package main

import (
	"context"
	"fmt"
	"io"
	"io/fs"
	"os"
	"os/signal"
	"path/filepath"
	"strings"
	"syscall"
	"time"
)

// watchSnapshot maps a file path to its modification time and size.
type watchSnapshot map[string]string

// takeWatchSnapshot records the state of every file under the given paths.
// Directories are walked recursively. Anything at or below the excluded path
// (typically the output path) is skipped so regenerating output does not
// retrigger the watcher. Missing paths are simply absent from the snapshot, so
// creating them later counts as a change.
func takeWatchSnapshot(paths []string, exclude string) watchSnapshot {
	snap := watchSnapshot{}
	exclude = absPathOrSelf(exclude)
	for _, root := range paths {
		_ = filepath.WalkDir(root, func(p string, d fs.DirEntry, err error) error {
			if err != nil {
				return nil
			}
			if exclude != "" && isWithin(absPathOrSelf(p), exclude) {
				if d.IsDir() {
					return filepath.SkipDir
				}
				return nil
			}
			if d.IsDir() {
				return nil
			}
			info, err := d.Info()
			if err != nil {
				return nil
			}
			snap[p] = fmt.Sprintf("%d-%d", info.ModTime().UnixNano(), info.Size())
			return nil
		})
	}
	return snap
}

func (s watchSnapshot) equal(o watchSnapshot) bool {
	if len(s) != len(o) {
		return false
	}
	for k, v := range s {
		if o[k] != v {
			return false
		}
	}
	return true
}

func absPathOrSelf(p string) string {
	if p == "" {
		return ""
	}
	if abs, err := filepath.Abs(p); err == nil {
		return abs
	}
	return p
}

// isWithin reports whether path equals dir or is located below it.
func isWithin(path, dir string) bool {
	if path == dir {
		return true
	}
	return strings.HasPrefix(path, dir+string(filepath.Separator))
}

// watchLoop runs fn once immediately and again whenever the watched paths
// change, until ctx is cancelled. Changes are debounced: fn runs only after
// the snapshot has stayed unchanged for one poll interval. Errors from fn are
// reported to w and do not stop the loop.
func watchLoop(ctx context.Context, w io.Writer, paths []string, exclude string, interval time.Duration, fn func() error) {
	run := func() {
		if err := fn(); err != nil {
			fmt.Fprintf(w, "Error: %v\n", err)
		}
		fmt.Fprintf(w, "Watching for changes (Ctrl-C to stop)...\n")
	}

	last := takeWatchSnapshot(paths, exclude)
	run()

	ticker := time.NewTicker(interval)
	defer ticker.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
		}
		cur := takeWatchSnapshot(paths, exclude)
		if cur.equal(last) {
			continue
		}
		// Debounce: wait until writes settle.
		for settled := false; !settled; {
			select {
			case <-ctx.Done():
				return
			case <-time.After(interval):
			}
			next := takeWatchSnapshot(paths, exclude)
			settled = next.equal(cur)
			cur = next
		}
		fmt.Fprintf(w, "\nChange detected, regenerating...\n")
		last = cur
		run()
	}
}

// runConvertWatch runs the conversion once and then again whenever the source
// files change, until interrupted.
func runConvertWatch(parent context.Context, w io.Writer, run func() error) error {
	var paths []string
	switch {
	case len(convertFromList) > 0:
		paths = convertFromList
	case convertSourcePath != "":
		paths = []string{convertSourcePath}
	default:
		return fmt.Errorf("--watch requires --from-path or --from-list (live database connections cannot be watched)")
	}
	if convertDryRun {
		return fmt.Errorf("--watch cannot be combined with --dry-run")
	}
	if convertWatchInterval <= 0 {
		return fmt.Errorf("--watch-interval must be positive")
	}
	if parent == nil {
		parent = context.Background()
	}

	ctx, stop := signal.NotifyContext(parent, os.Interrupt, syscall.SIGTERM)
	defer stop()
	watchLoop(ctx, w, paths, convertTargetPath, convertWatchInterval, run)
	return nil
}