From 40a0c8acfbfb88be791c295a14014468e2fd4298 Mon Sep 17 00:00:00 2001
From: mat <github@matdoes.dev>
Date: Sat, 11 Mar 2023 16:38:13 -0600
Subject: slightly optimize azalea_nbt::Tag::id

---
 azalea-nbt/src/tag.rs | 47 +++++++++++++++++++----------------------------
 1 file changed, 19 insertions(+), 28 deletions(-)

(limited to 'azalea-nbt/src')

diff --git a/azalea-nbt/src/tag.rs b/azalea-nbt/src/tag.rs
index 23bed1d4..87a7374d 100755
--- a/azalea-nbt/src/tag.rs
+++ b/azalea-nbt/src/tag.rs
@@ -5,43 +5,34 @@ use serde::{Deserialize, Serialize};
 
 /// An NBT value.
 #[derive(Clone, Debug, PartialEq, Default)]
+#[repr(u8)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(untagged))]
 pub enum Tag {
     #[default]
-    End, // 0
-    Byte(i8),                        // 1
-    Short(i16),                      // 2
-    Int(i32),                        // 3
-    Long(i64),                       // 4
-    Float(f32),                      // 5
-    Double(f64),                     // 6
-    ByteArray(Vec<u8>),              // 7
-    String(String),                  // 8
-    List(Vec<Tag>),                  // 9
-    Compound(AHashMap<String, Tag>), // 10
-    IntArray(Vec<i32>),              // 11
-    LongArray(Vec<i64>),             // 12
+    End = 0,
+    Byte(i8) = 1,
+    Short(i16) = 2,
+    Int(i32) = 3,
+    Long(i64) = 4,
+    Float(f32) = 5,
+    Double(f64) = 6,
+    ByteArray(Vec<u8>) = 7,
+    String(String) = 8,
+    List(Vec<Tag>) = 9,
+    Compound(AHashMap<String, Tag>) = 10,
+    IntArray(Vec<i32>) = 11,
+    LongArray(Vec<i64>) = 12,
 }
 
 impl Tag {
     /// Get the numerical ID of the tag type.
     #[inline]
     pub fn id(&self) -> u8 {
-        match self {
-            Tag::End => 0,
-            Tag::Byte(_) => 1,
-            Tag::Short(_) => 2,
-            Tag::Int(_) => 3,
-            Tag::Long(_) => 4,
-            Tag::Float(_) => 5,
-            Tag::Double(_) => 6,
-            Tag::ByteArray(_) => 7,
-            Tag::String(_) => 8,
-            Tag::List(_) => 9,
-            Tag::Compound(_) => 10,
-            Tag::IntArray(_) => 11,
-            Tag::LongArray(_) => 12,
-        }
+        // SAFETY: Because `Self` is marked `repr(u8)`, its layout is a `repr(C)`
+        // `union` between `repr(C)` structs, each of which has the `u8`
+        // discriminant as its first field, so we can read the discriminant
+        // without offsetting the pointer.
+        unsafe { *<*const _>::from(self).cast::<u8>() }
     }
 
     /// If the type is a byte, return the [`i8`].
-- 
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