Add errorSnippet method

Author: antonmedv

src/Maml.php

@@ -12,6 +12,7 @@
 use Maml\Ast\IntegerNode;
 use Maml\Ast\NullNode;
 use Maml\Ast\ObjectNode;
+use Maml\Ast\Position;
 use Maml\Ast\RawStringNode;
 use Maml\Ast\StringNode;
 
@@ -69,4 +70,18 @@ public static function toValue(
         }
         return $result;
     }
+
+    public static function errorSnippet(string $source, Position $pos, string $message): string
+    {
+        $offset = $pos->offset;
+        $pre = \substr($source, \max(0, $offset - 40), \min($offset, 40));
+        $lines = \explode("\n", $pre);
+        $lastLine = \end($lines) ?: '';
+        $postParts = \explode("\n", \substr($source, $offset, 40), 2);
+        $postfix = $postParts[0];
+
+        $snippet = "    {$lastLine}{$postfix}\n";
+        $pointer = '    ' . \str_repeat('.', \max(0, \strlen($lastLine) - 1)) . "^\n";
+        return "{$message} on line {$pos->line}.\n\n{$snippet}{$pointer}";
+    }
 }

tests/MamlTest.php

@@ -4,6 +4,7 @@
 
 namespace Maml\Tests;
 
+use Maml\Ast\ObjectNode;
 use Maml\Maml;
 use PHPUnit\Framework\TestCase;
 
@@ -20,4 +21,44 @@ public function testParseAndStringify(): void
         $reparsed = Maml::parse($stringified);
         $this->assertSame($parsed, $reparsed);
     }
+
+    public function testErrorSnippetPointsAtPosition(): void
+    {
+        $source = "{\n  name: \"test\"\n  timeout: -1\n}";
+        $doc = Maml::parseAst($source);
+        $this->assertInstanceOf(ObjectNode::class, $doc->value);
+        $node = $doc->value->properties[1]->value;
+
+        $result = Maml::errorSnippet($source, $node->span->start, 'Invalid value');
+        $this->assertStringContainsString('Invalid value on line 3.', $result);
+        $this->assertStringContainsString('timeout: -1', $result);
+        $this->assertStringContainsString('^', $result);
+    }
+
+    public function testErrorSnippetAtStartOfSource(): void
+    {
+        $source = 'null';
+        $doc = Maml::parseAst($source);
+
+        $result = Maml::errorSnippet($source, $doc->value->span->start, 'Expected object');
+        $this->assertStringContainsString('Expected object on line 1.', $result);
+        $this->assertStringContainsString('null', $result);
+        $this->assertStringContainsString('^', $result);
+    }
+
+    public function testErrorSnippetOnNestedNode(): void
+    {
+        $source = "{\n  items: [\n    {name: \"x\", count: 0}\n  ]\n}";
+        $doc = Maml::parseAst($source);
+        $this->assertInstanceOf(ObjectNode::class, $doc->value);
+        $items = $doc->value->properties[0]->value;
+        $this->assertInstanceOf(\Maml\Ast\ArrayNode::class, $items);
+        $inner = $items->elements[0]->value;
+        $this->assertInstanceOf(ObjectNode::class, $inner);
+        $countNode = $inner->properties[1]->value;
+
+        $result = Maml::errorSnippet($source, $countNode->span->start, 'Count must be positive');
+        $this->assertStringContainsString('Count must be positive on line 3.', $result);
+        $this->assertStringContainsString('^', $result);
+    }
 }