From c4ad74c9b2329b299f41b34820c7d55668c3c2b6 Mon Sep 17 00:00:00 2001
From: Ian Preston <ianopolous@protonmail.com>
Date: Wed, 31 Jan 2024 19:21:59 +0000
Subject: [PATCH] Use perfect hash for temperature lookup

---
 .../CalculateAverage_ianopolousfast.java      | 44 +++++++++++++------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/src/main/java/dev/morling/onebrc/CalculateAverage_ianopolousfast.java b/src/main/java/dev/morling/onebrc/CalculateAverage_ianopolousfast.java
index 92e2f6ecb8..c7426c15fd 100644
--- a/src/main/java/dev/morling/onebrc/CalculateAverage_ianopolousfast.java
+++ b/src/main/java/dev/morling/onebrc/CalculateAverage_ianopolousfast.java
@@ -23,6 +23,7 @@
 import java.lang.foreign.MemorySegment;
 import java.nio.ByteOrder;
 import java.nio.channels.*;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardOpenOption;
@@ -44,7 +45,7 @@ public class CalculateAverage_ianopolousfast {
 
     public static final int MAX_LINE_LENGTH = 107;
     public static final int MAX_STATIONS = 1 << 14;
-    private static final OfLong LONG_LAYOUT = JAVA_LONG_UNALIGNED.withOrder(ByteOrder.BIG_ENDIAN);
+    private static final OfLong LONG_LAYOUT_LE = JAVA_LONG_UNALIGNED.withOrder(ByteOrder.LITTLE_ENDIAN);
     private static final VectorSpecies<Byte> BYTE_SPECIES = ByteVector.SPECIES_PREFERRED.length() >= 16
             ? ByteVector.SPECIES_128
             : ByteVector.SPECIES_64;
@@ -131,18 +132,31 @@ public static short getMinus(long d) {
         return ((d & 0xff00000000000000L) ^ 0x2d00000000000000L) != 0 ? 0 : (short) -1;
     }
 
+    private static final long PERFECT_HASH_SEED = -1982870890352534081L;
+    static final short[] temperatureLookup = new short[5003];
+    static {
+        // use perfect hash from hundredwatt
+        for (int i = -999; i < 1000; i++) {
+            String s = Integer.toString(i);
+            String withDot = s.substring(0, s.length() - 1) + (Math.abs(i) < 10 ? "0" : "") + "." + s.charAt(s.length() - 1);
+
+            byte[] bytes = withDot.getBytes(StandardCharsets.UTF_8);
+            MemorySegment mem = MemorySegment.ofArray(Arrays.copyOfRange(bytes, 0, 8));
+            long d = mem.get(LONG_LAYOUT_LE, 0);
+            long hash = (d * PERFECT_HASH_SEED) & ~(1L << 63);
+            int index = (int) (hash % temperatureLookup.length);
+            if (temperatureLookup[index] != 0)
+                throw new IllegalStateException("Perfect hash is not perfect!");
+            temperatureLookup[index] = (short) i;
+        }
+    }
+
     public static void processTemperature(long lineSplit, int size, MemorySegment buffer, Stat station) {
-        long d = buffer.get(LONG_LAYOUT, lineSplit);
-        // negative is either 0 or -1
-        short negative = getMinus(d);
-        d = d << (negative * -8);
-        int dotIndex = size - 2 + negative;
-        d = (d >> 8) | 0x30000000_00000000L; // add a leading 0 digit
-        d = d >> 8 * (5 - dotIndex);
-        short temperature = (short) ((byte) d - '0' +
-                10 * (((byte) (d >> 16)) - '0') +
-                100 * (((byte) (d >> 24)) - '0'));
-        temperature = (short) ((temperature ^ negative) - negative); // negative treatment inspired by merkitty
+        long d = buffer.get(LONG_LAYOUT_LE, lineSplit);
+        d = d & (-1L >>> 8 * (8 - size));
+        long hash = (d * PERFECT_HASH_SEED) & ~(1L << 63);
+        int temperatureIndex = (int) (hash % temperatureLookup.length);
+        short temperature = temperatureLookup[temperatureIndex];
         station.add(temperature);
     }
 
@@ -204,6 +218,11 @@ public static Stat[] parseStats(long start1, long end2, MemorySegment buffer) {
             }
         }
 
+        innerLoop(start1, end2, buffer, stations);
+        return stations;
+    }
+
+    private static void innerLoop(long start1, long end2, MemorySegment buffer, Stat[] stations) {
         while (start1 < end2) {
             int lineSize1 = lineSize(start1, buffer);
             long start2 = start1 + lineSize1 + 1;
@@ -220,7 +239,6 @@ public static Stat[] parseStats(long start1, long end2, MemorySegment buffer) {
             else
                 start1 += lineSize1 + 1;
         }
-        return stations;
     }
 
     public static class Stat {