Add `bufio.Lookahead_Reader`

core/bufio/lookahead_reader.odin

@@ -0,0 +1,83 @@
+package bufio
+
+import "core:io"
+
+// Loadahead_Reader provides io lookahead.
+// This is useful for tokenizers/parsers.
+// Loadahead_Reader is similar to bufio.Reader, but unlike bufio.Reader, Loadahead_Reader's buffer size
+// will EXACTLY match the specified size, whereas bufio.Reader's buffer size may differ from the specified size.
+// This makes sure that the buffer will not be accidentally read beyond the expected size.
+Loadahead_Reader :: struct {
+	r:   io.Reader,
+	buf: []byte,
+	n:   int,
+}
+
+lookahead_reader_init :: proc(lr: ^Loadahead_Reader, r: io.Reader, buf: []byte) -> ^Loadahead_Reader {
+	lr.r = r;
+	lr.buf = buf;
+	lr.n = 0;
+	return lr;
+}
+
+lookahead_reader_buffer :: proc(lr: ^Loadahead_Reader) -> []byte {
+	return lr.buf[:lr.n];
+}
+
+
+// lookahead_reader_peek returns a slice of the Lookahead_Reader which holds n bytes
+// If the Lookahead_Reader cannot hold enough bytes, it will read from the underlying reader to populate the rest.
+// NOTE: The returned buffer is not a copy of the underlying buffer
+lookahead_reader_peek :: proc(lr: ^Loadahead_Reader, n: int) -> ([]byte, io.Error) {
+	switch {
+	case n < 0:
+		return nil, .Negative_Read;
+	case n > len(lr.buf):
+		return nil, .Buffer_Full;
+	}
+
+	n := n;
+	err: io.Error;
+	read_count: int;
+
+	if lr.n < n {
+		read_count, err = io.read_at_least(lr.r, lr.buf[lr.n:], n-lr.n);
+		if err == .Unexpected_EOF {
+			err = .EOF;
+		}
+	}
+
+	lr.n += read_count;
+
+	if n > lr.n {
+		n = lr.n;
+	}
+	return lr.buf[:n], err;
+}
+
+// lookahead_reader_peek_all returns a slice of the Lookahead_Reader populating the full buffer
+// If the Lookahead_Reader cannot hold enough bytes, it will read from the underlying reader to populate the rest.
+// NOTE: The returned buffer is not a copy of the underlying buffer
+lookahead_reader_peek_all :: proc(lr: ^Loadahead_Reader) -> ([]byte, io.Error) {
+	return lookahead_reader_peek(lr, len(lr.buf));
+}
+
+
+// lookahead_reader_consume drops the first n populated bytes from the Lookahead_Reader.
+lookahead_reader_consume :: proc(lr: ^Loadahead_Reader, n: int) -> io.Error {
+	switch {
+	case n == 0:
+		return nil;
+	case n < 0:
+		return .Negative_Read;
+	case lr.n < n:
+		return .Short_Buffer;
+	}
+	copy(lr.buf, lr.buf[n:lr.n]);
+	lr.n -= n;
+	return nil;
+}
+
+lookahead_reader_consume_all :: proc(lr: ^Loadahead_Reader) -> io.Error {
+	return lookahead_reader_consume(lr, lr.n);
+}