fix: Optimize sha256ToLong with ThreadLocal (#419)

context:

https://statsig.unthread.io/dashboard/inbox/me/conversations/d24094ee-3afb-4643-9b6e-3df24500cf7b

Author: weihao-statsig

src/main/kotlin/com/statsig/sdk/Hashing.kt

@@ -14,6 +14,10 @@ enum class HashAlgo {
 
 class Hashing {
     companion object {
+        private val threadLocalDigest: ThreadLocal<MessageDigest> = ThreadLocal.withInitial {
+            MessageDigest.getInstance("SHA-256")
+        }
+
         fun djb2(input: String): String {
             var hash = 0
             for (element in input) {
@@ -36,7 +40,8 @@ class Hashing {
         }
 
         fun sha256ToLong(input: String): Long {
-            val md = MessageDigest.getInstance("SHA-256")
+            val md = threadLocalDigest.get()
+            md.reset()
             val hashBytes = md.digest(input.toByteArray(Charsets.UTF_8))
             // Combine more bytes to increase randomness
             val buffer = ByteBuffer.wrap(hashBytes)

src/test/java/com/statsig/sdk/ThreadLocalHashingTest.kt

@@ -0,0 +1,48 @@
+package com.statsig.sdk
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import java.nio.ByteBuffer
+import java.security.MessageDigest
+import java.util.concurrent.Executors
+import java.util.concurrent.TimeUnit
+import kotlin.math.absoluteValue
+
+class ThreadLocalHashingTest {
+
+    @Test
+    fun testSingleSha256ToLong() {
+        val input = "test"
+        val expected = generateSha256ToLongExpected(input)
+        val result = Hashing.sha256ToLong(input)
+        assertEquals(expected, result)
+    }
+
+    @Test
+    fun testConcurrentSha256ToLong() {
+        val executor = Executors.newFixedThreadPool(10)
+        val tasks = List(10) {
+            Runnable {
+                repeat(100) {
+                    val input = "test$it"
+                    val expected = generateSha256ToLongExpected(input)
+                    val result = Hashing.sha256ToLong(input)
+                    assertEquals(expected, result)
+                }
+            }
+        }
+        tasks.forEach { executor.submit(it) }
+        executor.shutdown()
+        executor.awaitTermination(1, TimeUnit.MINUTES)
+    }
+
+    private fun generateSha256ToLongExpected(input: String): Long {
+        val md = MessageDigest.getInstance("SHA-256")
+        val hashBytes = md.digest(input.toByteArray(Charsets.UTF_8))
+        val buffer = ByteBuffer.wrap(hashBytes)
+        val high = buffer.long
+        val low = buffer.long
+
+        return (high xor low).absoluteValue
+    }
+}