From c06cdf2108e43adbf78231f5e57a89e068901778 Mon Sep 17 00:00:00 2001
From: Travis Brown <travisbrown@stripe.com>
Date: Thu, 8 Jun 2017 16:33:47 -0500
Subject: [PATCH] Support merge keys

---
 .../scala/io/circe/yaml/parser/package.scala  | 14 ++++++-
 src/test/resources/test-yamls/merge-key.json  | 40 +++++++++++++++++++
 src/test/resources/test-yamls/merge-key.yml   | 27 +++++++++++++
 3 files changed, 80 insertions(+), 1 deletion(-)
 create mode 100644 src/test/resources/test-yamls/merge-key.json
 create mode 100644 src/test/resources/test-yamls/merge-key.yml

diff --git a/src/main/scala/io/circe/yaml/parser/package.scala b/src/main/scala/io/circe/yaml/parser/package.scala
index d6039819..a7e3d969 100644
--- a/src/main/scala/io/circe/yaml/parser/package.scala
+++ b/src/main/scala/io/circe/yaml/parser/package.scala
@@ -4,6 +4,7 @@ import cats.syntax.either._
 import io.circe._
 import java.io.{Reader, StringReader}
 import org.yaml.snakeyaml.Yaml
+import org.yaml.snakeyaml.constructor.SafeConstructor
 import org.yaml.snakeyaml.nodes._
 import scala.collection.JavaConverters._
 
@@ -38,6 +39,15 @@ package object parser {
       None
   }
 
+  private[this] class FlatteningConstructor extends SafeConstructor {
+    def flatten(node: MappingNode): MappingNode = {
+      flattenMapping(node)
+      node
+    }
+  }
+
+  private[this] val flattener: FlatteningConstructor = new FlatteningConstructor
+
   private[this] def yamlToJson(node: Node): Either[ParsingFailure, Json] = {
 
     def convertScalarNode(node: ScalarNode) = Either.catchNonFatal(node.getTag match {
@@ -61,7 +71,9 @@ package object parser {
 
     node match {
       case mapping: MappingNode =>
-        mapping.getValue.asScala.foldLeft(Either.right[ParsingFailure, JsonObject](JsonObject.empty)) {
+        flattener.flatten(mapping).getValue.asScala.foldLeft(
+          Either.right[ParsingFailure, JsonObject](JsonObject.empty)
+        ) {
           (objEither, tup) => for {
             obj <- objEither
             key <- convertKeyNode(tup.getKeyNode)
diff --git a/src/test/resources/test-yamls/merge-key.json b/src/test/resources/test-yamls/merge-key.json
new file mode 100644
index 00000000..dae8042a
--- /dev/null
+++ b/src/test/resources/test-yamls/merge-key.json
@@ -0,0 +1,40 @@
+[
+  {
+    "x" : 1,
+    "y" : 2
+  },
+  {
+    "x" : 0,
+    "y" : 2
+  },
+  {
+    "r" : 1e1
+  },
+  {
+    "r" : 1
+  },
+  {
+    "x" : 1,
+    "y" : 2,
+    "r" : 1e1,
+    "label" : "center/big"
+  },
+  {
+    "x" : 1,
+    "y" : 2,
+    "r" : 1e1,
+    "label" : "center/big"
+  },
+  {
+    "x" : 1,
+    "y" : 2,
+    "r" : 1e1,
+    "label" : "center/big"
+  },
+  {
+    "r" : 1e1,
+    "x" : 1,
+    "y" : 2,
+    "label" : "center/big"
+  }
+]
\ No newline at end of file
diff --git a/src/test/resources/test-yamls/merge-key.yml b/src/test/resources/test-yamls/merge-key.yml
new file mode 100644
index 00000000..ee4a48fe
--- /dev/null
+++ b/src/test/resources/test-yamls/merge-key.yml
@@ -0,0 +1,27 @@
+---
+- &CENTER { x: 1, y: 2 }
+- &LEFT { x: 0, y: 2 }
+- &BIG { r: 10 }
+- &SMALL { r: 1 }
+
+# All the following maps are equal:
+
+- # Explicit keys
+  x: 1
+  y: 2
+  r: 10
+  label: center/big
+
+- # Merge one map
+  << : *CENTER
+  r: 10
+  label: center/big
+
+- # Merge multiple maps
+  << : [ *CENTER, *BIG ]
+  label: center/big
+
+- # Override
+  << : [ *BIG, *LEFT, *SMALL ]
+  x: 1
+  label: center/big