fix(lexer): Handle incomplete tokens

Fixes #135

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 - Fixes a potential crash when using the `{% for %}` template tag with the
   incorrect amount of arguments.
+- Fixes a potential crash when using incomplete tokens in a template for
+  example, `{%%}` or `{{}}`.
 
 
 ## 0.9.0

Sources/Lexer.swift

@@ -7,6 +7,7 @@ struct Lexer {
 
   func createToken(string: String) -> Token {
     func strip() -> String {
+      guard string.characters.count > 4 else { return "" }
       let start = string.index(string.startIndex, offsetBy: 2)
       let end = string.index(string.endIndex, offsetBy: -2)
       return string[start..<end].trim(character: " ")

Tests/StencilTests/LexerSpec.swift

@@ -54,5 +54,15 @@ func testLexer() {
       try expect(tokens[0]) == Token.variable(value: "thing")
       try expect(tokens[1]) == Token.variable(value: "name")
     }
+
+    $0.it("can tokenize an unclosed block") {
+      let lexer = Lexer(templateString: "{%}")
+      let _ = lexer.tokenize()
+    }
+
+    $0.it("can tokenize an empty variable") {
+      let lexer = Lexer(templateString: "{{}}")
+      let _ = lexer.tokenize()
+    }
   }
 }