#32 start writing JsonPath dsl

modules/core/src/main/scala/tethys/readers/JsonPath.scala

@@ -0,0 +1,32 @@
+package tethys.readers
+
+import tethys.JsonReader
+import tethys.readers.JsonPath.JsonObjectPart
+import tethys.readers.instances.SingleValueObjectReader
+
+import scala.language.dynamics
+
+
+sealed trait JsonPath extends Dynamic {
+  def as[A](implicit reader: JsonReader[A]): JsonReader[A] = buildReader(reader)
+  def array()
+
+  def selectDynamic(name: String): JsonObjectPart = new JsonObjectPart(name, this)
+  def applyDynamic(index: Int): JsonObjectPart = ???
+
+  protected def buildReader[A](inner: JsonReader[A]): JsonReader[A]
+}
+
+
+object JsonPath extends JsonPath {
+
+  override protected def buildReader[A](inner: JsonReader[A]): JsonReader[A] = inner
+
+  final class JsonObjectPart(name: String, outer: JsonPath) extends JsonPath {
+    override protected def buildReader[A](inner: JsonReader[A]): JsonReader[A] = {
+      outer.buildReader(new SingleValueObjectReader[A](name, inner))
+    }
+  }
+
+
+}

modules/core/src/main/scala/tethys/readers/instances/SingleValueObjectReader.scala

@@ -0,0 +1,54 @@
+package tethys.readers.instances
+
+import tethys.JsonReader
+import tethys.readers.tokens.TokenIterator
+import tethys.readers.{FieldName, ReaderError}
+
+import scala.annotation.tailrec
+import scala.reflect.ClassTag
+
+class SingleValueObjectReader[A: ClassTag](name: String, reader: JsonReader[A]) extends JsonReader[A] {
+
+
+  override def read(it: TokenIterator)(implicit fieldName: FieldName): A = {
+    if(!it.currentToken().isObjectStart) ReaderError.wrongType[A]
+    else readField(it)
+  }
+
+  @tailrec
+  private def readField(it: TokenIterator)(implicit fieldName: FieldName): A = {
+    it.currentToken() match {
+      case token if token.isObjectEnd =>
+        it.nextToken()
+        reader.defaultValue.getOrElse(ReaderError.wrongJson(s"'$name' field is missing"))
+      case token if token.isFieldName =>
+        val currentName = it.fieldName()
+        it.nextToken()
+        if(currentName == name) {
+          val res = reader.read(it)(fieldName.appendFieldName(name))
+          skipRest(it)
+          res
+        } else {
+          readField(it)
+        }
+
+      case token => ReaderError.wrongJson(s"Expect end of object or field name but '$token' found")
+    }
+  }
+
+  @tailrec
+  private def skipRest(it: TokenIterator)(implicit fieldName: FieldName): Unit = {
+    it.currentToken() match {
+      case token if token.isObjectEnd =>
+        it.nextToken()
+
+      case token if token.isFieldName =>
+        it.nextToken()
+        it.skipExpression()
+        skipRest(it)
+
+      case token =>
+        ReaderError.wrongJson(s"Expect end of object or field name but '$token' found")
+    }
+  }
+}