DivineMC/patches/server/0023-lithium-math.sine_lut.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: NONPLAYT <76615486+NONPLAYT@users.noreply.github.com>
Date: Sat, 25 May 2024 18:39:17 +0300
Subject: [PATCH] lithium: math.sine_lut


diff --git a/src/main/java/net/minecraft/util/Mth.java b/src/main/java/net/minecraft/util/Mth.java
index f298cdfcf1539e467f57f9f7789de3cf2ca54665..f83a544be66206ddd52f11524e84e821eb15343c 100644
--- a/src/main/java/net/minecraft/util/Mth.java
+++ b/src/main/java/net/minecraft/util/Mth.java
@@ -29,7 +29,7 @@ public class Mth {
     public static final Vector3f Y_AXIS = new Vector3f(0.0F, 1.0F, 0.0F);
     public static final Vector3f X_AXIS = new Vector3f(1.0F, 0.0F, 0.0F);
     public static final Vector3f Z_AXIS = new Vector3f(0.0F, 0.0F, 1.0F);
-    private static final float[] SIN = Util.make(new float[65536], sineTable -> {
+    public static final float[] SIN = Util.make(new float[65536], sineTable -> { // DivineMC - lithium: math.sine_lut
         for (int ix = 0; ix < sineTable.length; ix++) {
             sineTable[ix] = (float)Math.sin((double)ix * Math.PI * 2.0 / 65536.0);
         }
@@ -46,11 +46,11 @@ public class Mth {
     private static final double[] COS_TAB = new double[257];
 
     public static float sin(float value) {
-        return SIN[(int)(value * 10430.378F) & 65535];
+        return space.bxteam.divinemc.util.lithium.CompactSineLUT.sin(value); // DivineMC - lithium: math.sine_lut
     }
 
     public static float cos(float value) {
-        return SIN[(int)(value * 10430.378F + 16384.0F) & 65535];
+        return space.bxteam.divinemc.util.lithium.CompactSineLUT.cos(value); // DivineMC - lithium: math.sine_lut
     }
 
     public static float sqrt(float value) {
diff --git a/src/main/java/space/bxteam/divinemc/util/lithium/CompactSineLUT.java b/src/main/java/space/bxteam/divinemc/util/lithium/CompactSineLUT.java
new file mode 100644
index 0000000000000000000000000000000000000000..79a49c3e99ab069172f2fd85a942474f0c872fc9
--- /dev/null
+++ b/src/main/java/space/bxteam/divinemc/util/lithium/CompactSineLUT.java
@@ -0,0 +1,88 @@
+package space.bxteam.divinemc.util.lithium;
+
+import net.minecraft.util.Mth;
+
+/**
+ * A replacement for the sine angle lookup table used in {@link Mth}, both reducing the size of LUT and improving
+ * the access patterns for common paired sin/cos operations.
+ * <p>
+ *  sin(-x) = -sin(x)
+ *    ... to eliminate negative angles from the LUT.
+ * <p>
+ *  sin(x) = sin(pi/2 - x)
+ *    ... to eliminate supplementary angles from the LUT.
+ * <p>
+ * Using these identities allows us to reduce the LUT from 64K entries (256 KB) to just 16K entries (64 KB), enabling
+ * it to better fit into the CPU's caches at the expense of some cycles on the fast path. The implementation has been
+ * tightly optimized to avoid branching where possible and to use very quick integer operations.
+ * <p>
+ * Generally speaking, reducing the size of a lookup table is always a good optimization, but since we need to spend
+ * extra CPU cycles trying to maintain parity with vanilla, there is the potential risk that this implementation ends
+ * up being slower than vanilla when the lookup table is able to be kept in cache memory.
+ * <p>
+ * Unlike other "fast math" implementations, the values returned by this class are *bit-for-bit identical* with those
+ * from {@link Mth}. Validation is performed during runtime to ensure that the table is correct.
+ *
+ * @author coderbot16   Author of the original (and very clever) <a href="https://gitlab.com/coderbot16/i73/-/tree/master/i73-trig/src">implementation</a> in Rust
+ * @author jellysquid3  Additional optimizations, port to Java
+ */
+public class CompactSineLUT {
+    private static final int[] SIN_INT = new int[16384 + 1];
+    private static final float SIN_MIDPOINT;
+
+    static {
+        // Copy the sine table, covering to raw int bits
+        for (int i = 0; i < SIN_INT.length; i++) {
+            SIN_INT[i] = Float.floatToRawIntBits(Mth.SIN[i]);
+        }
+
+        SIN_MIDPOINT = Mth.SIN[Mth.SIN.length / 2];
+
+        // Test that the lookup table is correct during runtime
+        for (int i = 0; i < Mth.SIN.length; i++) {
+            float expected = Mth.SIN[i];
+            float value = lookup(i);
+
+            if (expected != value) {
+                throw new IllegalArgumentException(String.format("LUT error at index %d (expected: %s, found: %s)", i, expected, value));
+            }
+        }
+    }
+
+    // [VanillaCopy] Mth#sin(float)
+    public static float sin(float f) {
+        return lookup((int) (f * 10430.378f) & 0xFFFF);
+    }
+
+    // [VanillaCopy] Mth#cos(float)
+    public static float cos(float f) {
+        return lookup((int) (f * 10430.378f + 16384.0f) & 0xFFFF);
+    }
+
+    private static float lookup(int index) {
+        // A special case... Is there some way to eliminate this?
+        if (index == 32768) {
+            return SIN_MIDPOINT;
+        }
+
+        // Trigonometric identity: sin(-x) = -sin(x)
+        // Given a domain of 0 <= x <= 2*pi, just negate the value if x > pi.
+        // This allows the sin table size to be halved.
+        int neg = (index & 0x8000) << 16;
+
+        // All bits set if (pi/2 <= x), none set otherwise
+        // Extracts the 15th bit from 'half'
+        int mask = (index << 17) >> 31;
+
+        // Trigonometric identity: sin(x) = sin(pi/2 - x)
+        int pos = (0x8001 & mask) + (index ^ mask);
+
+        // Wrap the position in the table. Moving this down to immediately before the array access
+        // seems to help the Hotspot compiler optimize the bit math better.
+        pos &= 0x7fff;
+
+        // Fetch the corresponding value from the LUT and invert the sign bit as needed
+        // This directly manipulate the sign bit on the float bits to simplify logic
+        return Float.intBitsToFloat(SIN_INT[pos] ^ neg);
+    }
+}