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package main

// File watching.
//
// We watch the *directory*, not the file. Editors save by writing a temporary
// file and renaming it over the target, which replaces the inode; a watch on the
// file itself follows the old inode and goes silent after the first save.
//
// Changes are debounced because a single save can produce several events
// (CREATE + WRITE + RENAME + CHMOD), and re-solving on each would start work we
// immediately throw away.

import (
	"log"
	"path/filepath"
	"time"

	"github.com/fsnotify/fsnotify"
)

// debounceInterval is how long to wait for the event storm of one save to
// settle. Long enough to coalesce an editor's write+rename, short enough to feel
// immediate.
const debounceInterval = 150 * time.Millisecond

// watchFile calls onChange after the file at path (or its theme sibling) is
// modified. It blocks until the watcher fails.
func watchFile(path string, onChange func()) error {
	w, err := fsnotify.NewWatcher()
	if err != nil {
		return err
	}
	defer w.Close()

	dir := filepath.Dir(path)
	if err := w.Add(dir); err != nil {
		return err
	}
	log.Printf("watching %s", dir)

	// Editing the theme should re-render too, and it is a sibling of the model.
	stem := path[:len(path)-len(filepath.Ext(path))]
	themePath := stem + ".thm"

	var timer *time.Timer
	for {
		select {
		case ev, ok := <-w.Events:
			if !ok {
				return nil
			}
			// Ignore CHMOD-only events: some editors touch permissions without
			// changing content, and re-solving on that is pure waste.
			if ev.Op == fsnotify.Chmod {
				continue
			}
			if ev.Name != path && ev.Name != themePath {
				continue
			}
			if timer != nil {
				timer.Stop()
			}
			timer = time.AfterFunc(debounceInterval, onChange)

		case err, ok := <-w.Errors:
			if !ok {
				return nil
			}
			log.Printf("watcher: %v", err)
		}
	}
}