Branchless buffer refills for vector decoding

This reduces the number instructions emitted by nearly half when we
check if there is enough data to blindly refill the lookahead. The best
part, we don't even need to use the unchecked API to blindly refill --
somehow the compiler can figured this out when we ensure there is at
least 16 bytes left unbuffered. Crazy.

Cargo.toml

@@ -16,7 +16,7 @@ serde = { version = "1", features = ["derive"] }
 encoding_rs = "0.8"
 phf = { version = "0.11", features = ["macros"] }
 fnv = "1.0"
-bitter = "0.7"
+bitter = "0.7.1"
 
 [dev-dependencies]
 serde_json = "1"

src/network/models.rs

@@ -29,6 +29,32 @@ pub struct Vector3i {
 
 impl Vector3i {
     pub fn decode(bits: &mut LittleEndianReader<'_>, net_version: i32) -> Option<Vector3i> {
+        // Do we have enough data available to blindly refill the lookahead twice?
+        // Note: this code doesn't actually use the unchecked bitter API as the
+        // compiler was able to emit the same code with both as long as we
+        // ensured there was 16 bytes left (even though a `Vector3i` will never
+        // need that many bytes).
+        if bits.unbuffered_bytes_remaining() >= 16 {
+            bits.refill_lookahead();
+            let size_bits = bits.peek_bits_max_computed(4, if net_version >= 7 { 22 } else { 20 });
+            let bias = 1 << (size_bits + 1);
+            let bit_limit = (size_bits + 2) as u32;
+            let dx = bits.peek_and_consume(bit_limit) as u32;
+            bits.refill_lookahead();
+            let dy = bits.peek_and_consume(bit_limit) as u32;
+            let dz = bits.peek_and_consume(bit_limit) as u32;
+            Some(Vector3i {
+                x: (dx as i32) - bias,
+                y: (dy as i32) - bias,
+                z: (dz as i32) - bias,
+            })
+        } else {
+            Vector3i::eof_decode(bits, net_version)
+        }
+    }
+
+    #[cold]
+    pub fn eof_decode(bits: &mut LittleEndianReader<'_>, net_version: i32) -> Option<Vector3i> {
         bits.refill_lookahead();
         if bits.lookahead_bits() < 5 {
             return None;