From 672e7c4b66468828d982aae72ee0f22fe3d2ca09 Mon Sep 17 00:00:00 2001
From: Anuraag Agrawal <anuraaga@gmail.com>
Date: Tue, 14 May 2019 16:42:49 +0900
Subject: [PATCH] Let Java copy arrays which it can sometimes do faster than
 manual loops. (#2591)

---
 .../src/main/java/zipkin2/SpanBenchmarks.java | 20 +++++++++++++++++++
 zipkin/src/main/java/zipkin2/Span.java        | 16 ++++++++++++---
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/benchmarks/src/main/java/zipkin2/SpanBenchmarks.java b/benchmarks/src/main/java/zipkin2/SpanBenchmarks.java
index a30a1d2333a..182c7aeb120 100644
--- a/benchmarks/src/main/java/zipkin2/SpanBenchmarks.java
+++ b/benchmarks/src/main/java/zipkin2/SpanBenchmarks.java
@@ -143,6 +143,26 @@ public byte[] deserialize_kryo() {
     return output.getBuffer();
   }
 
+  @Benchmark
+  public String padLeft_1Char() {
+    return Span.padLeft("1", 16);
+  }
+
+  @Benchmark
+  public String padLeft_15Chars() {
+    return Span.padLeft("123456789012345", 16);
+  }
+
+  @Benchmark
+  public String padLeft_17Chars() {
+    return Span.padLeft("12345678901234567", 32);
+  }
+
+  @Benchmark
+  public String padLeft_31Chars() {
+    return Span.padLeft("1234567890123456789012345678901", 32);
+  }
+
   // Convenience main entry-point
   public static void main(String[] args) throws RunnerException {
     Options opt = new OptionsBuilder()
diff --git a/zipkin/src/main/java/zipkin2/Span.java b/zipkin/src/main/java/zipkin2/Span.java
index 6ea685d4b1d..8d22bce5e9b 100644
--- a/zipkin/src/main/java/zipkin2/Span.java
+++ b/zipkin/src/main/java/zipkin2/Span.java
@@ -659,11 +659,21 @@ public static String normalizeTraceId(String traceId) {
     }
   }
 
+  static final String THIRTY_TWO_ZEROS;
+  static {
+    char[] zeros = new char[32];
+    Arrays.fill(zeros, '0');
+    THIRTY_TWO_ZEROS = new String(zeros);
+  }
+
   static String padLeft(String id, int desiredLength) {
+    int length = id.length();
+    int numZeros = desiredLength - length;
+
     char[] data = Platform.shortStringBuffer();
-    int i = 0, length = id.length(), offset = desiredLength - length;
-    for (; i < offset; i++) data[i] = '0';
-    for (int j = 0; j < length; j++) data[i++] = id.charAt(j);
+    THIRTY_TWO_ZEROS.getChars(0, numZeros, data, 0);
+    id.getChars(0, length, data, numZeros);
+
     return new String(data, 0, desiredLength);
   }