Further optimize StringSequence.startsWith

See gh-21259

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

@@ -108,17 +108,7 @@ final class StringSequence implements CharSequence {
 		if (length - prefixLength - offset < 0) {
 			return false;
 		}
-		if (length == this.source.length()) {
-			return this.source.startsWith(prefix, offset);
-		}
-		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 @@ class StringSequenceTests {
 		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();
+	}
+
 }