//go:build js

package main

import (
	"bytes"
	"encoding/json"
	"syscall/js"
	"text/template"
)

func main() {
	regenChan := make(chan struct{})
	go executeTemplate(regenChan)

	eventListener := func(this js.Value, args []js.Value) interface{} {
		ev := args[0]
		ev.Call("preventDefault")

		regenChan <- struct{}{}

		return nil
	}

	querySelector("#template").Call("addEventListener", "keyup", js.FuncOf(eventListener))
	querySelector("#data").Call("addEventListener", "keyup", js.FuncOf(eventListener))

	regenChan <- struct{}{}
	<-make(chan struct{})
}

func executeTemplate(regenChan chan struct{}) {
	templateEl := querySelector("#template")
	dataEl := querySelector("#data")
	outputEl := querySelector("#output")

	tmplMessageEl := querySelector("#template-message")
	dataMessageEl := querySelector("#data-message")
	displayErrors := func(dataErr error, tmplErr error) {
		if dataErr != nil {
			dataMessageEl.Set("innerText", dataErr.Error())
		} else {
			dataMessageEl.Set("innerText", "")
		}
		if tmplErr != nil {
			tmplMessageEl.Set("innerText", tmplErr.Error())
		} else {
			tmplMessageEl.Set("innerText", "")
		}
	}

	var templateSource string
	var tmpl *template.Template
	setTemplateSource := func(newTemplateSource string) (*template.Template, error) {
		if tmpl == nil || templateSource != newTemplateSource {
			newTmpl := template.New("")
			if _, err := newTmpl.Parse(newTemplateSource); err != nil {
				return nil, err
			}
			tmpl = newTmpl
		}
		return tmpl, nil
	}

	var outBuf bytes.Buffer
	for range regenChan {
		templateValue := templateEl.Get("value").String()
		dataValue := dataEl.Get("value").String()

		var dataJson interface{}
		dataErr := json.Unmarshal([]byte(dataValue), &dataJson)
		tmplInst, tmplErr := setTemplateSource(templateValue)

		if dataErr != nil || tmplErr != nil {
			displayErrors(dataErr, tmplErr)
			continue
		}

		outBuf.Reset()
		if err := tmplInst.Execute(&outBuf, dataJson); err != nil {
			displayErrors(nil, err)
			continue
		}

		outputEl.Set("innerText", outBuf.String())
		displayErrors(nil, nil)
	}
}

func querySelector(query string) js.Value {
	return js.Global().Get("document").Call("querySelector", query)
}