// Scala Lecture 5
//=================

for (n <- (1 to 10).toList) yield {
  val add = 10
  n + add
}

println(add)

List(1,2,3,4).sum

// extension methods
// implicit conversions
// (Immutable) OOP

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


// Extensions 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 extensions.

extension (s: String) {
  def increment = s.map(c => (c + 1).toChar)
}

"HAL".increment



// a more relevant example

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)
}

extension (that: Int) {
  def seconds = Duration(that, SECONDS)
  def minutes = Duration(that, MINUTES)
}

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


// Implicit Conversions
//=====================



// 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*

val r = STAR(CHAR('a'))


// 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))
}

given Conversion[String, Rexp] = (s => charlist2rexp(s.toList))

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

val r1 = CHAR('a') | CHAR('b') | CHAR('c')
val r2 = CHAR('a') ~  CHAR('b')

val r3 : Rexp = "hello world"
      

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




// Object Oriented Programming in Scala
// =====================================


abstract class Animal 
case class Bird(name: String) extends Animal {
   override def toString = name
}
case class Mammal(name: String) extends Animal
case class Reptile(name: String) extends Animal

Mammal("Zebra")
println(Mammal("Zebra"))
println(Mammal("Zebra").toString)


Bird("Sparrow")
println(Bird("Sparrow"))
println(Bird("Sparrow").toString)

Bird("Sparrow").copy(name = "House Sparrow")

def group(a : Animal) = a match {
  case Bird(_) => "It's a bird"
  case Mammal(_) => "It's a mammal"
}


// There is a very convenient short-hand notation
// for constructors:

class Fraction(x: Int, y: Int) {
  def numer = x
  def denom = y
}

val half = Fraction(1, 2)
half.numer

// does not work with "vanilla" classes
half match {
  case Fraction(x, y) => x / y
}


case class Fraction(numer: Int, denom: Int)

val half = Fraction(1, 2)

half.numer
half.denom

// works with case classes
half match {
  case Fraction(x, y) => x / y
}


// In mandelbrot.scala I used complex (imaginary) numbers 
// and implemented the usual arithmetic operations for complex 
// numbers.

case class Complex(re: Double, im: Double) { 
  // represents the complex number re + im * i
  def +(that: Complex) = Complex(this.re + that.re, this.im + that.im)
  def -(that: Complex) = Complex(this.re - that.re, this.im - that.im)
  def *(that: Complex) = Complex(this.re * that.re - this.im * that.im,
                                 this.re * that.im + that.re * this.im)
  def *(that: Double) = Complex(this.re * that, this.im * that)
  def abs = Math.sqrt(this.re * this.re + this.im * this.im)
}

// usual way to reference methods
//object.method(....)

val test = Complex(1, 2) + (Complex (3, 4))


import scala.language.postfixOps
(List(5,4,3,2,1) sorted) reverse

// this could have equally been written as
val test = Complex(1, 2).+(Complex (3, 4))

// this applies to all methods, but requires
import scala.language.postfixOps

List(5, 2, 3, 4).sorted
List(5, 2, 3, 4) sorted


// ...to allow the notation n + m * i
import scala.language.implicitConversions   

val i = Complex(0, 1)

given Conversion[Double, Complex] = (re => Complex(re, 0))

val inum1 = -2.0 + -1.5 * i
val inum2 =  1.0 +  1.5 * i



// All is public by default....so no public is needed.
// You can have the usual restrictions about private 
// values and methods, if you are MUTABLE !!!

case class BankAccount(init: Int) {

  private var balance = init

  def deposit(amount: Int): Unit = {
    if (amount > 0) balance = balance + amount
  }

  def withdraw(amount: Int): Int =
    if (0 < amount && amount <= balance) {
      balance = balance - amount
      balance
    } else throw new Error("insufficient funds")
}

// BUT since we are completely IMMUTABLE, this is 
// virtually of no concern to us.



// another example about Fractions
import scala.language.implicitConversions
import scala.language.reflectiveCalls

case class Fraction(numer: Int, denom: Int) {
  override def toString = numer.toString + "/" + denom.toString

  def +(other: Fraction) = 
    Fraction(numer * other.denom + other.numer * denom, 
             denom * other.denom)
  def *(other: Fraction) = 
    Fraction(numer * other.numer, denom * other.denom)
 }

given Conversion[Int, Fraction] = (x => Fraction(x, 1))

val half = Fraction(1, 2)
val third = Fraction (1, 3)

half + third
half * third

1 + half




// DFAs in Scala  
//===============
import scala.util.Try


// A is the state type
// C is the input (usually characters)

case class DFA[A, C](start: A,              // starting state
                     delta: (A, C) => A,    // transition function
                     fins:  A => Boolean) { // final states (Set)

  def deltas(q: A, s: List[C]) : A = s match {
    case Nil => q
    case c::cs => deltas(delta(q, c), cs)
  }

  def accepts(s: List[C]) : Boolean = 
    Try(fins(deltas(start, s))).getOrElse(false)
}

// the example shown in the handout 
abstract class State
case object Q0 extends State
case object Q1 extends State
case object Q2 extends State
case object Q3 extends State
case object Q4 extends State

val delta : (State, Char) => State = 
  { case (Q0, 'a') => Q1
    case (Q0, 'b') => Q2
    case (Q1, 'a') => Q4
    case (Q1, 'b') => Q2
    case (Q2, 'a') => Q3
    case (Q2, 'b') => Q2
    case (Q3, 'a') => Q4
    case (Q3, 'b') => Q0
    case (Q4, 'a') => Q4
    case (Q4, 'b') => Q4 
    case _ => throw new Exception("Undefined") }

val dfa = DFA(Q0, delta, Set[State](Q4))

dfa.accepts("abaaa".toList)     // true
dfa.accepts("bbabaab".toList)   // true
dfa.accepts("baba".toList)      // false
dfa.accepts("abc".toList)       // false


// NFAs (Nondeterministic Finite Automata)


case class NFA[A, C](starts: Set[A],          // starting states
                     delta: (A, C) => Set[A], // transition function
                     fins:  A => Boolean) {   // final states 

  // given a state and a character, what is the set of 
  // next states? if there is none => empty set
  def next(q: A, c: C) : Set[A] = 
    Try(delta(q, c)).getOrElse(Set[A]()) 

  def nexts(qs: Set[A], c: C) : Set[A] =
    qs.flatMap(next(_, c))

  // depth-first version of accepts
  def search(q: A, s: List[C]) : Boolean = s match {
    case Nil => fins(q)
    case c::cs => next(q, c).exists(search(_, cs))
  }

  def accepts(s: List[C]) : Boolean =
    starts.exists(search(_, s))
}



// NFA examples

val nfa_trans1 : (State, Char) => Set[State] = 
  { case (Q0, 'a') => Set(Q0, Q1) 
    case (Q0, 'b') => Set(Q2) 
    case (Q1, 'a') => Set(Q1) 
    case (Q2, 'b') => Set(Q2) }

val nfa = NFA(Set[State](Q0), nfa_trans1, Set[State](Q2))

nfa.accepts("aa".toList)             // false
nfa.accepts("aaaaa".toList)          // false
nfa.accepts("aaaaab".toList)         // true
nfa.accepts("aaaaabbb".toList)       // true
nfa.accepts("aaaaabbbaaa".toList)    // false
nfa.accepts("ac".toList)             // false


// Q: Why the kerfuffle about the polymorphic types in DFAs/NFAs?
// A: Subset construction. Here the state type for the DFA is
//    sets of states.


def subset[A, C](nfa: NFA[A, C]) : DFA[Set[A], C] = {
  DFA(nfa.starts, 
      { case (qs, c) => nfa.nexts(qs, c) }, 
      _.exists(nfa.fins))
}

subset(nfa).accepts("aa".toList)             // false
subset(nfa).accepts("aaaaa".toList)          // false
subset(nfa).accepts("aaaaab".toList)         // true
subset(nfa).accepts("aaaaabbb".toList)       // true
subset(nfa).accepts("aaaaabbbaaa".toList)    // false
subset(nfa).accepts("ac".toList)             // false


// Laziness with style
//=====================

// The concept of lazy evaluation doesn’t really 
// exist in non-functional languages. C-like languages
// are (sort of) strict. To see the difference, consider

def square(x: Int) = x * x

square(42 + 8)

// This is called "strict evaluation".

// In contrast say we have a pretty expensive operation:

def peop(n: BigInt): Boolean = peop(n + 1) 

val a = "foo"
val b = "foo"

if (a == b || peop(0)) println("true") else println("false")

// This is called "lazy evaluation":
// you delay compuation until it is really 
// needed. Once calculated though, the result
// does not need to be re-calculated.

// A useful example is

def time_needed[T](i: Int, code: => T) = {
  val start = System.nanoTime()
  for (j <- 1 to i) code
  val end = System.nanoTime()
  f"${(end - start) / (i * 1.0e9)}%.6f secs"
}

// A slightly less obvious example: Prime Numbers.
// (I do not care how many) primes: 2, 3, 5, 7, 9, 11, 13 ....

def generatePrimes (s: LazyList[Int]): LazyList[Int] =
  s.head #:: generatePrimes(s.tail.filter(_ % s.head != 0))

val primes = generatePrimes(LazyList.from(2))

// the first 10 primes
primes.take(100).toList

time_needed(1, primes.filter(_ > 100).take(3000).toList)
time_needed(1, primes.filter(_ > 100).take(3000).toList)

// A Stream (LazyList) of successive numbers:

LazyList.from(2).take(10)
LazyList.from(2).take(10).force

// An Iterative version of the Fibonacci numbers
def fibIter(a: BigInt, b: BigInt): LazyList[BigInt] =
  a #:: fibIter(b, a + b)


fibIter(1, 1).take(10).force
fibIter(8, 13).take(10).force

fibIter(1, 1).drop(10000).take(1)
fibIter(1, 1).drop(10000).take(1).force


// LazyLists are good for testing


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

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*


// 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))
}

given Conversion[String, Rexp] = (s => charlist2rexp(s.toList))

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



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

// Task: enumerate exhaustively regular expressions
// starting from small ones towards bigger ones.

// 1st idea: enumerate them all in a Set
// up to a level

def enuml(l: Int, s: String) : Set[Rexp] = l match {
  case 0 => Set(ZERO, ONE) ++ s.map(CHAR).toSet
  case n =>  
    val rs = enuml(n - 1, s)
    rs ++
    (for (r1 <- rs; r2 <- rs) yield ALT(r1, r2)) ++
    (for (r1 <- rs; r2 <- rs) yield SEQ(r1, r2)) ++
    (for (r1 <- rs) yield STAR(r1))
}

enuml(1, "a")
enuml(1, "a").size
enuml(2, "a").size
enuml(3, "a").size 
enuml(4, "a").size // out of heap space


def enum(rs: LazyList[Rexp]) : LazyList[Rexp] = 
  rs #::: enum( (for (r1 <- rs; r2 <- rs) yield ALT(r1, r2)) #:::
                (for (r1 <- rs; r2 <- rs) yield SEQ(r1, r2)) #:::
                (for (r1 <- rs) yield STAR(r1)) )


enum(LazyList(ZERO, ONE, CHAR('a'), CHAR('b'))).take(200).force
enum(LazyList(ZERO, ONE, CHAR('a'), CHAR('b'))).take(5_000_000).force // out of memory


def depth(r: Rexp) : Int = r match {
  case ZERO => 0
  case ONE => 0
  case CHAR(_) => 0
  case ALT(r1, r2) => Math.max(depth(r1), depth(r2)) + 1
  case SEQ(r1, r2) => Math.max(depth(r1), depth(r2)) + 1 
  case STAR(r1) => depth(r1) + 1
}


val is = 
  (enum(LazyList(ZERO, ONE, CHAR('a'), CHAR('b')))
    .dropWhile(depth(_) < 3)
    .take(10).foreach(println))













// The End ... Almost Christmas
//===============================

// I hope you had fun!

// A function should do one thing, and only one thing.

// Make your variables immutable, unless there's a good 
// reason not to. Usually there is not.

// I did it once, but this is actually not a good reason:
// generating new labels:

var counter = -1

def Fresh(x: String) = {
  counter += 1
  x ++ "_" ++ counter.toString()
}

Fresh("x")
Fresh("x")



// I think you can be productive on Day 1, but the 
// language is deep.
//
// http://scalapuzzlers.com
//
// http://www.latkin.org/blog/2017/05/02/when-the-scala-compiler-doesnt-help/

val two   = 0.2
val one   = 0.1
val eight = 0.8
val six   = 0.6

two - one == one
eight - six == two
eight - six


// problems about equality and type-errors

List(1, 2, 3).contains("your cup")   // should not compile, but retruns false

List(1, 2, 3) == Vector(1, 2, 3)     // again should not compile, but returns true