strings: add a new module strings.textscanner to simplify writing small parsers

vlib/strings/textscanner/textscanner_test.v

@@ -0,0 +1,100 @@
+import strings.textscanner
+
+fn test_remaining() {
+	mut s := textscanner.new('abc')
+	assert s.remaining() == 3
+	s.next()
+	s.next()
+	assert s.remaining() == 1
+	s.next()
+	assert s.remaining() == 0
+	s.next()
+	s.next()
+	assert s.remaining() == 0
+	s.reset()
+	assert s.remaining() == 3
+}
+
+fn test_next() {
+	mut s := textscanner.new('abc')
+	assert s.next() == `a`
+	assert s.next() == `b`
+	assert s.next() == `c`
+	assert s.next() == -1
+	assert s.next() == -1
+	assert s.next() == -1
+}
+
+fn test_skip() {
+	mut s := textscanner.new('abc')
+	assert s.next() == `a`
+	s.skip()
+	assert s.next() == `c`
+	assert s.next() == -1
+}
+
+fn test_skip_n() {
+	mut s := textscanner.new('abc')
+	s.skip_n(2)
+	assert s.next() == `c`
+	assert s.next() == -1
+}
+
+fn test_peek() {
+	mut s := textscanner.new('abc')
+	assert s.peek() == `a`
+	assert s.peek() == `a`
+	assert s.peek() == `a`
+	//
+	assert s.next() == `a`
+	assert s.next() == `b`
+	assert s.next() == `c`
+	assert s.next() == -1
+}
+
+fn test_peek_n() {
+	mut s := textscanner.new('abc')
+	assert s.peek_n(0) == `a`
+	assert s.peek_n(1) == `b`
+	assert s.peek_n(2) == `c`
+	assert s.peek_n(3) == -1
+	assert s.peek_n(4) == -1
+	//
+	assert s.next() == `a`
+	assert s.next() == `b`
+	assert s.next() == `c`
+	assert s.next() == -1
+}
+
+fn test_back() {
+	mut s := textscanner.new('abc')
+	assert s.next() == `a`
+	s.back()
+	assert s.next() == `a`
+	assert s.next() == `b`
+	s.back()
+	assert s.next() == `b`
+	assert s.next() == `c`
+	assert s.next() == -1
+}
+
+fn test_back_n() {
+	mut s := textscanner.new('abc')
+	assert s.next() == `a`
+	s.back_n(10)
+	assert s.next() == `a`
+	assert s.next() == `b`
+	assert s.next() == `c`
+	s.back_n(2)
+	assert s.next() == `b`
+}
+
+fn test_reset() {
+	mut s := textscanner.new('abc')
+	assert s.next() == `a`
+	s.next()
+	s.next()
+	assert s.next() == -1
+	s.reset()
+	assert s.next() == `a`
+}