io: add a string_reader submodule (#20893)

vlib/io/string_reader/string_reader_test.v

@@ -0,0 +1,110 @@
+module string_reader
+
+import io
+
+struct Buf {
+pub:
+	bytes []u8
+mut:
+	i int
+}
+
+fn (mut b Buf) read(mut buf []u8) !int {
+	if !(b.i < b.bytes.len) {
+		return io.Eof{}
+	}
+	n := copy(mut buf, b.bytes[b.i..])
+	b.i += n
+	return n
+}
+
+fn test_read_all() {
+	mut reader := StringReader.new()
+
+	if _ := reader.read_all(false) {
+		assert false, 'should return io.Eof'
+	} else {
+		assert err is io.Eof
+	}
+
+	contents := 'test string'
+	buf := Buf{
+		bytes: contents.bytes()
+	}
+
+	reader = StringReader.new(reader: buf)
+	data := reader.read_all(false)!
+
+	assert data == contents
+}
+
+fn test_read_bytes() {
+	mut reader := StringReader.new()
+
+	if _ := reader.read_bytes(4) {
+		assert false, 'should return io.Eof'
+	} else {
+		assert err.msg() == 'reader is not set'
+	}
+
+	buf := Buf{
+		bytes: '12345678'.bytes()
+	}
+
+	reader = StringReader.new(reader: buf)
+	mut data := reader.read_bytes(4)!
+	assert data == buf.bytes[..4]
+
+	data = reader.read_bytes(4)!
+	assert data == buf.bytes[4..8]
+}
+
+fn test_read_line() {
+	mut reader := StringReader.new()
+
+	if _ := reader.read_line() {
+		assert false, 'should return io.Eof'
+	} else {
+		assert err is io.Eof
+	}
+
+	buf := Buf{
+		bytes: 'first line\r\nsecond line\n'.bytes()
+	}
+
+	reader = StringReader.new(reader: buf)
+
+	assert reader.read_line()! == 'first line'
+	assert reader.read_line()! == 'second line'
+}
+
+fn test_from_string() {
+	mut reader := StringReader.new(source: 'test')
+	assert reader.read_all(false)! == 'test'
+
+	if _ := reader.read_all(false) {
+		assert false, 'should return Eof'
+	} else {
+		assert err is io.Eof
+	}
+}
+
+fn test_from_string_and_reader() {
+	buf := Buf{
+		bytes: 'buffer'.bytes()
+	}
+
+	mut reader := StringReader.new(reader: buf, source: 'string')
+
+	assert reader.read_all(false)! == 'stringbuffer'
+}
+
+fn test_flush() {
+	mut reader := StringReader.new(source: 'flushed data')
+
+	str := reader.flush()
+	assert str == 'flushed data'
+
+	assert reader.offset == 0
+	assert reader.builder.len == 0
+}