Fix FqNameUnsafe.startsWith() when first segment of fqName is shorter than given segment

FqNameUnsafe("c.C").startsWith("cnames")
must return false
It's a refix of fdf826208f

compiler/tests/org/jetbrains/kotlin/name/FqNameUnsafeTest.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.name;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class FqNameUnsafeTest {
+    @Test
+    public void pathSegments() {
+        Assert.assertEquals(new ArrayList<Name>(), new FqNameUnsafe("").pathSegments());
+
+        for (String name : new String[] { "org", "org.jetbrains", "org.jetbrains.kotlin" }) {
+            List<Name> segments = new FqNameUnsafe(name).pathSegments();
+            List<String> segmentsStrings = new ArrayList<>();
+            for (Name segment : segments) {
+                segmentsStrings.add(segment.asString());
+            }
+            Assert.assertEquals(Arrays.asList(name.split("\\.")), segmentsStrings);
+        }
+    }
+
+    @Test
+    public void startsWithName() {
+        Assert.assertTrue(new FqNameUnsafe("abc.def").startsWith(Name.identifier("abc")));
+        Assert.assertTrue(new FqNameUnsafe("abc").startsWith(Name.identifier("abc")));
+        Assert.assertTrue(new FqNameUnsafe("abc.").startsWith(Name.identifier("abc")));
+        Assert.assertTrue(new FqNameUnsafe(".abc").startsWith(Name.identifier("")));
+
+        Assert.assertFalse(new FqNameUnsafe("").startsWith(Name.identifier("")));
+        Assert.assertFalse(new FqNameUnsafe("").startsWith(Name.identifier("id")));
+
+        Assert.assertFalse(new FqNameUnsafe("segment").startsWith(Name.identifier("")));
+        Assert.assertFalse(new FqNameUnsafe("abc.").startsWith(Name.identifier("abc.")));
+        Assert.assertFalse(new FqNameUnsafe(".abc").startsWith(Name.identifier("abc")));
+        Assert.assertFalse(new FqNameUnsafe(".abc").startsWith(Name.identifier(".")));
+        Assert.assertFalse(new FqNameUnsafe(".abc").startsWith(Name.identifier(".abc")));
+        Assert.assertFalse(new FqNameUnsafe("abc.def").startsWith(Name.identifier("abc.def")));
+        Assert.assertFalse(new FqNameUnsafe("abcdef").startsWith(Name.identifier("abc")));
+        Assert.assertFalse(new FqNameUnsafe("abc").startsWith(Name.identifier("abcdef")));
+        Assert.assertFalse(new FqNameUnsafe("abc.xyz").startsWith(Name.identifier("abcdef")));
+    }
+}

core/compiler.common/src/org/jetbrains/kotlin/name/FqNameUnsafe.java

@@ -163,8 +163,10 @@ public final class FqNameUnsafe {
             return false;
 
         int firstDot = fqName.indexOf('.');
+        int fqNameFirstSegmentLength = firstDot == -1 ? fqName.length() : firstDot;
         String segmentAsString = segment.asString();
-        return fqName.regionMatches(0, segmentAsString, 0, firstDot == -1 ? Math.max(fqName.length(), segmentAsString.length()) : firstDot);
+        return fqNameFirstSegmentLength == segmentAsString.length() &&
+                fqName.regionMatches(0, segmentAsString, 0, fqNameFirstSegmentLength);
     }
 
     public boolean startsWith(@NotNull FqNameUnsafe other) {