hundredwatt - 2nd submission (#338)

* check full hash before comparing

* implement merykitty suggestions to simplify temperature masking; required refactoring to little-endian

* standalone script for offline Perfect Hash seed searching

* stop using an oversized hash table

---------

Co-authored-by: Jason Nochlin <hundredwatt@users.noreply.github.com>

1 files changed, 25 insertions, 46 deletions

diff --git a/src/main/java/dev/morling/onebrc/CalculateAverage_hundredwatt.java b/src/main/java/dev/morling/onebrc/CalculateAverage_hundredwatt.java
index 051de9c..9d935ff 100644
--- a/src/main/java/dev/morling/onebrc/CalculateAverage_hundredwatt.java
+++ b/src/main/java/dev/morling/onebrc/CalculateAverage_hundredwatt.java
@@ -17,6 +17,7 @@ package dev.morling.onebrc;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
 import java.nio.MappedByteBuffer;
 import java.nio.channels.FileChannel;
 import java.nio.file.Files;
@@ -36,7 +37,7 @@ public class CalculateAverage_hundredwatt {
     private static final long FILE_CHUNK_SIZE = CHUNK_SIZE - MAX_ROW_SIZE;
     public static final int TEMPERATURE_SLOTS = 5003; // prime number
     private static final short[] TEMPERATURES = new short[TEMPERATURE_SLOTS];
-    private static final long PERFECT_HASH_SEED = -5353381455852817461L;
+    private static final long PERFECT_HASH_SEED = -1982870890352534081L;
 
     // Construct a perfect hash function mapping temperatures encoded as longs (e.g., 0x2d342e3000000000 for -4.3) to
     // the corresponding short integer (e.g., -43).
@@ -46,17 +47,17 @@ public class CalculateAverage_hundredwatt {
         Map<Long, Short> decodeTemperatureMap = new HashMap<>();
         for (short i = -999; i <= 999; i++) {
             long word = 0;
-            int shift = 56;
+            int shift = 0;
             if (i < 0) {
                 word |= ((long) '-') << shift;
-                shift -= 8;
+                shift += 8;
             }
             if (Math.abs(i) >= 100) {
                 int hh = Math.abs(i) / 100;
                 int tt = (Math.abs(i) - hh * 100) / 10;
 
                 word |= ((long) (hh + '0')) << shift;
-                shift -= 8;
+                shift += 8;
                 word |= ((long) (tt + '0')) << shift;
             }
             else {
@@ -64,9 +65,9 @@ public class CalculateAverage_hundredwatt {
                 // convert to ascii
                 word |= ((long) (tt + '0')) << shift;
             }
-            shift -= 8;
+            shift += 8;
             word |= ((long) '.') << shift;
-            shift -= 8;
+            shift += 8;
             int uu = Math.abs(i) % 10;
             word |= ((long) (uu + '0')) << shift;
 
@@ -74,31 +75,6 @@ public class CalculateAverage_hundredwatt {
             decodeTemperatureMap.put(word, i);
         }
 
-        // Brute force to find seed:
-        // Random rand = new Random(System.nanoTime());
-        // int max = 0;
-        // int attempts = 0;
-        // while (true) {
-        // BitSet bs = new BitSet(5003);
-        // var seed = rand.nextLong();
-        // seed |= 0b1; // make sure it's odd
-        // for (var word : decodeTemperatureMap.keySet()) {
-        // var h = (word * seed) & ~(1L << 63);
-        // var pos = (int) (h % 5003);
-        // bs.set(pos);
-        // }
-        // ;
-        // var c = bs.cardinality();
-        // if (c == decodeTemperatureMap.size()) {
-        // System.out.println("seed: " + seed + " cardinality: " + c + " max cardinality: " + max);
-        // break;
-        // }
-        // max = Math.max(max, c);
-        // if (attempts % 100_000 == 0)
-        // System.out.println("seed: " + seed + " cardinality: " + c + " max cardinality: " + max);
-        // attempts++;
-        // }
-
         decodeTemperatureMap.entrySet().stream().forEach(e -> {
             var word = e.getKey();
             var h = (word * PERFECT_HASH_SEED) & ~(1L << 63);
@@ -144,11 +120,12 @@ public class CalculateAverage_hundredwatt {
     }
 
     static class HashTable {
-        private static final int INITIAL_SIZE = 128 * 1024;
+        private static final int INITIAL_SIZE = 16 * 1024;
         private static final float LOAD_FACTOR = 0.75f;
         private static final int GROW_FACTOR = 4;
         private final long[][] KEYS = new long[INITIAL_SIZE][];
         private final Record[] VALUES = new Record[INITIAL_SIZE];
+        private final long[] HASHES = new long[INITIAL_SIZE];
         private int size = INITIAL_SIZE;
 
         public HashTable() {
@@ -162,13 +139,14 @@ public class CalculateAverage_hundredwatt {
 
             // linear probing
             int i = 0;
-            while (KEYS[idx] != null && (0 != Arrays.compareUnsigned(KEYS[idx], 0, KEYS[idx].length, key, 0, length))) {
+            while (KEYS[idx] != null && (HASHES[idx] != hash) && (0 != Arrays.compareUnsigned(KEYS[idx], 0, KEYS[idx].length, key, 0, length))) {
                 i++;
                 idx = (idx + 1) & (size - 1);
             }
 
             if (KEYS[idx] == null) {
                 KEYS[idx] = Arrays.copyOf(key, length);
+                HASHES[idx] = hash;
             }
 
             VALUES[idx].updateWith(value);
@@ -186,7 +164,7 @@ public class CalculateAverage_hundredwatt {
     }
 
     private static String keyToString(long[] key) {
-        ByteBuffer kb = ByteBuffer.allocate(8 * key.length);
+        ByteBuffer kb = ByteBuffer.allocate(8 * key.length).order(ByteOrder.LITTLE_ENDIAN);
         Arrays.stream(key).forEach(kb::putLong);
 
         // remove trailing '\0' bytes from kb and
@@ -194,7 +172,7 @@ public class CalculateAverage_hundredwatt {
         byte b;
         int limit = kb.position() - 8;
         kb.position(limit);
-        while ((b = kb.get()) != 0 && b != ';') {
+        while ((b = kb.get()) != 0 && b != ';' && limit < kb.capacity() - 1) {
             limit++;
         }
 
@@ -202,7 +180,7 @@ public class CalculateAverage_hundredwatt {
         byte[] bytes = new byte[limit];
         kb.get(bytes);
 
-        return new String(bytes).replace("\0", "");
+        return new String(bytes);
     }
 
     private static Record merge(Record v, Record value) {
@@ -215,6 +193,7 @@ public class CalculateAverage_hundredwatt {
     }
 
     private static int processChunk(ByteBuffer bb, HashTable hashTable, long start, long size) {
+        bb.order(ByteOrder.LITTLE_ENDIAN);
         // Find first entry
         while (start != 0 && bb.get() != '\n') {
         }
@@ -228,6 +207,7 @@ public class CalculateAverage_hundredwatt {
         long temperature_hash;
         int temperature_pos;
         short temperature_value;
+        int hashInt;
 
         int i = 0;
         int end = (int) (size - MAX_ROW_SIZE);
@@ -258,29 +238,28 @@ public class CalculateAverage_hundredwatt {
                 hasvalue = (op1 & op2 & 0x8080808080808080L);
             }
             hash ^= key[offset]; // unset last word since it will be updated
-            key[offset] = key[offset] & (-hasvalue); // ';' == 0x3b and -hasvalue is something like 0xff8000..., we can ignore the 0x80 byte since 0x3b & 0x80 == 0
+            key[offset] = key[offset] & ~(-(hasvalue >> 7));
             hash ^= key[offset];
 
-            position = position + offset * 8 + (Long.numberOfLeadingZeros(hasvalue)) / 8 + 1; // +1 for \n
+            position = position + offset * 8 + Long.numberOfTrailingZeros(hasvalue) / 8 + 1; // +1 for \n
 
             // Parse temperature
             word = bb.getLong(position);
-            arg = (word) ^ 0x0101010101010101L * ('\n');
-            op1 = (arg - 0x0101010101010101L);
-            op2 = ~(arg);
-            hasvalue = (op1 & op2 & 0x8080808080808080L);
-            word = word & ((-hasvalue));
+            hasvalue = (word - 0x0B0B0B0B0B0B0B0BL) & 0x8080808080808080L;
+            int newlinePos = Long.numberOfTrailingZeros(hasvalue) - 8;
+
+            word = word & (~(-(1L << newlinePos)));
 
             // Perfect hash lookup for temperature
             temperature_hash = (word * PERFECT_HASH_SEED) & ~(1L << 63);
             temperature_pos = (int) (temperature_hash % TEMPERATURE_SLOTS);
             temperature_value = TEMPERATURES[temperature_pos];
 
-            position = position + (Long.numberOfLeadingZeros(hasvalue)) / 8 + 1; // +1 for \n
+            position = position + newlinePos / 8 + 2; // +1 for \n
 
-            int hash2 = (int) (hash ^ (hash >> 32));
+            hashInt = (int) (hash ^ (hash >> 32));
 
-            hashTable.putOrMerge(hash2, offset + 1, key, temperature_value);
+            hashTable.putOrMerge(hashInt, offset + 1, key, temperature_value);
         }
         return i;
     }