lib::std::encoding: add varint::{read,write}

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

lib/std/io/stream.c3

@@ -383,3 +383,61 @@ macro usz! copy_through_buffer(Stream *self, Stream* dst, char[] buffer) @local
 		if (written != len) return IoError.INCOMPLETE_WRITE?;
 	}
 }
+
+const char[*] MAX_VARS @private = { [2] = 3, [4] = 5, [8] = 10 };
+
+/**
+ * @require $typeof(x_ptr).kindof == POINTER && $typeof(x_ptr).inner.kindof.is_int()
+ **/
+macro usz! Stream.read_varint(&self, x_ptr)
+{
+	var $Type = $typefrom($typeof(x_ptr).inner);
+	const MAX = MAX_VARS[$Type.sizeof];
+	$Type x;
+	uint shift;
+	usz n;
+	for (usz i = 0; i < MAX; i++)
+	{
+		char! c = self.read_byte();
+		if (catch err = c)
+		{
+			case IoError.EOF:
+				return IoError.UNEXPECTED_EOF?;
+			default:
+				return err?;
+		}
+		n++;
+		if (c & 0x80 == 0)
+		{
+			if (i + 1 == MAX && c > 1) break;
+			x |= c << shift;
+			$if $Type.kindof == SIGNED_INT:
+				x = x & 1 == 0 ? x >> 1 : ~(x >> 1);
+			$endif
+			*x_ptr = x;
+			return n;
+		}
+		x |= (c & 0x7F) << shift;
+		shift += 7;
+	}
+	return MathError.OVERFLOW?;
+}
+
+/**
+ * @require $typeof(x).kindof.is_int()
+ **/
+macro usz! Stream.write_varint(&self, x)
+{
+	var $Type = $typeof(x);
+	const MAX = MAX_VARS[$Type.sizeof];
+	char[MAX] buffer;
+	usz i;
+	while (x >= 0x80)
+	{
+		buffer[i] = (char)(x | 0x80);
+		x >>= 7;
+		i++;
+	}
+	buffer[i] = (char)x;
+	return self.write_all(buffer[:i + 1]);
+}

test/unit/stdlib/io/varint.c3

@@ -0,0 +1,54 @@
+module std::io::varint @test;
+import std::io;
+
+fn void! write_read()
+{
+	ByteBuffer buf;
+	buf.tinit(16)!;
+	usz n;
+	uint x;
+	uint y;
+
+	n = buf.write_varint(123)!;
+	assert(n == 1, "got %d; want 1", n);
+	buf.read_varint(&y)!;
+	assert(y == 123, "got %d; want 123", y);
+
+	n = buf.write_varint(123456789)!;
+	assert(n == 4, "got %d; want 4", n);
+	buf.read_varint(&y)!;
+	assert(y == 123456789, "got %d; want 123456789", y);
+}
+
+struct VarIntTest
+{
+	uint in;
+	char[] bytes;
+}
+
+fn void! samples()
+{
+	VarIntTest[] tcases = {
+		{ 0,          { 0x00 } },
+        { 100,        { 0x64 } },
+        { 127,        { 0x7F } },
+        { 128,        { 0x80, 0x01 } },
+        { 16271,      { 0x8F, 0x7F } },
+        { 16383,      { 0xFF, 0x7F } },
+        { 16384,      { 0x80, 0x80, 0x01 } },
+        { 1048576,    { 0x80, 0x80, 0x40 } },
+        { 2097151,    { 0xFF, 0xFF, 0x7F } },
+        { 2097152,    { 0x80, 0x80, 0x80, 0x01 } },
+        { 2147483648, { 0x80, 0x80, 0x80, 0x80, 0x08 } },
+        { 4294967295, { 0xFF, 0xFF, 0xFF, 0xFF, 0x0F } },
+	};
+	foreach (tc : tcases)
+	{
+		ByteWriter bw;
+		bw.tinit();
+		usz n = bw.write_varint(tc.in)!;
+		assert(n == tc.bytes.len, "got %d; want %d", n, tc.bytes.len);
+		char[] bytes = bw.bytes[:bw.index];
+		assert(bytes == tc.bytes, "got %d; want %d", bytes, tc.bytes);
+	}
+}