// Package task implements the agent runtime's task execution model
// (DevDocs/design/04-运行时.md §8.5). A task runs either in the foreground —
// where it is promoted to the background automatically if it outlives a short
// threshold — or in the background, where the agent can read its real-time
// output. When a task ends, its result is returned to the agent over the
// in-process bus on the subjects "task.output" and "task.done".
//
// A foreground task that completes within [autoBackgroundThreshold] is returned
// to Start already in a terminal state. One that exceeds the threshold is
// promoted: Start returns immediately with the task now KindBackground and
// StatusRunning, and the run keeps going on its own goroutine. Background tasks
// (KindBackground from the start) behave the same way but Start never waits.
//
// The task layer is built on top of [internal/runtime/exec].Provider — the
// grant-based sandbox entry point — and never runs a command itself. Because the
// Provider's Execute returns only the final [exec.ExecResult], the captured
// Output is materialized when the task reaches a terminal state; the Output/
// ReadOutput methods are safe to poll at any point and a future streaming exec
// API can be layered onto the same Task and Manager surface.
package task

import (
	"context"
	"errors"
	"fmt"
	"sync"
	"time"

	"git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/exec"
)

// Kind is how a task runs relative to the caller.
type Kind string

const (
	// KindForeground runs synchronously: Start waits up to the auto-background
	// threshold, then either returns the finished task or promotes it.
	KindForeground Kind = "foreground"
	// KindBackground runs asynchronously: Start returns immediately and the agent
	// reads output through Output/ReadOutput.
	KindBackground Kind = "background"
)

// Status is the lifecycle state of a task.
type Status string

const (
	// StatusPending is the state before the task begins running.
	StatusPending Status = "pending"
	// StatusRunning is the state while the command executes.
	StatusRunning Status = "running"
	// StatusDone is the terminal state after a successful run.
	StatusDone Status = "done"
	// StatusFailed is the terminal state after a run that returned an error (a
	// non-zero exit code is NOT an error and is reported via ExitCode).
	StatusFailed Status = "failed"
	// StatusAborted is the terminal state after Abort or a parent context cancel.
	StatusAborted Status = "aborted"
)

// isTerminal reports whether a status no longer transitions.
func (s Status) isTerminal() bool {
	return s == StatusDone || s == StatusFailed || s == StatusAborted
}

// Output is a snapshot of a task's captured standard streams.
type Output struct {
	// Stdout is the captured standard output.
	Stdout string
	// Stderr is the captured standard error.
	Stderr string
}

// TaskSpec describes a work item to run. It maps directly onto the fields the
// [internal/runtime/exec] package needs.
type TaskSpec struct {
	// Command is the argv vector to execute. It must be non-empty.
	Command []string
	// Background selects async execution when true (KindBackground) and
	// foreground/auto-promoting execution when false (KindForeground).
	Background bool
	// WorkDir is the process working directory (empty lets the provider pick).
	WorkDir string
	// Timeout bounds the run; zero uses exec.DefaultTimeout.
	Timeout time.Duration
	// MaxOutputBytes caps combined output; zero uses exec.DefaultMaxOutputBytes.
	MaxOutputBytes int
}

// Snapshot is an atomic, read-only view of a task's mutable state, taken under
// the task's lock so callers observe a consistent state.
type Snapshot struct {
	ID         string
	Kind       Kind
	Cmd        []string
	Status     Status
	Output     Output
	StartedAt  time.Time
	FinishedAt time.Time
	ExitCode   int
	Truncated  bool
	Err        string
}

// Task is a single unit of work. ID, Kind, Cmd and StartedAt are fixed before
// Start returns (Kind may be flipped foreground→background during promotion,
// also before Start returns for a promoted task). The remaining state is mutated
// by the task's run goroutine, so it must be read through the accessor methods
// (Snapshot, Status, Output, ReadOutput, ...), which take the internal RWMutex.
type Task struct {
	// ID uniquely identifies the task within a Manager.
	ID string
	// Kind reports whether the task is running in the background.
	Kind Kind
	// Cmd is the argv vector being executed.
	Cmd []string
	// StartedAt is when the task began running.
	StartedAt time.Time

	mu         sync.RWMutex
	status     Status
	output     Output
	exitCode   int
	truncated  bool
	finErr     error
	finishedAt time.Time

	ctx    context.Context
	cancel context.CancelFunc

	done     chan struct{}
	doneOnce sync.Once

	// m is the owning Manager, used to publish bus events on completion.
	m *Manager
}

// Status returns the current lifecycle state.
func (t *Task) Status() Status {
	t.mu.RLock()
	defer t.mu.RUnlock()
	return t.status
}

// Output returns the current captured output. It never blocks and never waits
// for the task to finish, so it can be polled while a background task runs.
func (t *Task) Output() Output {
	t.mu.RLock()
	defer t.mu.RUnlock()
	return t.output
}

// FinishedAt returns when the task reached a terminal state, or the zero value
// while it is still running.
func (t *Task) FinishedAt() time.Time {
	t.mu.RLock()
	defer t.mu.RUnlock()
	return t.finishedAt
}

// ExitCode returns the process exit code (0 for success; -1 when an error is
// reported instead of an exit code).
func (t *Task) ExitCode() int {
	t.mu.RLock()
	defer t.mu.RUnlock()
	return t.exitCode
}

// Truncated reports whether output was dropped because MaxOutputBytes was reached.
func (t *Task) Truncated() bool {
	t.mu.RLock()
	defer t.mu.RUnlock()
	return t.truncated
}

// Error returns the error that caused a failed or aborted task, or nil.
func (t *Task) Error() error {
	t.mu.RLock()
	defer t.mu.RUnlock()
	return t.finErr
}

// Snapshot returns an atomic view of the task's current state.
func (t *Task) Snapshot() Snapshot {
	t.mu.RLock()
	defer t.mu.RUnlock()
	s := Snapshot{
		ID:         t.ID,
		Kind:       t.Kind,
		Cmd:        t.Cmd,
		Status:     t.status,
		Output:     t.output,
		StartedAt:  t.StartedAt,
		FinishedAt: t.finishedAt,
		ExitCode:   t.exitCode,
		Truncated:  t.truncated,
	}
	if t.finErr != nil {
		s.Err = t.finErr.Error()
	}
	return s
}

// ReadOutput waits until the task reaches a terminal state (or until ctx is
// canceled) and returns the final output together with any terminal error.
func (t *Task) ReadOutput(ctx context.Context) (Output, error) {
	select {
	case <-t.done:
		return t.Output(), t.Error()
	case <-ctx.Done():
		return Output{}, fmt.Errorf("task: read output %q: %w", t.ID, ctx.Err())
	}
}

// setRunning transitions the task to the running state. It is called by the
// Manager before exposing the task, so the status is always non-pending for a
// task the caller has been handed.
func (t *Task) setRunning() {
	t.mu.Lock()
	defer t.mu.Unlock()
	if !t.status.isTerminal() {
		t.status = StatusRunning
	}
}

// promoteBackground flips a foreground task to KindBackground once it has
// outlived the auto-background threshold. It never changes once Start returns.
func (t *Task) promoteBackground() {
	t.mu.Lock()
	defer t.mu.Unlock()
	if !t.status.isTerminal() {
		t.Kind = KindBackground
	}
}

// abort cancels the task, marking it aborted, and publishes its terminal state.
// It is idempotent for a task that is already terminal.
func (t *Task) abort() error {
	t.mu.Lock()
	if t.status.isTerminal() {
		t.mu.Unlock()
		return fmt.Errorf("task: abort %q: %w", t.ID, ErrAlreadyDone)
	}
	t.status = StatusAborted
	t.finishedAt = time.Now()
	t.mu.Unlock()

	t.cancel()
	t.finalize()
	return nil
}

// finish records the outcome of the execution and publishes the terminal state.
// A task that was already aborted keeps its aborted status.
func (t *Task) finish(res *exec.ExecResult, err error) {
	t.mu.Lock()
	if !t.status.isTerminal() {
		switch {
		case errors.Is(err, context.Canceled):
			t.status = StatusAborted
		case err != nil:
			t.status = StatusFailed
		default:
			t.status = StatusDone
		}
		t.finErr = err
		t.finishedAt = time.Now()
		if res != nil {
			t.output = Output{Stdout: res.Stdout, Stderr: res.Stderr}
			t.exitCode = res.ExitCode
			t.truncated = res.Truncated
		}
	}
	t.mu.Unlock()

	t.finalize()
}

// finalize closes the task's done channel and publishes its terminal output and
// done events. It runs at most once per task.
func (t *Task) finalize() {
	t.doneOnce.Do(func() {
		t.m.publishOutput(t)
		t.m.publishDone(t)
		close(t.done)
	})
}