// Scala Lecture 4
//=================


// expressions (essentially trees)

abstract class Exp
case class N(n: Int) extends Exp                  // for numbers
case class Plus(e1: Exp, e2: Exp) extends Exp
case class Times(e1: Exp, e2: Exp) extends Exp

def string(e: Exp) : String = e match {
  case N(n) => s"$n"
  case Plus(e1, e2) => s"(${string(e1)} + ${string(e2)})" 
  case Times(e1, e2) => s"(${string(e1)} * ${string(e2)})"
}

val e = Plus(N(9), Times(N(3), N(4)))
println(string(e))

def eval(e: Exp) : Int = e match {
  case N(n) => n
  case Plus(e1, e2) => eval(e1) + eval(e2) 
  case Times(e1, e2) => eval(e1) * eval(e2) 
}

println(eval(e))

// simplification rules:
// e + 0, 0 + e => e 
// e * 0, 0 * e => 0
// e * 1, 1 * e => e
//
// (....0  ....)

def simp(e: Exp) : Exp = e match {
  case N(n) => N(n)
  case Plus(e1, e2) => (simp(e1), simp(e2)) match {
    case (N(0), e2s) => e2s
    case (e1s, N(0)) => e1s
    case (e1s, e2s) => Plus(e1s, e2s)
  }  
  case Times(e1, e2) => (simp(e1), simp(e2)) match {
    case (N(0), _) => N(0)
    case (_, N(0)) => N(0)
    case (N(1), e2s) => e2s
    case (e1s, N(1)) => e1s
    case (e1s, e2s) => Times(e1s, e2s)
  }  
}


val e2 = Times(Plus(N(0), N(1)), Plus(N(0), N(9)))
println(string(e2))
println(string(simp(e2)))


// Tokens and Reverse Polish Notation
abstract class Token
case class T(n: Int) extends Token
case object PL extends Token
case object TI extends Token

// transfroming an Exp into a list of tokens
def rp(e: Exp) : List[Token] = e match {
  case N(n) => List(T(n))
  case Plus(e1, e2) => rp(e1) ::: rp(e2) ::: List(PL) 
  case Times(e1, e2) => rp(e1) ::: rp(e2) ::: List(TI) 
}
println(string(e2))
println(rp(e2))

def comp(ls: List[Token], st: List[Int] = Nil) : Int = (ls, st) match {
  case (Nil, st) => st.head 
  case (T(n)::rest, st) => comp(rest, n::st)
  case (PL::rest, n1::n2::st) => comp(rest, n1 + n2::st)
  case (TI::rest, n1::n2::st) => comp(rest, n1 * n2::st)
}

comp(rp(e))

def proc(s: String) : Token = s match {
  case  "+" => PL
  case  "*" => TI
  case  _ => T(s.toInt) 
}

comp("1 2 + 4 * 5 + 3 +".split(" ").toList.map(proc), Nil)


// Polymorphic Types
//===================

// You do not want to write functions like contains, first, 
// length and so on for every type of lists.

def length_int_list(lst: List[Int]): Int = lst match {
  case Nil => 0
  case x::xs => 1 + length_int_list(xs)
}

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

def length_string_list(lst: List[String]): Int = lst match {
  case Nil => 0
  case _::xs => 1 + length_string_list(xs)
}

length_string_list(List("1", "2", "3", "4"))


// you can make the function parametric in type(s)

def length[A](lst: List[A]): Int = lst match {
  case Nil => 0
  case x::xs => 1 + length(xs)
}
length(List("1", "2", "3", "4"))
length(List(1, 2, 3, 4))

length[Int](List(1, 2, 3, 4))


def map[A, B](lst: List[A], f: A => B): List[B] = lst match {
  case Nil => Nil
  case x::xs => f(x)::map(xs, f) 
}

map(List(1, 2, 3, 4), (x: Int) => x.toString)


// from knight1.scala
def first(xs: List[Pos], f: Pos => Option[Path]) : Option[Path] = ???

// should be
def first[A, B](xs: List[A], f: A => Option[B]) : Option[B] = ???

// Type inference is local in Scala

def id[T](x: T) : T = x

val x = id(322)          // Int
val y = id("hey")        // String
val z = id(Set(1,2,3,4)) // Set[Int]


// The type variable concept in Scala can get really complicated.
//
// - variance (OO)
// - bounds (subtyping)
// - quantification

// Java has issues with this too: Java allows
// to write the following incorrect code, and
// only recovers by raising an exception
// at runtime.

// Object[] arr = new Integer[10];
// arr[0] = "Hello World";


// Scala gives you a compile-time error, which
// is much better.

var arr = Array[Int]()
arr(0) = "Hello World"




// Function definitions again
//============================

// variable arguments

def printAll(strings: String*) = {
  strings.foreach(println)
}

printAll()
printAll("foo")
printAll("foo", "bar")
printAll("foo", "bar", "baz")

// pass a list to the varargs field
val fruits = List("apple", "banana", "cherry")

printAll(fruits: _*)


// you can also implement your own string interpolations
import scala.language.implicitConversions
import scala.language.reflectiveCalls

implicit def sring_inters(sc: StringContext) = new {
    def i(args: Any*): String = s"${sc.s(args:_*)}\n"
}

i"add ${3+2} ${3 * 3}" 


// default arguments

def length[A](xs: List[A]) : Int = xs match {
  case Nil => 0
  case _ :: tail => 1 + length(tail)
}

def lengthT[A](xs: List[A], acc : Int = 0) : Int = xs match {
  case Nil => acc
  case _ :: tail => lengthT(tail, 1 + acc)
}

lengthT(List.fill(100000)(1))


def fact(n: BigInt, acc: BigInt = 1): BigInt =
  if (n == 0) acc else fact(n - 1, n * acc)

fact(10)



// currying    (Haskell Curry)

def add(x: Int, y: Int) = x + y

List(1,2,3,4,5).map(x => add(3, x))

def add2(x: Int)(y: Int) = x + y

List(1,2,3,4,5).map(add2(3))

val a3 : Int => Int = add2(3)

// currying helps sometimes with type inference

def find[A](xs: List[A])(pred: A => Boolean): Option[A] = {
  xs match {
    case Nil => None
    case hd :: tl =>
      if (pred(hd)) Some(hd) else find(tl)(pred)
  }
}

find(List(1, 2, 3))(x => x % 2 == 0)

// Source.fromURL(url)(encoding)
// Source.fromFile(name)(encoding)


// 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", "")

candidates(game0, (0, 0))

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))

get_row(game0, 0)

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


// this is not mutable!!
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 {
    val cs = candidates(game, emptyPosition(game))
    cs.map(c => search(update(game, empty(game), c))).toList.flatten
  }
}

List(List("sol1"), List("sol2", "sol3")).flatten

search(game0).map(pretty)

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", "")

search(game1).map(pretty)

// a 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", "")

search(game2).map(pretty)

// 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(game3).map(pretty).foreach(println)

// 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()
  s"${(end - start) / 1.0e9} secs"
}

time_needed(1, search(game2))



// Tail recursion
//================

@tailrec
def fact(n: BigInt): BigInt = 
  if (n == 0) 1 else n * fact(n - 1)


fact(10)          
fact(1000)        
fact(100000)       

def factB(n: BigInt): BigInt = 
  if (n == 0) 1 else n * factB(n - 1)

def factT(n: BigInt, acc: BigInt): BigInt =
  if (n == 0) acc else factT(n - 1, n * acc)


factB(1000)




factT(10, 1)
println(factT(500000, 1))





// there is a flag for ensuring a function is tail recursive
import scala.annotation.tailrec

@tailrec
def factT(n: BigInt, acc: BigInt): BigInt =
  if (n == 0) acc else factT(n - 1, n * acc)

factT(100000, 1)

// for tail-recursive functions the Scala compiler
// generates loop-like code, which does not need
// to allocate stack-space in each recursive
// call; Scala can do this only for tail-recursive
// functions

// tail recursive version that searches 
// for all Sudoku solutions

@tailrec
def searchT(games: List[String], sols: List[String]): List[String] = games match {
  case Nil => sols
  case game::rest => {
    if (isDone(game)) searchT(rest, game::sols)
    else {
      val cs = candidates(game, emptyPosition(game))
      searchT(cs.map(c => update(game, empty(game), c)) ::: rest, sols)
    }
  }
}

searchT(List(game3), List()).map(pretty)


// tail recursive version that searches 
// for a single solution

def search1T(games: List[String]): Option[String] = games match {
  case Nil => None
  case game::rest => {
    if (isDone(game)) Some(game)
    else {
      val cs = candidates(game, emptyPosition(game))
      search1T(cs.map(c => update(game, empty(game), c)) ::: rest)
    }
  }
}

search1T(List(game3)).map(pretty)
time_needed(1, search1T(List(game3)))
time_needed(1, search1T(List(game2)))

// 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", "")

searchT(List(game3), Nil).map(pretty)
search1T(List(game3)).map(pretty)

// Moral: Whenever a recursive function is resource-critical
// (i.e. works with a large recursion depth), then you need to
// write it in tail-recursive fashion.
// 
// Unfortuantely, Scala because of current limitations in 
// the JVM is not as clever as other functional languages. It can 
// only optimise "self-tail calls". This excludes the cases of 
// multiple functions making tail calls to each other. Well,
// nothing is perfect. 







// Cool Stuff in Scala
//=====================


// Implicits or How to Pimp your Library
//======================================
//
// 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 = s.map(c => (c + 1).toChar) 
}

"HAL".increment


// Abstract idea:
// In that version implicit conversions were used to solve the 
// late extension problem; namely, given a class C and a class T, 
// how to have C extend T without touching or recompiling C. 
// Conversions add a wrapper when a member of T is requested 
// from an instance of C.

//Another example (TimeUnit in 2.13?)

import scala.concurrent.duration.{TimeUnit,SECONDS,MINUTES}

case class Duration(time: Long, unit: TimeUnit) {
  def +(o: Duration) = 
    Duration(time + unit.convert(o.time, o.unit), unit)
}

implicit class Int2Duration(that: Int) {
  def seconds = new Duration(that, SECONDS)
  def minutes = new Duration(that, MINUTES)
}

5.seconds + 2.minutes   //Duration(125L, SECONDS )
2.minutes + 60.seconds




// Regular expressions - the power of DSLs in Scala
//==================================================

abstract class Rexp
case object ZERO extends Rexp                     // nothing
case object ONE extends Rexp                      // the empty string
case class CHAR(c: Char) extends Rexp             // a character c
case class ALT(r1: Rexp, r2: Rexp) extends Rexp   // alternative  r1 + r2
case class SEQ(r1: Rexp, r2: Rexp) extends Rexp   // sequence     r1 . r2  
case class STAR(r: Rexp) extends Rexp             // star         r*



// writing (ab)* in the format above is 
// tedious
val r0 = STAR(SEQ(CHAR('a'), CHAR('b')))


// some convenience for typing in regular expressions
import scala.language.implicitConversions    
import scala.language.reflectiveCalls 

def charlist2rexp(s: List[Char]): Rexp = s match {
  case Nil => ONE
  case c::Nil => CHAR(c)
  case c::s => SEQ(CHAR(c), charlist2rexp(s))
}

implicit def string2rexp(s: String): Rexp = 
  charlist2rexp(s.toList)

"(a|b)"

val r1 = STAR("ab")
val r2 = (STAR("ab")) | (STAR("ba"))
val r3 = STAR(SEQ("ab", ALT("a", "b")))

implicit def RexpOps (r: Rexp) = new {
  def | (s: Rexp) = ALT(r, s)
  def % = STAR(r)
  def ~ (s: Rexp) = SEQ(r, s)
}

implicit def stringOps (s: String) = new {
  def | (r: Rexp) = ALT(s, r)
  def | (r: String) = ALT(s, r)
  def % = STAR(s)
  def ~ (r: Rexp) = SEQ(s, r)
  def ~ (r: String) = SEQ(s, r)
}

//example regular expressions
val digit = ("0" | "1" | "2" | "3" | "4" | 
              "5" | "6" | "7" | "8" | "9")
val sign = "+" | "-" | ""
val number = sign ~ digit ~ digit.% 



// Mind-Blowing Regular Expressions

// same examples using the internal regexes
val evil = "(a*)*b"


println("a" * 100)

("a" * 10000).matches(evil)
("a" * 10).matches(evil)
("a" * 10000).matches(evil)
("a" * 20000).matches(evil)
("a" * 50000).matches(evil)

time_needed(1, ("a" * 50000).matches(evil))