package main

import (
   "fmt"
   "strconv"
   "strings"

   "fyne.io/fyne/v2"
   "fyne.io/fyne/v2/app"
   "fyne.io/fyne/v2/container"
   "fyne.io/fyne/v2/layout"
   "fyne.io/fyne/v2/widget"
   "github.com/Knetic/govaluate"
)

var entry *widget.Entry
var filter = []string{"+", "-", "*", "/", "%", "."}
var lastInputIsFlag bool

func main() {
   a := app.New()
   w := a.NewWindow("Calculator")
   w.CenterOnScreen()
   w.Resize(fyne.NewSize(450, 300))

   entry = widget.NewEntry()
   entry.MultiLine = true

   digits := []string{
      "7", "8", "9", "*",
      "4", "5", "6", "-",
      "1", "2", "3", "+",
   }
   var digitBtns []fyne.CanvasObject
   digitBtns = append(digitBtns, widget.NewButton("c", func() {
      entry.SetText("")
      entry.Refresh()
   }))
   digitBtns = append(digitBtns, widget.NewButton("+/-", sign()))
   digitBtns = append(digitBtns, widget.NewButton("", percent()))
   digitBtns = append(digitBtns, widget.NewButton("/", input("/")))

   for _, v := range digits {
      val := v
      digitBtns = append(digitBtns, widget.NewButton(val, input(val)))
   }

   buts := container.New(layout.NewGridLayout(4), digitBtns...)

   equal := widget.NewButton("=", equals())

   b := container.New(
      layout.NewGridLayout(2),
      widget.NewButton("0", input("0")),
      container.New(layout.NewGridLayout(2), widget.NewButton(".", input(".")), equal))

   w.SetContent(container.New(layout.NewVBoxLayout(), entry, buts, b))
   w.ShowAndRun()
}

func percent() func() {
   return func() {

   }
}

func sign() func() {
   return func() {
      if strings.Contains(entry.Text, ".") {
         value, err := strconv.ParseFloat(entry.Text, 64)
         if err != nil {
            return
         }
         value = -value
         entry.Text = fmt.Sprint(value)
      } else {
         value, err := strconv.ParseInt(entry.Text, 10, 64)
         if err != nil {
            return
         }
         value = -value
         entry.Text = fmt.Sprint(value)
      }

      entry.Refresh()
   }
}

func equals() func() {
   return func() {
      lines := strings.Split(entry.Text, "\n")
      if len(lines) == 0 {
         return
      }

      line := lines[len(lines)-1]
      line = strings.Trim(line, "+/x")
      expr, _ := govaluate.NewEvaluableExpression(line)
      result, _ := expr.Evaluate(nil)
      line += "=\n"
      line += fmt.Sprint(result)
      entry.Text = line
      entry.Refresh()
   }
}

func input(val string) func() {
   return func() {
      var thisFlag bool
      for _, v := range filter {
         if v == val {
            thisFlag = true
         }
      }

      if thisFlag && lastInputIsFlag {
         return
      }

      lastInputIsFlag = thisFlag
      entry.SetText(entry.Text + val)
      entry.Refresh()
   }
}

go get -u github.com/Knetic/govaluate

go build -ldflags="-w -s -H windowsgui"