From 3516ab494570f077df967bfe1222acc0dfcdf5da Mon Sep 17 00:00:00 2001
From: Sam Anthony <sam@samanthony.xyz>
Date: Fri, 28 Jul 2023 20:46:24 -0230
Subject: arithmetic operators

---
 calculator.go | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++--------
 ui.go         |  6 ++++--
 2 files changed, 58 insertions(+), 10 deletions(-)

diff --git a/calculator.go b/calculator.go
index 08193b4..d2371b7 100644
--- a/calculator.go
+++ b/calculator.go
@@ -1,6 +1,9 @@
 package main
 
-import "strconv"
+import (
+	"fmt"
+	"strconv"
+)
 
 type Stack []float64
 
@@ -9,18 +12,61 @@ type Calculator struct {
 	buf   string
 }
 
-// add performs addition when the user inputs the '+' operator.
-func (c *Calculator) add() {
+// performOp performs the specified arithmetic operation and returns nil or
+// OpError if op is not a valid operator.
+func (c *Calculator) performOp(op byte) error {
 	if len(c.stack) < 1 {
-		return
+		return nil
 	}
+
+	fn, err := parseOp(op)
+	if err != nil {
+		return err
+	}
+
 	if con := parseConstant(c.buf); con != nil {
-		c.stack[len(c.stack)-1] += *con
-	} else if f, err := strconv.ParseFloat(c.buf, 64); err == nil {
-		c.stack[len(c.stack)-1] += f
+		fn(&c.stack[len(c.stack)-1], *con)
+	} else if fl, err := strconv.ParseFloat(c.buf, 64); err == nil {
+		fn(&c.stack[len(c.stack)-1], fl)
 	} else if len(c.stack) > 1 {
-		c.stack[len(c.stack)-2] += c.stack[len(c.stack)-1]
+		fn(&c.stack[len(c.stack)-2], c.stack[len(c.stack)-1])
 		c.stack = c.stack[:len(c.stack)-1]
 	}
 	c.buf = ""
+	return nil
+}
+
+// parseOp returns a closure that performs the specified arithmetic operation,
+// or OpError if op is not a valid operator.
+func parseOp(op byte) (func(lhs *float64, rhs float64), error) {
+	switch op {
+	case '+':
+		return func(lhs *float64, rhs float64) { *lhs += rhs }, nil
+	case '-':
+		return func(lhs *float64, rhs float64) { *lhs -= rhs }, nil
+	case '*':
+		return func(lhs *float64, rhs float64) { *lhs *= rhs }, nil
+	case '/':
+		return func(lhs *float64, rhs float64) {
+			if rhs != 0 {
+				*lhs /= rhs
+			}
+		}, nil
+	case '%':
+		return func(lhs *float64, rhs float64) {
+			if rhs != 0 {
+				*lhs = float64(int64(*lhs) % int64(rhs))
+			}
+		}, nil
+	}
+	return nil, OpError{op}
+}
+
+// OpError records an invalid arithmetic operator.
+type OpError struct {
+	c byte
+}
+
+func (e OpError) Error() string {
+	return fmt.Sprintf("invalid operator: %c", e.c)
 }
diff --git a/ui.go b/ui.go
index 931e8a5..5996a9c 100644
--- a/ui.go
+++ b/ui.go
@@ -37,8 +37,10 @@ func (ui UI) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		switch msg.String() {
 		case "ctrl+c", "Q":
 			return ui, tea.Quit
-		case "+":
-			ui.calc.add()
+		case "+", "-", "*", "/", "%":
+			if err := ui.calc.performOp(msg.String()[0]); err != nil {
+				panic(err)
+			}
 		case "backspace":
 			if len(ui.calc.buf) > 0 {
 				ui.calc.buf = ui.calc.buf[:len(ui.calc.buf)-1]
-- 
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