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

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

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);
+	}
+}