string: add filter method (#5812)

vlib/builtin/string.v

@@ -1382,6 +1382,20 @@ pub fn (s string) map(func fn(byte) byte) string {
 	return tos(res, s.len)
 }
 
+pub fn (s string) filter(func fn(b byte) bool) string {
+	mut new_len := 0
+	mut buf := malloc(s.len + 1)
+	for i in 0 .. s.len {
+		mut b := s[i]
+		if func(b) {
+			buf[new_len] = b
+			new_len++
+		}
+	}
+	buf[new_len] = 0
+	return string(buf, new_len)
+}
+
 // Allows multi-line strings to be formatted in a way that removes white-space
 // before a delimeter. by default `|` is used.
 // Note: the delimiter has to be a byte at this time. That means surrounding

vlib/builtin/string_test.v

@@ -784,6 +784,18 @@ fn foo(b byte) byte {
 	return b - 10
 }
 
+fn test_string_filter() {
+	foo := 'V is awesome!!!!'.filter(fn (b byte) bool {
+		return b != `!`
+	})
+	assert foo == 'V is awesome'
+	assert 'Alexander'.filter(filter) == 'Alexnder'
+}
+
+fn filter(b byte) bool {
+	return b != `a`
+}
+
 fn test_split_into_lines() {
 	line_content := 'Line'
 	text_crlf := '${line_content}\r\n${line_content}\r\n${line_content}'