package tests

import spray.json.JsValue
import language.experimental.macros
import reflect.macros.whitebox.Context


abstract class JsonLens[T] {
  def get(obj: T): JsValue
  def set(obj: T, value: JsValue): T
  def fields: Map[String, JsonLens[T]] = Map()
}


object JsonLens {

  def of[T]: JsonLens[T] = macro lensMacro[T]

  def lensMacro[T : c.WeakTypeTag](c: Context): c.Expr[JsonLens[T]] = {
    import c.universe._

    val lensType = weakTypeOf[T]

    val primaryCtor = lensType.decls.collectFirst {
      case m: MethodSymbol if m.isPrimaryConstructor => m
    }

    val fields = primaryCtor match {
      case Some(pc) => pc.paramLists.head
      case None => Nil
    }

    val fieldLenses = fields.map { field =>
      val fieldName = field.name.decodedName
      val fieldType = field.typeSignature
      q"""
        Map(
          ${fieldName.toString} -> new JsonLens[$lensType] {
            def get(obj: $lensType) = implicitly[JsonLens[$fieldType]].get(obj.${fieldName.toTermName})
            def set(obj: $lensType, value: JsValue) = obj.copy(${fieldName.toTermName} = implicitly[JsonLens[$fieldType]].set(obj.${fieldName.toTermName}, value))
          }) ++ implicitly[JsonLens[$fieldType]].fields.map({ kv =>
            (${fieldName.toString} + "." + kv._1 -> new JsonLens[$lensType] {
              def get(obj: $lensType): JsValue = kv._2.get(obj.${fieldName.toTermName})
              def set(obj: $lensType, value: JsValue): $lensType = obj.copy(${fieldName.toTermName} = kv._2.set(obj.${fieldName.toTermName}, value))
            })
          })
      """
    }

    val fieldLensMap = fieldLenses.fold(q"Map[String, JsonLens[$lensType]]()")((fieldLens1, fieldLens2) => q"$fieldLens1 ++ $fieldLens2")

    c.Expr[JsonLens[T]] {
      q"""
        new JsonLens[$lensType] {
          def get(obj: $lensType): JsValue = implicitly[JsonFormat[$lensType]].write(obj)
          def set(obj: $lensType, value: JsValue): $lensType = implicitly[JsonFormat[$lensType]].read(value)
          override def fields = $fieldLensMap
        }
      """
    }
  }

}