Improve ConcatSwitchSides mutator and fix diff printing

WIP.md

@@ -28,7 +28,7 @@
 
 # Backlog Prio 1
 - [ ] Fix PHPStorm indexing Issues. Maybe do not end cached files with .php?
-- [ ] Automatically empty cache when package version changes
+- [ ] Automatically empty cache when package version changes / Maybe there is another approach: Use the same cache key per php file, but store a hash of the file content and the package version in the cache. If the hash changes, the cache is invalid.
 - [ ] What should we do with interfaces? ignore them completely?
 - [ ] Finish: Disable mutations by annotation
 - [ ] Run mutations in a reasonable order: New, Survived, NotCovered, Skipped, Killed (Survived first, if --stop-on-survived or --bail; NotCovered first, if --stop-on-uncovered)

src/Mutation.php

@@ -12,6 +12,7 @@
 use PhpParser\PrettyPrinter\Standard;
 use SebastianBergmann\Diff\Differ;
 use SebastianBergmann\Diff\Output\UnifiedDiffOutputBuilder;
+use Symfony\Component\Console\Formatter\OutputFormatter;
 use Symfony\Component\Finder\SplFileInfo;
 
 class Mutation
@@ -89,11 +90,15 @@ private static function diff(string $originalSource, string $modifiedSource): st
         $diff = (new Differ(new UnifiedDiffOutputBuilder("\n--- Expected\n+++ Actual\n")))
             ->diff($originalSource, $modifiedSource);
 
+        if (! str_contains($diff, self::DIFF_SEPARATOR)) {
+            return '';
+        }
+
         $tmp = '';
         $lines = explode(PHP_EOL, explode(self::DIFF_SEPARATOR, $diff)[1]);
 
         foreach ($lines as $line) {
-            $tmp .= self::colorizeLine($line, str_starts_with($line, '-') ? 'red' : (str_starts_with($line, '+') ? 'green' : 'gray')).PHP_EOL;
+            $tmp .= self::colorizeLine(OutputFormatter::escape($line), str_starts_with($line, '-') ? 'red' : (str_starts_with($line, '+') ? 'green' : 'gray')).PHP_EOL;
         }
 
         $diff = str_replace(explode(self::DIFF_SEPARATOR, $diff)[1], $tmp, $diff);

src/Mutators/String/ConcatSwitchSides.php

@@ -7,6 +7,9 @@
 use Pest\Mutate\Mutators\Abstract\AbstractMutator;
 use PhpParser\Node;
 use PhpParser\Node\Expr\BinaryOp\Concat;
+use PhpParser\Node\Expr\ConstFetch;
+use PhpParser\Node\Expr\Variable;
+use PhpParser\Node\Scalar\String_;
 
 class ConcatSwitchSides extends AbstractMutator
 {
@@ -24,6 +27,33 @@ public static function nodesToHandle(): array
         return [Concat::class];
     }
 
+    public static function can(Node $node): bool
+    {
+        if (! $node instanceof Concat) {
+            return false;
+        }
+
+        if ($node->left->getType() !== $node->right->getType()) {
+            return true;
+        }
+
+        if ($node->left instanceof ConstFetch && $node->right instanceof ConstFetch) {
+            return $node->left->name->toString() !== $node->right->name->toString();
+        }
+
+        if ($node->left instanceof String_ && $node->right instanceof String_) {
+            return $node->left->value !== $node->right->value;
+        }
+        if (! $node->left instanceof Variable) {
+            return true;
+        }
+        if (! $node->right instanceof Variable) {
+            return true;
+        }
+
+        return $node->left->name !== $node->right->name;
+    }
+
     public static function mutate(Node $node): Node
     {
         /** @var Concat $node */

tests/Mutators/String/ConcatSwitchSidesTest.php

@@ -23,3 +23,27 @@
         return $a + $b;
         CODE);
 })->expectExceptionMessage('No mutation performed');
+
+it('does not mutate if both sides are equal strings', function (): void {
+    mutateCode(ConcatSwitchSides::class, <<<'CODE'
+        <?php
+
+        return 'a' . 'a';
+        CODE);
+})->expectExceptionMessage('No mutation performed');
+
+it('does not mutate if both sides are equal const fetches', function (): void {
+    mutateCode(ConcatSwitchSides::class, <<<'CODE'
+        <?php
+
+        return PHP_EOL . PHP_EOL;
+        CODE);
+})->expectExceptionMessage('No mutation performed');
+
+it('does not mutate if both sides are equal variables', function (): void {
+    mutateCode(ConcatSwitchSides::class, <<<'CODE'
+        <?php
+
+        return $a . $a;
+        CODE);
+})->expectExceptionMessage('No mutation performed');