negation command

2 files changed, 17 insertions, 3 deletions

diff --git a/calculator.go b/calculator.go
index f0ed7db..3493382 100644
--- a/calculator.go
+++ b/calculator.go
@@ -38,12 +38,24 @@ func (c *Calculator) swap() {
 		c.stack.push(f)
 	}
 	if st != nil {
-		c.buf = strings.TrimSpace(printStackVal(*st))
+		c.buf = strings.TrimSpace(printNum(*st))
 	} else {
 		c.buf = ""
 	}
 }
 
+// negate negates the number in the buffer, if any; or the bottom number on the
+// stack, if any.
+func (c *Calculator) negate() {
+	if con := parseConstant(c.buf); con != nil {
+		c.buf = strings.TrimSpace(printNum(-*con))
+	} else if f, err := strconv.ParseFloat(c.buf, 64); err == nil {
+		c.buf = strings.TrimSpace(printNum(-f))
+	} else if len(c.buf) == 0 && len(c.stack) > 0 {
+		c.stack[len(c.stack)-1] = -c.stack[len(c.stack)-1]
+	}
+}
+
 // 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 {
diff --git a/ui.go b/ui.go
index 6596b77..85919c4 100644
--- a/ui.go
+++ b/ui.go
@@ -44,6 +44,8 @@ func (ui UI) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		case "C":
 			ui.calc.buf = ""
 			ui.calc.stack = ui.calc.stack[:0]
+		case "N":
+			ui.calc.negate()
 		case "+", "-", "*", "/", "%", "^":
 			if err := ui.calc.performOp(msg.String()[0]); err != nil {
 				panic(err)
@@ -71,7 +73,7 @@ func (ui UI) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 func (ui UI) View() string {
 	var s string
 	for _, f := range ui.calc.stack {
-		s += printStackVal(f) + "\n"
+		s += printNum(f) + "\n"
 	}
 	s += boxTop(ui.windowWidth) + "\n"
 	s += fmt.Sprintf("%[1]c%-*s%[1]c\n", boxVertical, ui.windowWidth-2, ui.calc.buf)
@@ -79,7 +81,7 @@ func (ui UI) View() string {
 	return s
 }
 
-func printStackVal(v float64) string {
+func printNum(v float64) string {
 	return fmt.Sprintf(" %.*g", sigDigs, v)
 }