2nd task done

02/practice/src/main/scala/org/spbsu/mkn/scala/IntList.scala

@@ -2,20 +2,80 @@ package org.spbsu.mkn.scala
 
 import org.spbsu.mkn.scala.IntList._
 
+import scala.annotation.tailrec
+
 sealed trait IntList {
   def head: Int
+
   def tail: IntList
+
   def drop(n: Int): IntList
+
   def take(n: Int): IntList
+
   def map(f: Int => Int): IntList
-  def ::(elem: Int): IntList = ???
+
+  def ::(elem: Int): IntList = IntCons(elem, this)
+
 }
 
 object IntList {
   def undef: Nothing = throw new UnsupportedOperationException("operation is undefined")
-  def fromSeq(seq: Seq[Int]): IntList = ???
-  def sum(intList: IntList): Int      = ???
-  def size(intList: IntList): Int     = ???
-  // extra task: implement sum using foldLeft
-  // def foldLeft(???)(???): ??? = ???
+
+  @tailrec
+  def foldLeft(f: Int => Int => Int)(intList: IntList)(init: Int): Int = intList match {
+    case IntNil => init
+    case _ => foldLeft(f)(intList.tail)(f(intList.head)(init))
+  }
+
+  def fromSeq(seq: Seq[Int]): IntList = seq match {
+    case _ if seq.isEmpty => IntNil
+    case _ => seq.foldRight(IntNil: IntList)((elem: Int, intList: IntList) => IntCons(elem, intList))
+  }
+
+  def sum(intList: IntList): Int = intList match {
+    case IntNil => undef
+    case _ => foldLeft(x => _ + x)(intList)(0)
+  }
+
+  def size(intList: IntList): Int = intList match {
+    case IntNil => 0
+    case _ => foldLeft(_ => _ + 1)(intList)(0)
+  }
+
 }
+
+case object IntNil extends IntList {
+  override def head: Int = undef
+
+  override def tail: IntList = undef
+
+  override def drop(n: Int): IntList = n match {
+    case 0 => this
+    case _ => undef
+  }
+
+  override def take(n: Int): IntList = n match {
+    case 0 => this
+    case _ => undef
+  }
+
+  override def map(f: Int => Int): IntList = this
+}
+
+case class IntCons(override val head: Int, override val tail: IntList) extends IntList {
+  override def drop(n: Int): IntList = n match {
+    case 0 => this
+    case _ => tail.drop(n - 1)
+  }
+
+  override def take(n: Int): IntList = n match {
+    case 0 => IntNil
+    case _ => IntCons(head, tail.take(n - 1))
+
+  }
+
+  override def map(f: Int => Int): IntList = IntCons(f(head), tail.map(f))
+
+}
+