package config

import "os"
import "log"
import "sync"
import "slices"
import "path/filepath"
import "github.com/fsnotify/fsnotify"
import "git.tebibyte.media/tomo/tomo"
import "git.tebibyte.media/tomo/tomo/event"

// Goroutine model:
// All private methods (except for lockAndProcessEvent) do not lock the config,
// but all public methods do. Private methods may not call public methods.
// Locking must always be method-level, with a call to Lock at the start,
// directly followed by a deferred call to Unlock.

type config struct {
	open                 bool
	watcher              *fsnotify.Watcher
	lock                 sync.RWMutex
	ignoreNextUserUpdate bool

	paths struct {
		user     string
		system   []string
		watching map[string] struct { }
	}

	data struct {
		user   *File
		system []map[string] Value
	}
	
	on struct {
		change event.Broadcaster[func (string)]
	}
}

// NewConfig creates a new Config using paths to the user-level config file, and
// a set of system config files. These files need not exist: the user-level file
// will be created when Set is called for the first time if it does not exist
// already, and nonexistent system files are simply ignored (unless the Config
// finds them at any point to have been spontaneously created).
//
// The user file is written to when Set is called, and the system files are only
// read from. Values in the user file override those in the system files, and
// system files specified nearer to the start of the vararg list will override
// those farther down.
func NewConfig (user string, system ...string) (ConfigCloser, error) {
	conf := new(config)
	conf.paths.user   = user
	conf.paths.system = system
	err := conf.init()
	if err != nil { return nil, err }
	go func () {
		for event := range conf.watcher.Events {
			conf.lockAndProcessEvent(event)
		}
	} ()
	return conf, nil
}

// this method may only be run in the goroutine spawned by NewConfig.
func (this *config) lockAndProcessEvent (event fsnotify.Event) {
	this.lock.Lock()
	defer this.lock.Unlock()
	if !(event.Has(fsnotify.Write))                  { return }
	if _, ok := this.paths.watching[event.Name]; !ok { return }

	if event.Name == this.paths.user {
		if !this.ignoreNextUserUpdate {
			previousUser := this.data.user
			this.reloadUser()
			newUser := this.data.user
			this.processUserDiff(newUser.Diff(previousUser))
		}
		this.ignoreNextUserUpdate = false
	} else {
		index := slices.Index(this.paths.system, event.Name)
		if index > 0 {
			previousSystem := this.data.system[index]
			this.reloadSystem(index)
			newSystem := this.data.system[index]
			this.processSystemDiff(index, diffValueMaps(newSystem, previousSystem))
		}
	}
}

func (this *config) init () error {
	watcher, err := fsnotify.NewWatcher()
	if err != nil { return err }
	this.watcher = watcher
	
	this.paths.watching = make(map[string] struct { })
	this.watcher.Add(filepath.Dir(this.paths.user))
	this.paths.watching[this.paths.user] = struct { } { }
	this.reloadUser()
	for index, path := range this.paths.system {
		this.watcher.Add(filepath.Dir(path))
		this.paths.watching[path] = struct { } { }
		this.reloadSystem(index)
	}

	return nil
}

func (this *config) errIfClosed () error {
	if this.open {
		return nil
	} else {
		return ErrClosed
	}
}

func (this *config) reloadUser () {
	file, err := os.Open(this.paths.user)
	if err != nil { return }
	defer file.Close()
	
	userFile, err := Parse(file)
	if err != nil {
		log.Printf("nasin: problems loading user config file %s: %v", this.paths.user, err)
		return
	}
	this.data.user = userFile
}

func (this *config) reloadSystem (index int) {
	path := this.paths.system[index]
	file, err := os.Open(path)
	if err != nil { return }
	defer file.Close()
	
	systemFile, err := Parse(file)
	if err != nil {
		log.Printf("nasin: problems loading system config file %s: %v", path, err)
		return
	}
	this.data.system[index] = systemFile.Map()
}

func (this *config) saveUser () error {
	// TODO set some sort of flag to ignore the next inotify event for the
	// user file so we dont reload it immediately after. also need to
	// broadacast Changed event.
	enclosingDir := filepath.Dir(this.paths.user)
	err := os.MkdirAll(enclosingDir, 755)
	if err != nil { return err }
	file, err := os.Create(this.paths.user)
	if err != nil { return err }
	defer file.Close()
	_, err = this.data.user.WriteTo(file)
	if err != nil { return err }
	this.ignoreNextUserUpdate = true
	return nil
}

func (this *config) processUserDiff (changed map[string] struct { }) {
	for key := range changed {
		// this is the user file, and nothing has precedence over it, so
		// the change always matters
		this.broadcastChange(key)
	}
}

func (this *config) processSystemDiff (index int, changed map[string] struct { }) {
	for key := range changed {
		// if specified in the user file, the change doesn't matter
		if this.data.user != nil {
			if has, _ := this.data.user.Has(key); has {
				continue
			}
		}
		
		// if specified in any system files with precedence greater than
		// this one, the change doesn't matter
		for _, system := range this.data.system[:index] {
			if _, has := system[key]; has {
				continue
			}
		}

		// the change does matter
		this.broadcastChange(key)
	}
}

func (this *config) broadcastChange (key string) {
	for _, listener := range this.on.change.Listeners() {
		tomo.Do(func () { listener(key) })
	}
}

func (this *config) get (key string, fallback Value) (Value, error) {
	// try user config
	if !KeyValid(key) { return nil, ErrMalformedKey }

	if this.data.user != nil {
		value, err := this.data.user.Get(key)
		if err == nil && err != ErrNonexistentEntry {
			return value, nil
		}
	}

	// try system configs
	for _, config := range this.data.system {
		if value, ok := config[key]; ok {
			return value, nil
		}
	}

	// use fallback
	return fallback, nil
	
}

func (this *config) Get (key string, fallback Value) (Value, error) {
	this.lock.Lock()
	defer this.lock.Unlock()
	return this.get(key, fallback)
}

func (this *config) Close () error {
	this.lock.Lock()
	defer this.lock.Unlock()
	this.open = false
	return this.watcher.Close()
}

func (this *config) GetString (key string, fallback string) (string, error) {
	this.lock.Lock()
	defer this.lock.Unlock()
	value, err := this.get(key, ValueString(fallback))
	if err != nil { return "", err }
	if value, ok := value.(ValueString); ok {
		return string(value), nil
	}
	return fallback, nil
}

func (this *config) GetNumber (key string, fallback float64) (float64, error) {
	this.lock.Lock()
	defer this.lock.Unlock()
	value, err := this.get(key, ValueNumber(fallback))
	if err != nil { return 0, err }
	if value, ok := value.(ValueNumber); ok {
		return float64(value), nil
	}
	return fallback, nil
}

func (this *config) GetBool (key string, fallback bool) (bool, error) {
	this.lock.Lock()
	defer this.lock.Unlock()
	value, err := this.get(key, ValueBool(fallback))
	if err != nil { return false, err }
	if value, ok := value.(ValueBool); ok {
		return bool(value), nil
	}
	return fallback, nil
}

func (this *config) Set (key string, value Value) error {
	this.lock.Lock()
	defer this.lock.Unlock()
	if this.data.user == nil { this.data.user = NewFile() }
	err := this.data.user.Set(key, value)
	if err != nil { return err }
	err = this.saveUser()
	if err != nil { return err }
	this.broadcastChange(key)
	return nil
}

func (this *config) Reset (key string) error {
	this.lock.Lock()
	defer this.lock.Unlock()
	if this.data.user == nil { this.data.user = NewFile() }
	err := this.data.user.Reset(key)
	if err != nil { return err }
	err = this.saveUser()
	if err != nil { return err }
	this.broadcastChange(key)
	return nil
}

func (this *config) OnChange (callback func (string)) event.Cookie {
	this.lock.Lock()
	defer this.lock.Unlock()
	return this.on.change.Connect(callback)
}

func diffValueMaps (first, second map[string] Value) map[string] struct { } {
	diff := make(map[string] struct { })
	
	// - keys only first has
	// - keys both have, but are different
	for key, firstValue := range first {
		secondValue, ok := second[key]
		if !ok {
			diff[key] = struct { } { }
			continue
		}
		if !firstValue.Equals(secondValue) {
			diff[key] = struct { } { }
		}
	}

	// - keys only second has
	for key := range second {
		if _, has := first[key]; !has {
			diff[key] = struct { } { }
		}
	}

	return diff
}