From d7cc6f5cf1ce712053b303183da827ef585401eb Mon Sep 17 00:00:00 2001
From: Kim Shrier <gitkim@westryn.net>
Date: Sun, 17 Mar 2024 03:11:10 -0600
Subject: [PATCH] strings.textscanner: fix off-by-one error in skip method
 (#21045)

---
 vlib/strings/textscanner/textscanner.v        |  2 +-
 .../strings/textscanner/textscanner_test.js.v | 26 +++++++++++++++++++
 vlib/strings/textscanner/textscanner_test.v   | 26 +++++++++++++++++++
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/vlib/strings/textscanner/textscanner.v b/vlib/strings/textscanner/textscanner.v
index be748b5d527725..65a38820bf874b 100644
--- a/vlib/strings/textscanner/textscanner.v
+++ b/vlib/strings/textscanner/textscanner.v
@@ -50,7 +50,7 @@ pub fn (mut ss TextScanner) next() int {
 // `skip()` does not return a result.
 @[inline]
 pub fn (mut ss TextScanner) skip() {
-	if ss.pos + 1 < ss.ilen {
+	if ss.pos < ss.ilen {
 		ss.pos++
 	}
 }
diff --git a/vlib/strings/textscanner/textscanner_test.js.v b/vlib/strings/textscanner/textscanner_test.js.v
index e9d248769a5892..ef9f7b7cc46623 100644
--- a/vlib/strings/textscanner/textscanner_test.js.v
+++ b/vlib/strings/textscanner/textscanner_test.js.v
@@ -31,6 +31,15 @@ fn test_skip() {
 	s.skip()
 	assert s.next() == `c`
 	assert s.next() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip()
+	assert s.peek() == `b`
+	s.skip()
+	assert s.peek() == `c`
+	s.skip()
+	assert s.peek() == -1
 }
 
 fn test_skip_n() {
@@ -38,6 +47,23 @@ fn test_skip_n() {
 	s.skip_n(2)
 	assert s.next() == `c`
 	assert s.next() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip_n(2)
+	assert s.peek() == `c`
+	s.skip_n(2)
+	assert s.peek() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip_n(3)
+	assert s.peek() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip_n(4)
+	assert s.peek() == -1
 }
 
 fn test_peek() {
diff --git a/vlib/strings/textscanner/textscanner_test.v b/vlib/strings/textscanner/textscanner_test.v
index e9d248769a5892..ef9f7b7cc46623 100644
--- a/vlib/strings/textscanner/textscanner_test.v
+++ b/vlib/strings/textscanner/textscanner_test.v
@@ -31,6 +31,15 @@ fn test_skip() {
 	s.skip()
 	assert s.next() == `c`
 	assert s.next() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip()
+	assert s.peek() == `b`
+	s.skip()
+	assert s.peek() == `c`
+	s.skip()
+	assert s.peek() == -1
 }
 
 fn test_skip_n() {
@@ -38,6 +47,23 @@ fn test_skip_n() {
 	s.skip_n(2)
 	assert s.next() == `c`
 	assert s.next() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip_n(2)
+	assert s.peek() == `c`
+	s.skip_n(2)
+	assert s.peek() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip_n(3)
+	assert s.peek() == -1
+
+	s.reset()
+	assert s.peek() == `a`
+	s.skip_n(4)
+	assert s.peek() == -1
 }
 
 fn test_peek() {