From bacbfcc517c73202ef54f1d855793af38bfcfa02 Mon Sep 17 00:00:00 2001
From: Thomas von Dein <tom@vondein.org>
Date: Thu, 9 Nov 2023 18:47:31 +0100
Subject: [PATCH] added debug output to Backup()

---
 calc.go      | 4 ++++
 calc_test.go | 5 +++++
 stack.go     | 4 ++++
 3 files changed, 13 insertions(+)

diff --git a/calc.go b/calc.go
index 7177a2e..82bda08 100644
--- a/calc.go
+++ b/calc.go
@@ -393,6 +393,10 @@ func (c *Calc) DoFuncall(funcname string) error {
 		return R.Err
 	}
 
+	// don't forget to backup!
+	c.stack.Backup()
+
+	// "pop"
 	if batch {
 		// get rid of stack
 		c.stack.Clear()
diff --git a/calc_test.go b/calc_test.go
index 9ffe0cd..a5e0958 100644
--- a/calc_test.go
+++ b/calc_test.go
@@ -236,6 +236,11 @@ func TestCalc(t *testing.T) {
 			exp:   2,
 			batch: true,
 		},
+		{
+			name: "undo",
+			cmd:  `4 4 + undo *`,
+			exp:  16,
+		},
 	}
 
 	for _, tt := range tests {
diff --git a/stack.go b/stack.go
index 3d73fd3..c853db0 100644
--- a/stack.go
+++ b/stack.go
@@ -198,10 +198,14 @@ func (s *Stack) Backup() {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
+	s.Debug(fmt.Sprintf("backing up %d items from rev %d",
+		s.linklist.Len(), s.rev))
+
 	s.backup = list.List{}
 	for e := s.linklist.Front(); e != nil; e = e.Next() {
 		s.backup.PushBack(e.Value.(float64))
 	}
+
 	s.backuprev = s.rev
 }