package cc

import (
	"bufio"
	"context"
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"

	"github.com/fsnotify/fsnotify"
)

// rolloutUUIDRegex matches the UUID at the tail of rollout-<ISO>-<UUID>.jsonl.
// Matches v7 UUIDs (still 8-4-4-4-12 hex groups).
var rolloutUUIDRegex = regexp.MustCompile(`-([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})\.jsonl$`)

// extractIDFromRolloutFilename tries to extract the thread_id UUID from a
// rollout filename. Returns "" when the name doesn't match the expected pattern.
func extractIDFromRolloutFilename(filename string) string {
	m := rolloutUUIDRegex.FindStringSubmatch(strings.ToLower(filename))
	if m == nil {
		return ""
	}
	return m[1]
}

// CodexSessionsDir returns the codex sessions directory for a given date:
// <homeDir>/.codex/sessions/YYYY/MM/DD. The time.Time parameter makes this
// testable without calling time.Now() inside the library.
func CodexSessionsDir(homeDir string, t time.Time) string {
	return filepath.Join(homeDir, ".codex", "sessions",
		fmt.Sprintf("%04d", t.Year()),
		fmt.Sprintf("%02d", t.Month()),
		fmt.Sprintf("%02d", t.Day()),
	)
}

// CodexSessionWatchOpts configures WatchForNewCodexSession.
type CodexSessionWatchOpts struct {
	SessionsDir string
	Timeout     time.Duration
}

// WatchForNewCodexSession watches SessionsDir for the next new rollout-*.jsonl
// file. It prefers the UUID embedded in the filename; falls back to parsing
// payload.id from the first JSON line. Returns ("", nil) on timeout or context
// cancellation. MkdirAll ensures the leaf dir exists so the first-of-day
// session (when codex creates the YYYY/MM/DD dir from scratch) is captured.
//
// Known limitation: a session file landing in a different day-dir than the one
// watched (watcher started just before midnight) will not be caught.
func WatchForNewCodexSession(ctx context.Context, opts CodexSessionWatchOpts) (string, error) {
	// Ensure the target dir exists so fsnotify can watch it even before codex
	// creates it for the first time today.
	if err := os.MkdirAll(opts.SessionsDir, 0o755); err != nil {
		return "", nil
	}

	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		return "", nil
	}
	defer watcher.Close()

	if err := watcher.Add(opts.SessionsDir); err != nil {
		return "", nil
	}

	// Deduplicate: fsnotify may fire multiple events for the same file.
	seen := make(map[string]bool)

	timer := time.NewTimer(opts.Timeout)
	defer timer.Stop()

	for {
		select {
		case <-ctx.Done():
			return "", nil
		case <-timer.C:
			return "", nil
		case event, ok := <-watcher.Events:
			if !ok {
				return "", nil
			}
			if event.Op&(fsnotify.Create|fsnotify.Rename) == 0 {
				continue
			}
			name := filepath.Base(event.Name)
			if !strings.HasPrefix(name, "rollout-") || !strings.HasSuffix(name, ".jsonl") {
				continue
			}
			if seen[name] {
				continue
			}
			seen[name] = true

			// Confirm the file exists (rename fires for deletes too).
			if _, err := os.Stat(event.Name); err != nil {
				continue
			}

			// Prefer UUID from filename; fall back to first-line JSON payload.id.
			if id := extractIDFromRolloutFilename(name); id != "" {
				return id, nil
			}
			if id := extractIDFromFirstLine(event.Name); id != "" {
				return id, nil
			}
			// Could not extract an id — keep watching.

		case _, ok := <-watcher.Errors:
			if !ok {
				return "", nil
			}
		}
	}
}

// sessionMetaLine is the shape of the first JSON line in a codex rollout file.
type sessionMetaLine struct {
	Payload *struct {
		ID string `json:"id"`
	} `json:"payload"`
	ThreadID  string `json:"thread_id"`
	SessionID string `json:"session_id"`
	ID        string `json:"id"`
}

// extractIDFromFirstLine reads the first JSON line of filePath and returns the
// thread id found in payload.id, thread_id, session_id, or id (in that order).
func extractIDFromFirstLine(filePath string) string {
	f, err := os.Open(filePath)
	if err != nil {
		return ""
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	if !scanner.Scan() {
		return ""
	}
	line := strings.TrimSpace(scanner.Text())
	if line == "" {
		return ""
	}

	var meta sessionMetaLine
	if err := json.Unmarshal([]byte(line), &meta); err != nil {
		return ""
	}

	if meta.Payload != nil && meta.Payload.ID != "" {
		return meta.Payload.ID
	}
	if meta.ThreadID != "" {
		return meta.ThreadID
	}
	if meta.SessionID != "" {
		return meta.SessionID
	}
	return meta.ID
}