io: fix not reading the last byte of a stream with `.read(mut buf)`

vlib/io/buffered_reader.v

@@ -22,7 +22,9 @@ pub struct BufferedReaderConfig {
 
 // new_buffered_reader creates new BufferedReader
 pub fn new_buffered_reader(o BufferedReaderConfig) &BufferedReader {
-	assert o.cap >= 2
+	if o.cap <= 0 {
+		panic('new_buffered_reader should be called with a positive `cap`')
+	}
 	// create
 	r := &BufferedReader{
 		reader: o.reader
@@ -35,6 +37,9 @@ pub fn new_buffered_reader(o BufferedReaderConfig) &BufferedReader {
 
 // read fufills the Reader interface
 pub fn (mut r BufferedReader) read(mut buf []byte) ?int {
+	if r.end_of_stream {
+		return none
+	}
 	// read data out of the buffer if we dont have any
 	if r.needs_fill() {
 		if !r.fill_buffer() {
@@ -43,6 +48,9 @@ pub fn (mut r BufferedReader) read(mut buf []byte) ?int {
 		}
 	}
 	read := copy(buf, r.buf[r.offset..r.len])
+	if read == 0 {
+		return none
+	}
 	r.offset += read
 	return read
 }
@@ -79,7 +87,7 @@ fn (mut r BufferedReader) fill_buffer() bool {
 
 // needs_fill returns whether the buffer needs refilling
 fn (r BufferedReader) needs_fill() bool {
-	return r.offset >= r.len - 1
+	return r.offset >= r.len
 }
 
 // end_of_stream returns whether the end of the stream was reached

vlib/io/custom_string_reading_test.v

@@ -0,0 +1,57 @@
+import io
+
+struct StringReader {
+	text string
+mut:
+	place int
+}
+
+fn imin(a int, b int) int {
+	return if a < b { a } else { b }
+}
+
+fn (mut s StringReader) read(mut buf []byte) ?int {
+	$if debug {
+		eprintln('>>>> StringReader.read output buf.len: $buf.len')
+	}
+	if s.place > s.text.len + 1 {
+		return none
+	}
+	mut howmany := imin(buf.len, s.text.len - s.place)
+	xxx := s.text[s.place..s.place + howmany].bytes()
+	read := copy(buf, xxx)
+	s.place += read
+	return read
+}
+
+fn read_from_string(text string, capacity int) []byte {
+	mut str := StringReader{
+		text: text
+	}
+	mut stream := io.new_buffered_reader(reader: io.make_reader(str), cap: capacity)
+	//
+	mut buf := []byte{len: 1}
+	mut res := []byte{}
+	mut i := 0
+	for {
+		z := stream.read(mut buf) or { break }
+		res << buf
+		$if debug {
+			println('capacity: $capacity, i: $i, buf: $buf | z: $z')
+		}
+		i++
+	}
+	return res
+}
+
+pub fn test_reading_from_a_string() {
+	for capacity in 1 .. 1000 {
+		assert read_from_string('a', capacity) == [byte(`a`)]
+		assert read_from_string('ab', capacity) == [byte(`a`), `b`]
+		assert read_from_string('abc', capacity) == [byte(`a`), `b`, `c`]
+		assert read_from_string('abcde', capacity) == [byte(`a`), `b`, `c`, `d`, `e`]
+		large_string_bytes := []byte{len: 1000, init: `x`}
+		large_string := large_string_bytes.bytestr()
+		assert read_from_string(large_string, capacity) == large_string_bytes
+	}
+}