Fail on duplicate keys in a mapping

CHANGELOG.md

@@ -11,7 +11,9 @@
 
 ##### Bug Fixes
 
-* None.
+* Yams will now correctly error when it tries to decode a mapping with duplicate keys.  
+  [Tejas Sharma](https://github.com/tejassharma96)
+  [#415](https://github.com/jpsim/Yams/issues/415)
 
 ## 5.1.3
 

Sources/Yams/Parser.swift

@@ -355,13 +355,31 @@ private extension Parser {
             pairs.append((key, value))
             event = try parse()
         }
+        let keys = pairs.map { $0.0 }
+        try checkDuplicates(mappingKeys: keys)
         let node = Node.mapping(.init(pairs, tag(firstEvent.mappingTag), event.mappingStyle, firstEvent.startMark))
         if let anchor = firstEvent.mappingAnchor {
             anchors[anchor] = node
         }
         return node
     }
 
+    private func checkDuplicates(mappingKeys: [Node]) throws {
+        var duplicates: [String: [Node]] = [:]
+        for key in mappingKeys {
+            guard let keyString = key.string else { continue }
+            if duplicates.keys.contains(keyString) {
+                duplicates[keyString]?.append(key)
+            } else {
+                duplicates[keyString] = [key]
+            }
+        }
+        duplicates = duplicates.filter { $1.count > 1 }
+        guard duplicates.isEmpty else {
+            throw YamlError.duplicatedKeysInMapping(duplicates: duplicates, yaml: yaml)
+        }
+    }
+
     func tag(_ string: String?) -> Tag {
         let tagName = string.map(Tag.Name.init(rawValue:)) ?? .implicit
         return Tag(tagName, resolver, constructor)

Sources/Yams/YamlError.swift

@@ -75,6 +75,12 @@ public enum YamlError: Error {
     /// - parameter encoding: The string encoding used to decode the string data.
     case dataCouldNotBeDecoded(encoding: String.Encoding)
 
+    /// Multiple uses of the same key detected in a mapping
+    ///
+    /// - parameter duplicates: A dictionary keyed by the duplicated node value, with all nodes that duplicate the value
+    /// - parameter yaml:       YAML String which the problem occured while reading. 
+    case duplicatedKeysInMapping(duplicates: [String: [Node]], yaml: String)
+
     /// The error context.
     public struct Context: CustomStringConvertible {
         /// Context text.
@@ -175,6 +181,20 @@ extension YamlError: CustomStringConvertible {
             return problem
         case .dataCouldNotBeDecoded(encoding: let encoding):
             return "String could not be decoded from data using '\(encoding)' encoding"
+        case let .duplicatedKeysInMapping(duplicates, yaml):
+            return duplicates.duplicatedKeyErrorDescription(yaml: yaml)
+        }
+    }
+}
+
+private extension Dictionary where Key == String, Value == [Node] {
+    func duplicatedKeyErrorDescription(yaml: String) -> String {
+        var error = "error: parser: expected all keys to be unique but found the following duplicated key(s):"
+        for key in self.keys.sorted() {
+            let duplicatedNodes = self[key]!
+            let marks = duplicatedNodes.compactMap { $0.mark }
+            error += "\n\(key) (\(marks)):\n\(marks.map { $0.snippet(from: yaml) }.joined(separator: "\n"))"
         }
+        return error
     }
 }

Tests/YamsTests/YamlErrorTests.swift

@@ -149,6 +149,52 @@ class YamlErrorTests: XCTestCase {
             )
         }
     }
+
+    func testDuplicateKeysCannotBeParsed() throws {
+        let yamlString = """
+            a: value
+            a: different_value
+        """
+        XCTAssertThrowsError(try Parser(yaml: yamlString).singleRoot()) { error in
+            XCTAssertTrue(error is YamlError)
+            XCTAssertEqual("\(error)", """
+                error: parser: expected all keys to be unique but found the following duplicated key(s):
+                a ([1:5, 2:5]):
+                    a: value
+                    ^
+                    a: different_value
+                    ^
+                """)
+        }
+    }
+
+    func testDuplicatedKeysCannotBeParsed_MultipleDuplicates() throws {
+        let yamlString = """
+            a: value
+            a: different_value
+            b: value
+            b: different_value
+            b: different_different_value
+        """
+        XCTAssertThrowsError(try Parser(yaml: yamlString).singleRoot()) { error in
+            XCTAssertTrue(error is YamlError)
+            XCTAssertEqual("\(error)", """
+                error: parser: expected all keys to be unique but found the following duplicated key(s):
+                a ([1:5, 2:5]):
+                    a: value
+                    ^
+                    a: different_value
+                    ^
+                b ([3:5, 4:5, 5:5]):
+                    b: value
+                    ^
+                    b: different_value
+                    ^
+                    b: different_different_value
+                    ^
+                """)
+        }
+    }
 }
 
 extension YamlErrorTests {