package downloader

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"math"
	"net/http"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"time"

	"rssd/poller"
	"rssd/state"
	"rssd/ytdlp"
)

// DownloadJob carries all information needed for a single download.
type DownloadJob struct {
	FeedURL      string
	EnclosureURL string
	DestPath     string

	// YouTube-only fields. IsYouTube selects the yt-dlp download path.
	IsYouTube  bool
	VideoID    string  // YouTube video ID
	Duration   float64 // expected video duration in seconds (0 = unknown)
	YTSettings ytdlp.Settings
}

// Pool manages a fixed-size worker pool for downloading files.
type Pool struct {
	poolSize    int
	backoffBase time.Duration
	state       *state.Manager
}

// NewPool creates a download pool with the given parameters.
func NewPool(poolSize int, backoffBase time.Duration, sm *state.Manager) *Pool {
	return &Pool{poolSize: poolSize, backoffBase: backoffBase, state: sm}
}

// liveWaitInterval is how often a still-live YouTube stream is re-checked.
// While a stream is live, the static formats bestaudio requests may not be
// available ("Requested format is not available"); once the stream ends,
// YouTube converts it to a regular VOD whose formats download normally.
const liveWaitInterval = 10 * time.Minute

// maxLiveWaits bounds the live wait: after this many 10-minute waits
// (~4 hours) the job falls back to a regular retry attempt with the
// normal exponential backoff, so a video that never converts still ends
// in a permanent failure rather than retrying forever.
const maxLiveWaits = 24

// errLiveWait is returned by downloadYT when the video is still live and
// has been rescheduled; it is not a download failure.
var errLiveWait = errors.New("video is live, waiting for stream to end")

// Run starts the worker goroutines. It returns immediately after spawning the workers.
func (p *Pool) Run(jobs <-chan DownloadJob) {
	for i := 0; i < p.poolSize; i++ {
		go func() {
			for job := range jobs {
				p.worker(job)
			}
		}()
	}
}

func (p *Pool) worker(job DownloadJob) {
	fmt.Fprintf(os.Stderr, "rssd: worker started download %s\n", job.EnclosureURL)
	var err error
	if job.IsYouTube {
		err = p.downloadYT(job)
	} else {
		err = p.download(job)
	}
	if err != nil {
		if errors.Is(err, errLiveWait) {
			fmt.Fprintf(os.Stderr, "rssd: %s still live, retrying in %s\n", job.EnclosureURL, liveWaitInterval)
		} else {
			fmt.Fprintf(os.Stderr, "ERROR: download %s failed: %v\n", job.EnclosureURL, err)
		}
	} else {
		fmt.Fprintf(os.Stderr, "OK: downloaded %s -> %s\n", job.EnclosureURL, job.DestPath)
	}
	fmt.Fprintf(os.Stderr, "rssd: worker finished download %s\n", job.EnclosureURL)
}

// downloadYT downloads the native audio stream of a YouTube video via yt-dlp
// (bestaudio, no -x), i.e. exactly what YouTube serves (typically Opus in
// WebM) with no re-encoding. The file lands at destPath + ".<actual-ext>";
// state is updated to the real path.
func (p *Pool) downloadYT(job DownloadJob) error {
	// While the video is still a live stream, the static formats we request
	// are not available yet. Instead of counting a failed attempt, reschedule
	// the job in liveWaitInterval; once the stream ends and converts to a
	// regular VOD the normal download succeeds.
	if p.isStillLive(job) {
		p.markLiveWait(job.EnclosureURL)
		return errLiveWait
	}

	// Update state to downloading.
	if !p.state.UpdateJob(job.EnclosureURL, func(j *state.Job) {
		j.Status = state.StatusDownloading
		j.AttemptCount++
	}) {
		return fmt.Errorf("job not found for %s", job.EnclosureURL)
	}
	if err := p.state.Save(); err != nil {
		return fmt.Errorf("save state (downloading): %w", err)
	}

	// Ensure output directory exists.
	if err := os.MkdirAll(filepath.Dir(job.DestPath), 0755); err != nil {
		p.markRetry(job.EnclosureURL, fmt.Errorf("create dest dir: %w", err))
		return err
	}

	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Minute)
	defer cancel()

	// Download the native best audio stream. yt-dlp writes the file to
	// destPath with the source extension (e.g. .webm, .m4a) appended.
	if err := p.runYT(ctx, job, "-f", "bestaudio", "-o", job.DestPath+".%(ext)s"); err != nil {
		p.markRetry(job.EnclosureURL, err)
		return err
	}

	// Find the actual file: destPath with the real extension appended.
	actual, err := findDownloadedFile(job.DestPath)
	if err != nil {
		p.markRetry(job.EnclosureURL, err)
		return err
	}

	// Duration sanity check: fail fast on videos that only offer short
	// clips (e.g. some shorts where only a partial audio stream exists).
	if err := checkDuration(actual, job.Duration); err != nil {
		os.Remove(actual)
		p.markRetry(job.EnclosureURL, err)
		return err
	}

	// Mark as downloaded and record the real destination path (extension
	// was only known after the download).
	p.state.UpdateJob(job.EnclosureURL, func(j *state.Job) {
		j.Status = state.StatusDownloaded
		j.DestPath = actual
		j.LiveWaitCount = 0
	})
	if err := p.state.Save(); err != nil {
		return fmt.Errorf("save state (downloaded): %w", err)
	}
	job.DestPath = actual

	return nil
}

// isStillLive reports whether the video is a live stream that has not ended
// yet (or is not even started). If the probe itself fails, false is returned
// so the normal download path (with its own error handling) can proceed.
func (p *Pool) isStillLive(job DownloadJob) bool {
	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
	defer cancel()
	info, err := ytdlp.Info(ctx, job.YTSettings, job.VideoID)
	if err != nil {
		return false
	}
	return info.LiveStatus == "is_live" || info.LiveStatus == "upcoming"
}

// markLiveWait reschedules a job whose video is still live. The wait does
// not count against the job's retry budget; after maxLiveWaits consecutive
// waits the next check falls back to a regular retry attempt.
func (p *Pool) markLiveWait(enclosureURL string) {
	p.state.UpdateJob(enclosureURL, func(j *state.Job) {
		j.Status = state.StatusRetrying
		j.LiveWaitCount++
		t := time.Now()
		if j.LiveWaitCount <= maxLiveWaits {
			j.Error = fmt.Sprintf("video is live (wait %d/%d), retrying after stream ends", j.LiveWaitCount, maxLiveWaits)
			t = t.Add(liveWaitInterval)
		} else {
			j.LiveWaitCount = 0
			j.AttemptCount++
			delay := p.backoffBase * time.Duration(1<<uint(j.AttemptCount-1))
			j.Error = fmt.Sprintf("video still live after %d waits; falling back to regular retry", maxLiveWaits)
			t = t.Add(delay)
		}
		j.NextAttemptAt = &t
	})
	p.state.Save()
}

// runYT executes yt-dlp for a single video, downloading audio only.
func (p *Pool) runYT(ctx context.Context, job DownloadJob, args ...string) error {
	cmd := ytdlp.CommandFor(ctx, job.YTSettings)
	cmd.Args = append(cmd.Args, args...)
	cmd.Args = append(cmd.Args, "--no-playlist", job.EnclosureURL)

	var stderr bytes.Buffer
	cmd.Stderr = &stderr
	if err := cmd.Run(); err != nil {
		return fmt.Errorf("yt-dlp download %s: %v: %s", job.VideoID, err, ytdlp.LastLines(stderr.String(), 3))
	}
	return nil
}

// findDownloadedFile locates the file yt-dlp produced for the given output
// base path. yt-dlp appends the source extension, so we look for files
// starting with base + "." in the same directory (preferring the most
// recently modified match).
func findDownloadedFile(base string) (string, error) {
	dir := filepath.Dir(base)
	entries, err := os.ReadDir(dir)
	if err != nil {
		return "", fmt.Errorf("read output dir: %w", err)
	}
	var best string
	var bestMod time.Time
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		name := e.Name()
		if name == filepath.Base(base) {
			return filepath.Join(dir, name), nil
		}
		if strings.HasPrefix(name, filepath.Base(base)+".") {
			if best == "" {
				best = filepath.Join(dir, name)
				info, err := e.Info()
				if err == nil {
					bestMod = info.ModTime()
				}
				continue
			}
			info, err := e.Info()
			if err == nil && info.ModTime().After(bestMod) {
				best = filepath.Join(dir, name)
				bestMod = info.ModTime()
			}
		}
	}
	if best == "" {
		return "", fmt.Errorf("downloaded file not found for %s", base)
	}
	return best, nil
}

// checkDuration verifies the extracted audio's duration is within tolerance
// of the expected video duration. This catches videos where yt-dlp fell back
// to a shorter clip (e.g. some shorts only offer partial audio).
func checkDuration(path string, expected float64) error {
	if expected <= 0 {
		return nil // no expectation to check against
	}
	act, err := ffprobeDuration(path)
	if err != nil {
		// If we can't probe, don't block the download.
		return nil
	}
	// Allow a 10% tolerance plus a 5s floor for rounding/formatting.
	tol := expected * 0.10
	if tol < 5 {
		tol = 5
	}
	if math.Abs(act-expected) > tol {
		return fmt.Errorf("duration mismatch: extracted %.0fs vs expected %.0fs (video may be unavailable in full length)", act, expected)
	}
	return nil
}

func (p *Pool) download(job DownloadJob) error {
	// Update state to downloading.
	if !p.state.UpdateJob(job.EnclosureURL, func(j *state.Job) {
		j.Status = state.StatusDownloading
		j.AttemptCount++
	}) {
		return fmt.Errorf("job not found for %s", job.EnclosureURL)
	}
	if err := p.state.Save(); err != nil {
		return fmt.Errorf("save state (downloading): %w", err)
	}

	// Ensure output directory exists.
	if err := os.MkdirAll(filepath.Dir(job.DestPath), 0755); err != nil {
		p.markRetry(job.EnclosureURL, fmt.Errorf("create dest dir: %w", err))
		return err
	}

	// Create destination file.
	dstFile, err := os.Create(job.DestPath)
	if err != nil {
		p.markRetry(job.EnclosureURL, fmt.Errorf("create dest file: %w", err))
		return err
	}

	// Download with timeout.
	client := &http.Client{Timeout: 30 * time.Minute}
	req, err := http.NewRequest("GET", job.EnclosureURL, nil)
	if err != nil {
		dstFile.Close()
		os.Remove(job.DestPath)
		p.markRetry(job.EnclosureURL, fmt.Errorf("create request: %w", err))
		return err
	}
	req.Header.Set("User-Agent", poller.UserAgent)

	resp, err := client.Do(req)
	if err != nil {
		dstFile.Close()
		os.Remove(job.DestPath)
		p.markRetry(job.EnclosureURL, fmt.Errorf("fetch: %w", err))
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		dstFile.Close()
		os.Remove(job.DestPath)
		p.markRetry(job.EnclosureURL, fmt.Errorf("HTTP %d", resp.StatusCode))
		return fmt.Errorf("HTTP %d", resp.StatusCode)
	}

	// Copy body to file.
	if _, err := io.Copy(dstFile, resp.Body); err != nil {
		dstFile.Close()
		os.Remove(job.DestPath)
		p.markRetry(job.EnclosureURL, fmt.Errorf("write file: %w", err))
		return err
	}
	if err := dstFile.Close(); err != nil {
		os.Remove(job.DestPath)
		p.markRetry(job.EnclosureURL, fmt.Errorf("close file: %w", err))
		return err
	}

	// Mark as downloaded.
	p.state.UpdateJob(job.EnclosureURL, func(j *state.Job) {
		j.Status = state.StatusDownloaded
	})
	if err := p.state.Save(); err != nil {
		return fmt.Errorf("save state (downloaded): %w", err)
	}

	return nil
}

// ffprobeDuration returns the duration of a media file in seconds.
func ffprobeDuration(path string) (float64, error) {
	out, err := exec.Command("ffprobe", "-v", "error",
		"-show_entries", "format=duration",
		"-of", "json", path).Output()
	if err != nil {
		return 0, fmt.Errorf("ffprobe %s: %w", filepath.Base(path), err)
	}
	var res struct {
		Format struct {
			Duration string `json:"duration"`
		} `json:"format"`
	}
	if err := json.Unmarshal(out, &res); err != nil {
		return 0, err
	}
	var d float64
	if _, err := fmt.Sscanf(res.Format.Duration, "%g", &d); err != nil {
		return 0, err
	}
	return d, nil
}

// markRetry updates a job to retrying status with the given error and computes backoff delay.
func (p *Pool) markRetry(enclosureURL string, err error) {
	p.state.UpdateJob(enclosureURL, func(j *state.Job) {
		j.Status = state.StatusRetrying
		j.LiveWaitCount = 0
		j.Error = err.Error()
		// Exponential backoff: base * 2^attemptCount (attempt already incremented).
		delay := p.backoffBase * time.Duration(1<<uint(j.AttemptCount-1))
		t := time.Now().Add(delay)
		j.NextAttemptAt = &t
	})
	p.state.Save()
}