strings.textscanner: fix off-by-one error in skip method (#21045)

Author: kimshrier

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++
 	}
 }

vlib/strings/textscanner/textscanner_test.js.v

@@ -31,13 +31,39 @@ 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() {
 	mut s := textscanner.new('abc')
 	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() {

vlib/strings/textscanner/textscanner_test.v

@@ -31,13 +31,39 @@ 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() {
 	mut s := textscanner.new('abc')
 	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() {