Include context in parse errors

Author: theodorejb

src/ParserAbstract.php

@@ -380,10 +380,21 @@ protected function getErrorMessage(int $symbol, int $state, int $line): string
     {
         $expectedString = '';
         if ($expected = $this->getExpectedTokens($state)) {
-            $expectedString = ': Expecting ' . implode(', ', $expected);
+            $expectedString = ' Expecting ' . implode(', ', $expected);
         }
 
-        return "Parse error on line {$line}{$expectedString}, got {$this->symbolToName[$symbol]}";
+        // get up to 22 chars of context starting 3 tokens before error
+        $startPos = max(0, $this->tokenPos - 3);
+        $context = '';
+        for ($pos = $startPos; $pos <= $this->tokenPos; $pos++) {
+            $context .= $this->tokens[$pos]->text;
+        }
+        if (strlen($context) > 22) {
+            $context = '...' . substr($context, -22);
+        }
+        $context = str_replace(["\r\n", "\n"], '', $context);
+
+        return "Parse error on line {$line}: {$context}{$expectedString}, got {$this->symbolToName[$symbol]}";
     }
 
     private function getNodeError(string $message, Node $node): string

tests/ParserTest.php

@@ -31,6 +31,18 @@ public function testParse(): void
         $this->assertSame(file_get_contents('tests/test1.json'), $actual);
     }
 
+    public function testException(): void
+    {
+        try {
+            $parser = (new ParserFactory())->create();
+            $parser->parse("some\nlong\ncontext\n{{{foo}}");
+            $this->fail('Failed to throw exception for invalid template');
+        } catch (\Exception $e) {
+            $expected = 'Parse error on line 4: ...longcontext{{{foo}} Expecting CLOSE_UNESCAPED, got CLOSE';
+            $this->assertSame($expected, $e->getMessage());
+        }
+    }
+
     /**
      * @return \Generator<array{0: SpecArr}>
      */