Merge pull request #21259 from dreis2211

* pr/21259:
  Further optimize StringSequence.startsWith
  Optimize StringSequence.startsWith

Closes gh-21259

Author: philwebb

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/StringSequence.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -98,23 +98,17 @@ int indexOf(String str, int fromIndex) {
 		return this.source.indexOf(str, this.start + fromIndex) - this.start;
 	}
 
-	boolean startsWith(CharSequence prefix) {
+	boolean startsWith(String prefix) {
 		return startsWith(prefix, 0);
 	}
 
-	boolean startsWith(CharSequence prefix, int offset) {
+	boolean startsWith(String prefix, int offset) {
 		int prefixLength = prefix.length();
-		if (length() - prefixLength - offset < 0) {
+		int length = length();
+		if (length - prefixLength - offset < 0) {
 			return false;
 		}
-		int prefixOffset = 0;
-		int sourceOffset = offset;
-		while (prefixLength-- != 0) {
-			if (charAt(sourceOffset++) != prefix.charAt(prefixOffset++)) {
-				return false;
-			}
-		}
-		return true;
+		return this.source.startsWith(prefix, this.start + offset);
 	}
 
 	@Override

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/StringSequenceTests.java

@@ -203,4 +203,18 @@ void startsWithOffsetWhenShorterAndDoesNotStartWith() {
 		assertThat(new StringSequence("xab").startsWith("c", 1)).isFalse();
 	}
 
+	@Test
+	void startsWithOnSubstringTailWhenMatch() {
+		StringSequence subSequence = new StringSequence("xabc").subSequence(1);
+		assertThat(subSequence.startsWith("abc")).isTrue();
+		assertThat(subSequence.startsWith("abcd")).isFalse();
+	}
+
+	@Test
+	void startsWithOnSubstringMiddleWhenMatch() {
+		StringSequence subSequence = new StringSequence("xabc").subSequence(1, 3);
+		assertThat(subSequence.startsWith("ab")).isTrue();
+		assertThat(subSequence.startsWith("abc")).isFalse();
+	}
+
 }