add more efficient bit reader / writer

pkg/bits/write.go

@@ -0,0 +1,26 @@
+package bits
+
+// WriteBits writes N bits.
+func WriteBits(buf []byte, pos *int, bits uint64, n int) {
+	res := 8 - (*pos & 0x07)
+	if n < res {
+		buf[*pos>>0x03] |= byte(bits << (res - n))
+		*pos += n
+		return
+	}
+
+	buf[*pos>>3] |= byte(bits >> (n - res))
+	*pos += res
+	n -= res
+
+	for n >= 8 {
+		buf[*pos>>3] = byte(bits >> (n - 8))
+		*pos += 8
+		n -= 8
+	}
+
+	if n > 0 {
+		buf[*pos>>3] = byte((bits & (1<<n - 1)) << (8 - n))
+		*pos += n
+	}
+}