil {
		return false
	}
	select {
	case r.jobs <- invocation:
		return true
	default:
		return false
	}
}

func (r *Runner) Results() <-chan Result { return r.results }
func (r *Runner) Close() {
	r.cancel()
	// A kernel task stuck in uninterruptible filesystem I/O cannot be reaped
	// until the kernel releases it. It still must not delay reader shutdown.
	select {
	case <-r.done:
	case <-time.After(750 * time.Millisecond):
	}
}

// Cancel invalidates queued work and cancels active invocations without waiting
// on the caller. Session handoff therefore never waits for a script to exit.
func (r *Runner) Cancel() uint64 {
	r.mu.Lock()
	defer r.mu.Unlock()
	r.generation++
	for _, cancel := range r.active {
		cancel()
	}
	return r.generation
}

func (r *Runner) work() {
	for {
		select {
		case <-r.ctx.Done():
			return
		case invocation := <-r.jobs:
			if r.ctx.Err() != nil {
				return
			}
			result := r.execute(invocation)
			select {
			case r.results <- result:
			case <-r.ctx.Done():
				return
			}
		}
	}
}

type boundedOutput struct {
	data      []byte
	limit     int
	truncated bool
}

func (b *boundedOutput) Write(data []byte) (int, error) {
	count := min(len(data), b.limit-len(b.data))
	b.data = append(b.data, data[:count]...)
	b.truncated = b.truncated || count < len(data)
	return len(data), nil // Keep draining after reaching the retention limit.
}

func (b *boundedOutput) text() string {
	text := string(b.data)
	if b.truncated {
		text = strings.ToValidUTF8(text, "")
	}
	return strings.ReplaceAll(strings.ReplaceAll(text, "\r\n", "\n"), "\r", "\n")
}

func (r *Runner) execute(invocation Invocation) Result {
	result := Result{Invocation: invocation}
	r.mu.Lock()
	if invocation.Generation != r.generation {
		r.mu.Unlock()
		result.Err = context.Canceled
		return result
	}
	ctx, cancelInvocation := context.WithCancel(r.ctx)
	r.active[&invocation] = cancelInvocation
	r.mu.Unlock()
	defer func() { cancelInvocation(); r.mu.Lock(); delete(r.active, &invocation); r.mu.Unlock() }()
	info, err := os.Stat(invocation.Script.Path)
	switch {
	case errors.Is(err, os.ErrNotExist):
		result.Err = errors.New("Script file not found")
		return result
	case err != nil:
		result.Err = err
		return result
	case !info.Mode().IsRegular():
		result.Err = errors.New("Script source is not a valid file")
		return result
	case syscall.Access(invocation.Script.Path, 1) != nil:
		result.Err = errors.New("Script file is not executable")
		return result
	}
	if r.limits.Timeout > 0 {
		var cancel context.CancelFunc
		ctx, cancel = context.WithTimeout(ctx, r.limits.Timeout)
		defer cancel()
	}
	// Preserve Python's [script_path, current_user], inherited identity,
	// environment, cwd and stdin. No shell command string is constructed.
	name := ""
	if r.username != nil {
		name = r.username(invocation.Terminal)
	}
	cmd := exec.CommandContext(ctx, invocation.Script.Path, name)
	cmd.Stdin = os.Stdin
	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
	stdout, stderr := &boundedOutput{limit: r.limits.OutputBytes}, &boundedOutput{limit: r.limits.OutputBytes}
	cmd.Stdout, cmd.Stderr = stdout, stderr
	// CommandContext's default cancellation kills only the shell. Signal its
	// process group, allow bounded cleanup, then kill any remaining group members.
	cmd.Cancel = func() error { return syscall.Kill(-cmd.Process.Pid, syscall.SIGTERM) }
	cmd.WaitDelay = 250 * time.Millisecond
	err = cmd.Run()
	if (errors.Is(err, exec.ErrWaitDelay) || ctx.Err() != nil) && cmd.Process != nil {
		_ = syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
	}
	result.Stdout, result.Stderr = stdout.text(), stderr.text()
	result.Truncated = stdout.truncated || stderr.truncated
	// Python presents stderr then stdout even for a nonzero exit status.
	var exitError *exec.ExitError
	if err != nil && !errors.As(err, &exitError) {
		result.Err = err
	}
	if ctx.Err() != nil {
		result.Err = ctx.Err()
	}
	if !utf8.Valid(stdout.data) && !stdout.truncated || !utf8.Valid(stderr.data) && !stderr.truncated {
		result.Stdout, result.Stderr, result.Err = "", "", fmt.Errorf("script output is not UTF-8")
	}
	return result
}