Supported OpenApi 3.1.0 nullable changes. (#263)

* Supported OpenApi 3.1.0 nullable changes.
Fabrikt will now detect of the newer 3.1.0 specification is declared and automatically downgrade any of the json schema nullable types to the previous format.
EG, this:
```
    NewNullableFormat:
      type: object
      required:
        - version
      properties:
        version:
          type:
            - string
            - 'null'
```
Will get converted to this:
```
    NewNullableFormat:
      type: object
      required:
        - version
      properties:
        version:
          type: string
          nullable: true
```

* improve code

src/main/kotlin/com/cjbooms/fabrikt/util/YamlUtils.kt

@@ -19,33 +19,62 @@ object YamlUtils {
         ObjectMapper(
             YAMLFactory()
                 .disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER)
-                .enable(YAMLGenerator.Feature.MINIMIZE_QUOTES)
+                .enable(YAMLGenerator.Feature.MINIMIZE_QUOTES),
         )
             .registerKotlinModule()
             .configure(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES, true)
     private val internalMapper: ObjectMapper =
         ObjectMapper(YAMLFactory().disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER))
 
+    private val NULL_TYPE: JsonNode = objectMapper.valueToTree("null")
+
     fun mergeYamlTrees(mainTree: String, updateTree: String) =
         internalMapper.writeValueAsString(
             mergeNodes(
                 internalMapper.readTree(mainTree),
-                internalMapper.readTree(updateTree)
-            )
+                internalMapper.readTree(updateTree),
+            ),
         )!!
 
     fun parseOpenApi(input: String, inputDir: Path = Paths.get("").toAbsolutePath()): OpenApi3 =
         try {
-            OpenApi3Parser().parse(input, inputDir.toUri().toURL())
+            val root: JsonNode = objectMapper.readTree(input)
+            if (root["openapi"].asText() == "3.1.0") {
+                downgradeNullableSyntax(root)
+            }
+            OpenApi3Parser().parse(root, inputDir.toUri().toURL())
         } catch (ex: NullPointerException) {
             throw IllegalArgumentException(
                 "The Kaizen openapi-parser library threw a NPE exception when parsing this API. " +
                     "This is commonly due to an external schema reference that is unresolvable, " +
                     "possibly due to a lack of internet connection",
-                ex
+                ex,
             )
         }
 
+    private fun downgradeNullableSyntax(node: JsonNode) {
+        when {
+            node.isObject -> {
+                var requiresNullable = false
+                node.fields().forEach { (key, maybeTypeArray) ->
+                    if (key == "type" && maybeTypeArray.isArray && maybeTypeArray.contains(NULL_TYPE)) {
+                        val nonNullType = maybeTypeArray.first { it != NULL_TYPE }
+                        (node as ObjectNode).replace("type", nonNullType)
+                        requiresNullable = true
+                    }
+                    downgradeNullableSyntax(maybeTypeArray)
+                }
+                if (requiresNullable) {
+                    (node as ObjectNode).put("nullable", true)
+                }
+            }
+
+            node.isArray -> {
+                node.forEach { downgradeNullableSyntax(it) }
+            }
+        }
+    }
+
     /**
      * The below merge function has been shamelessly stolen from Stackoverflow: https://stackoverflow.com/a/32447591/1026785
      * and converted to much nicer Kotlin implementation
@@ -56,14 +85,20 @@ object YamlUtils {
             val incomingNode = incomingTree.get(fieldName)
             if (currentNode is ArrayNode && incomingNode is ArrayNode) {
                 incomingNode.forEach {
-                    if (currentNode.contains(it)) mergeNodes(
-                        currentNode.get(currentNode.indexOf(it)),
-                        it
-                    )
-                    else currentNode.add(it)
+                    if (currentNode.contains(it)) {
+                        mergeNodes(
+                            currentNode.get(currentNode.indexOf(it)),
+                            it,
+                        )
+                    } else {
+                        currentNode.add(it)
+                    }
                 }
-            } else if (currentNode is ObjectNode) mergeNodes(currentNode, incomingNode)
-            else (currentTree as? ObjectNode)?.replace(fieldName, incomingNode)
+            } else if (currentNode is ObjectNode) {
+                mergeNodes(currentNode, incomingNode)
+            } else {
+                (currentTree as? ObjectNode)?.replace(fieldName, incomingNode)
+            }
         }
         return currentTree
     }

src/test/kotlin/com/cjbooms/fabrikt/generators/ModelGeneratorTest.kt

@@ -54,6 +54,7 @@ class ModelGeneratorTest {
         "webhook",
         "instantDateTime",
         "discriminatedOneOf",
+        "openapi310",
     )
 
     @BeforeEach

src/test/resources/examples/openapi310/api.yaml

@@ -0,0 +1,14 @@
+openapi: 3.1.0
+components:
+  schemas:
+    NewNullableFormat:
+      type: object
+      required:
+        - version
+      properties:
+        version:
+          type:
+            - string
+            - 'null'
+          description: The resolved version or `null` if there is no matching version.
+

src/test/resources/examples/openapi310/models/Models.kt

@@ -0,0 +1,12 @@
+package examples.openapi310.models
+
+import com.fasterxml.jackson.`annotation`.JsonProperty
+import javax.validation.constraints.NotNull
+import kotlin.String
+
+public data class NewNullableFormat(
+    @param:JsonProperty("version")
+    @get:JsonProperty("version")
+    @get:NotNull
+    public val version: String,
+)