// Scala Lecture 2
//=================


// Option type
//=============

//in Java if something unusually happens, you return null;
//in Scala you use Option
//   - if the value is present, you use Some(value)
//   - if no value is present, you use None


List(7,2,3,4,5,6).find(_ < 4)
List(5,6,7,8,9).find(_ < 4)


// Values in types
//
// Boolean: 
// Int: 
// String: 
//
// Option[String]:
//   


val lst = List(None, Some(1), Some(2), None, Some(3))

lst.flatten

Some(1).get

Some(1).isDefined
None.isDefined

val ps = List((3, 0), (3, 2), (4, 2), (2, 0), (1, 0), (1, 1))

for ((x, y) <- ps) yield {
  if (y == 0) None else Some(x / y)
}

// getOrElse is for setting a default value

val lst = List(None, Some(1), Some(2), None, Some(3))
for (x <- lst) yield x.getOrElse(0)




// error handling with Option (no exceptions)
//
//  Try(something).getOrElse(what_to_do_in_an_exception)
//
import scala.util._
import io.Source

Source.fromURL("""http://www.inf.kcl.ac.uk/staff/urbanc/""").mkString

Try(Source.fromURL("""http://www.inf.kcl.ac.uk/staff/urbanc/""").mkString).getOrElse("")

Try(Some(Source.fromURL("""http://www.inf.kcl.ac.uk/staff/urbanc/""").mkString)).getOrElse(None)

// a function that turns strings into numbers
Integer.parseInt("12u34")

def get_me_an_int(s: String): Option[Int] = 
 Try(Some(Integer.parseInt(s))).getOrElse(None)

val lst = List("12345", "foo", "5432", "bar", "x21")
for (x <- lst) yield get_me_an_int(x)

// summing all the numbers
val sum = lst.flatMap(get_me_an_int(_)).sum


// This may not look any better than working with null in Java, but to
// see the value, you have to put yourself in the shoes of the
// consumer of the get_me_an_int function, and imagine you didn't
// write that function.
//
// In Java, if you didn't write this function, you'd have to depend on
// the Javadoc of the get_me_an_int. If you didn't look at the Javadoc, 
// you might not know that get_me_an_int could return a null, and your 
// code could potentially throw a NullPointerException.



// even Scala is not immune to problems like this:

List(5,6,7,8,9).indexOf(7)





// Type abbreviations
//====================

// some syntactic convenience
type Pos = (int, Int)

type Board = List[List[Int]]



// Implicits
//===========
//
// for example adding your own methods to Strings:
// imagine you want to increment strings, like
//
//     "HAL".increment
//
// you can avoid ugly fudges, like a MyString, by
// using implicit conversions


implicit class MyString(s: String) {
  def increment = for (c <- s) yield (c + 1).toChar 
}

"HAL".increment


// No return in Scala
//====================

//You should not use "return" in Scala:
//
// A return expression, when evaluated, abandons the 
// current computation and returns to the caller of the 
// function in which return appears."

def sq1(x: Int): Int = x * x
def sq2(x: Int): Int = return x * x

def sumq(ls: List[Int]): Int = {
  (for (x <- ls) yield (return x * x)).sum[Int]
}

sumq(List(1,2,3,4))


// last expression in a function is the return statement
def square(x: Int): Int = {
  println(s"The argument is ${x}.")
  x * x
}



// Pattern Matching
//==================

// A powerful tool which is supposed to come to Java in a few years
// time (https://www.youtube.com/watch?v=oGll155-vuQ)...Scala already
// has it for many years ;o)

// The general schema:
//
//    expression match {
//       case pattern1 => expression1
//       case pattern2 => expression2
//       ...
//       case patternN => expressionN
//    }


// remember
val lst = List(None, Some(1), Some(2), None, Some(3)).flatten


def my_flatten(xs: List[Option[Int]]): List[Int] = {
  ...
}






def my_flatten(lst: List[Option[Int]]): List[Int] = lst match {
  case Nil => Nil
  case None::xs => my_flatten(xs)
  case Some(n)::xs => n::my_flatten(xs)
}


// another example
def get_me_a_string(n: Int): String = n match {
  case 0 => "zero"
  case 1 => "one"
  case 2 => "two"
  case _ => "many"
}

get_me_a_string(0)

// you can also have cases combined
def season(month: String) = month match {
  case "March" | "April" | "May" => "It's spring"
  case "June" | "July" | "August" => "It's summer"
  case "September" | "October" | "November" => "It's autumn"
  case "December" | "January" | "February" => "It's winter"
}
 
println(season("November"))

// What happens if no case matches?

println(season("foobar"))

// User-defined Datatypes
//========================

abstract class Tree
case class Node(elem: Int, left: Tree, right: Tree) extends Tree
case class Leaf() extends Tree


def insert(tr: Tree, n: Int): Tree = tr match {
  case Leaf() => Node(n, Leaf(), Leaf())
  case Node(m, left, right) => 
    if (n == m) Node(m, left, right) 
    else if (n < m) Node(m, insert(left, n), right)
    else Node(m, left, insert(right, n))
}


val t1 = Node(4, Node(2, Leaf(), Leaf()), Node(7, Leaf(), Leaf()))
insert(t1, 3)

def depth(tr: Tree): Int = tr match {
  case Leaf() => 0
  case Node(_, left, right) => 1 + List(depth(left), depth(right)).max
}


def balance(tr: Tree): Int = tr match {
  case Leaf() => 0
  case Node(_, left, right) => depth(left) - depth(right)
}

balance(insert(t1, 3))

// another example

abstract class Person
case class King() extends Person
case class Peer(deg: String, terr: String, succ: Int) extends Person
case class Knight(name: String) extends Person
case class Peasant(name: String) extends Person
case class Clown() extends Person

def title(p: Person): String = p match {
  case King() => "His Majesty the King"
  case Peer(deg, terr, _) => s"The ${deg} of ${terr}"
  case Knight(name) => s"Sir ${name}"
  case Peasant(name) => name
}

def superior(p1: Person, p2: Person): Boolean = (p1, p2) match {
  case (King(), _) => true
  case (Peer(_,_,_), Knight(_)) => true
  case (Peer(_,_,_), Peasant(_)) => true
  case (Peer(_,_,_), Clown()) => true
  case (Knight(_), Peasant(_)) => true
  case (Knight(_), Clown()) => true
  case (Clown(), Peasant(_)) => true
  case _ => false
}

val people = List(Knight("David"), 
                  Peer("Duke", "Norfolk", 84), 
                  Peasant("Christian"), 
                  King(), 
                  Clown())

println(people.sortWith(superior(_, _)).mkString(", "))



// Higher-Order Functions
//========================

// functions can take functions as arguments

val lst = (1 to 10).toList

def even(x: Int): Boolean = x % 2 == 0
def odd(x: Int): Boolean = x % 2 == 1

lst.filter(x => even(x))
lst.filter(even(_))
lst.filter(even)

lst.find(_ > 8)

def square(x: Int): Int = x * x

lst.map(square)

lst.map(square).filter(_ > 4)

lst.map(square).filter(_ > 4).map(square)

// in my collatz.scala
//(1 to bnd).map(i => (collatz(i), i)).maxBy(_._1)


// type of functions, for example f: Int => Int

def my_map_int(lst: List[Int], f: Int => Int): List[Int] = lst match {
  case Nil => Nil
  case x::xs => f(x)::my_map_int(xs, f)
}

my_map_int(lst, square)

// other function types
//
// f1: (Int, Int) => Int
// f2: List[String] => Option[Int]
// ... 


def sumOf(f: Int => Int, lst: List[Int]): Int = lst match {
  case Nil => 0
  case x::xs => f(x) + sumOf(f, xs)
}

def sum_squares(lst: List[Int]) = sumOf(square, lst)
def sum_cubes(lst: List[Int])   = sumOf(x => x * x * x, lst)

sum_squares(lst)
sum_cubes(lst)

// lets try it factorial
def fact(n: Int): Int = ...

def sum_fact(lst: List[Int]) = sumOf(fact, lst)
sum_fact(lst)

// Avoid being mutable
//=====================

// a student showed me...
import scala.collection.mutable.ListBuffer



def collatz_max(bnd: Long): (Long, Long) = {
  val colNos = ListBuffer[(Long, Long)]()
  for (i <- (1L to bnd).toList) colNos += ((collatz(i), i))
  colNos.max
}

def collatz_max(bnd: Long): (Long, Long) = {
  (1L to bnd).map((i) => (collatz(i), i)).maxBy(_._1)
}

//views -> lazy collection
def collatz_max(bnd: Long): (Long, Long) = {
  (1L to bnd).view.map((i) => (collatz(i), i)).maxBy(_._1)
}

// raises a GC exception
(1 to 1000000000).filter(_ % 2 == 0).take(10).toList
// ==> java.lang.OutOfMemoryError: GC overhead limit exceeded

(1 to 1000000000).view.filter(_ % 2 == 0).take(10).toList



// Sudoku
//========

// THE POINT OF THIS CODE IS NOT TO BE SUPER
// EFFICIENT AND FAST, just explaining exhaustive
// depth-first search


val game0 = """.14.6.3..
              |62...4..9
              |.8..5.6..
              |.6.2....3
              |.7..1..5.
              |5....9.6.
              |..6.2..3.
              |1..5...92
              |..7.9.41.""".stripMargin.replaceAll("\\n", "")

type Pos = (Int, Int)
val EmptyValue = '.'
val MaxValue = 9

val allValues = "123456789".toList
val indexes = (0 to 8).toList




def empty(game: String) = game.indexOf(EmptyValue)
def isDone(game: String) = empty(game) == -1 
def emptyPosition(game: String) = (empty(game) % MaxValue, empty(game) / MaxValue)


def get_row(game: String, y: Int) = indexes.map(col => game(y * MaxValue + col))
def get_col(game: String, x: Int) = indexes.map(row => game(x + row * MaxValue))

def get_box(game: String, pos: Pos): List[Char] = {
    def base(p: Int): Int = (p / 3) * 3
    val x0 = base(pos._1)
    val y0 = base(pos._2)
    val ys = (y0 until y0 + 3).toList
    (x0 until x0 + 3).toList.flatMap(x => ys.map(y => game(x + y * MaxValue)))
}


//get_row(game0, 0)
//get_row(game0, 1)
//get_box(game0, (3,1))

def update(game: String, pos: Int, value: Char): String = game.updated(pos, value)

def toAvoid(game: String, pos: Pos): List[Char] = 
  (get_col(game, pos._1) ++ get_row(game, pos._2) ++ get_box(game, pos))

def candidates(game: String, pos: Pos): List[Char] = allValues diff toAvoid(game,pos)

//candidates(game0, (0,0))

def pretty(game: String): String = "\n" + (game sliding (MaxValue, MaxValue) mkString "\n")

def search(game: String): List[String] = {
  if (isDone(game)) List(game)
  else 
    candidates(game, emptyPosition(game)).map(c => search(update(game, empty(game), c))).toList.flatten
}


val game1 = """23.915...
              |...2..54.
              |6.7......
              |..1.....9
              |89.5.3.17
              |5.....6..
              |......9.5
              |.16..7...
              |...329..1""".stripMargin.replaceAll("\\n", "")


// game that is in the hard category
val game2 = """8........
              |..36.....
              |.7..9.2..
              |.5...7...
              |....457..
              |...1...3.
              |..1....68
              |..85...1.
              |.9....4..""".stripMargin.replaceAll("\\n", "")

// game with multiple solutions
val game3 = """.8...9743
              |.5...8.1.
              |.1.......
              |8....5...
              |...8.4...
              |...3....6
              |.......7.
              |.3.5...8.
              |9724...5.""".stripMargin.replaceAll("\\n", "")


search(game0).map(pretty)
search(game1).map(pretty)

// for measuring time
def time_needed[T](i: Int, code: => T) = {
  val start = System.nanoTime()
  for (j <- 1 to i) code
  val end = System.nanoTime()
  ((end - start) / i / 1.0e9) + " secs"
}

search(game2).map(pretty)
search(game3).distinct.length
time_needed(3, search(game2))
time_needed(3, search(game3))