perf: optimize EvaluatorBucketing (#88)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes the core bucketing hash computation used for
rollouts/experiments; any mismatch would alter variation assignments.
Added a regression test to confirm the new byte-based SHA-1 parsing
matches the previous hex/substring logic.
> 
> **Overview**
> Optimizes `computeBucketValue` to reduce allocations by building the
hash input with a single `StringBuilder` (including `append(int)` where
possible) and by computing the 15-hex-digit bucket value directly from
the raw SHA-1 bytes instead of `sha1Hex(...).substring(...)/parseLong`.
> 
> Updates bucketing attribute handling to append directly into the
builder (returning a boolean for bucketability), and adds a test
(`optimizedHashText`) to assert the optimized path produces the same
bucket value as the prior hex-based algorithm.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
cd1884c. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Todd Anderson <127344469+tanderson-ld@users.noreply.github.com>

Author: yuzawa-san

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/EvaluatorBucketing.java

@@ -47,30 +47,42 @@ static float computeBucketValue(
       }
     }
 
-    String idHash = getBucketableStringValue(contextValue);
-    if (idHash == null) {
+    StringBuilder keyBuilder = new StringBuilder();
+    if (seed != null) {
+      keyBuilder.append(seed.intValue());
+    } else {
+      keyBuilder.append(flagOrSegmentKey).append('.').append(salt);
+    }
+    keyBuilder.append('.');
+    if (!getBucketableStringValue(keyBuilder, contextValue)) {
       return 0;
     }
 
-    String prefix;
-    if (seed != null) {
-      prefix = seed.toString();
-    } else {
-      prefix = flagOrSegmentKey + "." + salt;
+    // turn the first 15 hex digits of this into a long
+    byte[] hash = DigestUtils.sha1(keyBuilder.toString());
+    long longVal = 0;
+    for (int i = 0; i < 7; i++) {
+      longVal <<= 8;
+      longVal |= (hash[i] & 0xff);
     }
-    String hash = DigestUtils.sha1Hex(prefix + "." + idHash).substring(0, 15);
-    long longVal = Long.parseLong(hash, 16);
+    longVal <<= 4;
+    longVal |= ((hash[7] >> 4) & 0xf);
     return (float) longVal / LONG_SCALE;
   }
 
-  private static String getBucketableStringValue(LDValue userValue) {
+  private static boolean getBucketableStringValue(StringBuilder keyBuilder, LDValue userValue) {
     switch (userValue.getType()) { 
     case STRING:
-      return userValue.stringValue();
+      keyBuilder.append(userValue.stringValue());
+      return true;
     case NUMBER:
-      return userValue.isInt() ? String.valueOf(userValue.intValue()) : null;
+      if (userValue.isInt()) {
+        keyBuilder.append(userValue.intValue());
+        return true;
+      }
+      return false;
     default:
-      return null;
+      return false;
     }
   }
 }

lib/sdk/server/src/test/java/com/launchdarkly/sdk/server/EvaluatorBucketingTest.java

@@ -8,6 +8,8 @@
 import com.launchdarkly.sdk.server.DataModel.RolloutKind;
 import com.launchdarkly.sdk.server.DataModel.WeightedVariation;
 
+import org.apache.commons.codec.digest.DigestUtils;
+
 import org.junit.Test;
 
 import java.util.Arrays;
@@ -132,6 +134,18 @@ public void cannotBucketByBooleanAttribute() {
     assertEquals(0f, result, Float.MIN_VALUE);
   }
 
+  @Test
+  public void optimizedHashText() {
+    LDContext context = LDContext.builder("key")
+        .set("stringattr", "33333")
+        .build();
+    float result = computeBucketValue(false, noSeed, context, null, "key", AttributeRef.fromLiteral("stringattr"), "salt");
+    String hash = DigestUtils.sha1Hex("key.salt.33333").substring(0, 15);
+    long longVal = Long.parseLong(hash, 16);
+    float expectedResult = longVal / (float) 0xFFFFFFFFFFFFFFFL;
+    assertEquals(expectedResult, result, Float.MIN_VALUE);
+  }
+
   private static void assertVariationIndexFromRollout(
       int expectedVariation,
       Rollout rollout,