Support $schema property to identify schema draft to use

README.md

@@ -155,7 +155,7 @@ val valid = schema.validate(elementToValidate, errors::add)
 
 ## Future plans
 
-- [ ] Add `$schema` property validation (if not set the latest supported will be used)
+- [x] Add `$schema` property validation (if not set the latest supported will be used)
 - [ ] Add proper `$id` support (for nested schemas and for referencing)
 - [ ] Add support for newer drafts
   - [ ] [Draft 2019-09 (Draft 8)](https://json-schema.org/specification-links.html#draft-2019-09-formerly-known-as-draft-8)

gradle.properties

@@ -3,5 +3,5 @@ kotlin.js.compiler=ir
 org.gradle.jvmargs=-Xmx1G
 org.gradle.java.installations.auto-download=false
 
-version=0.0.1-SNAPSHOT
+version=0.0.2-SNAPSHOT
 group=io.github.optimumcode

src/commonMain/kotlin/io/github/optimumcode/json/schema/internal/SchemaLoader.kt

@@ -71,11 +71,13 @@ private val factories: List<AssertionFactory> = listOf(
 private const val DEFINITIONS_PROPERTY: String = "definitions"
 private const val ID_PROPERTY: String = "\$id"
 private const val REF_PROPERTY: String = "\$ref"
+private const val SCHEMA_PROPERTY: String = "\$schema"
 
 internal const val ROOT_REFERENCE = '#'
 
 internal class SchemaLoader {
   fun load(schemaDefinition: JsonElement): JsonSchema {
+    extractSchemaType(schemaDefinition)
     val baseId = extractBaseID(schemaDefinition)
     val context = defaultLoadingContext(baseId)
     loadDefinitions(schemaDefinition, context)
@@ -84,6 +86,17 @@ internal class SchemaLoader {
     return JsonSchema(schemaAssertion, context.references)
   }
 
+  private fun extractSchemaType(schemaDefinition: JsonElement): SchemaType {
+    return if (schemaDefinition is JsonObject) {
+      schemaDefinition[SCHEMA_PROPERTY]?.let {
+        require(it is JsonPrimitive && it.isString) { "$SCHEMA_PROPERTY must be a string" }
+        SchemaType.find(it.content) ?: throw IllegalArgumentException("unsupported schema type ${it.content}")
+      } ?: SchemaType.values().last()
+    } else {
+      SchemaType.values().last()
+    }
+  }
+
   private fun loadDefinitions(schemaDefinition: JsonElement, context: DefaultLoadingContext) {
     if (schemaDefinition !is JsonObject) {
       return

src/commonMain/kotlin/io/github/optimumcode/json/schema/internal/SchemaType.kt

@@ -0,0 +1,19 @@
+package io.github.optimumcode.json.schema.internal
+
+import kotlin.jvm.JvmStatic
+
+internal enum class SchemaType(
+  private val schemaId: String,
+) {
+  DRAFT_7("json-schema.org/draft-07/schema"),
+  ;
+
+  companion object {
+    @JvmStatic
+    fun find(schemaId: String): SchemaType? {
+      val id = schemaId.substringAfter("://")
+        .substringBeforeLast(ROOT_REFERENCE)
+      return values().find { it.schemaId == id }
+    }
+  }
+}

src/commonTest/kotlin/io/github/optimumcode/json/schema/base/JsonSchemaTest.kt

@@ -148,5 +148,43 @@ class JsonSchemaTest : FunSpec() {
         )
       }
     }
+
+    listOf(
+      "http://json-schema.org/draft-07/schema#",
+      "http://json-schema.org/draft-07/schema",
+      "https://json-schema.org/draft-07/schema#",
+      "https://json-schema.org/draft-07/schema",
+    ).forEach {
+      test("loads schema with supported '$it' \$schema property") {
+        shouldNotThrowAny {
+          JsonSchema.fromDefinition(
+            """
+            {
+              "${KEY}schema": "$it",
+              "type": "string"
+            }
+            """.trimIndent(),
+          )
+        }
+      }
+    }
+
+    listOf(
+      "https://json-schema.org/draft/2020-12/schema",
+      "http://json-schema.org/draft-07/schema/",
+    ).forEach {
+      test("reports unsupported '$it' \$schema property") {
+        shouldThrow<IllegalArgumentException> {
+          JsonSchema.fromDefinition(
+            """
+            {
+              "${KEY}schema": "$it",
+              "type": "string"
+            }
+            """.trimIndent(),
+          )
+        }.message shouldBe "unsupported schema type $it"
+      }
+    }
   }
 }