// Scala Lecture 1
//=================



// Value assignments
// (their names should be lower case)
//====================================


val x = 42
val y = 3 + 4 
val z = x / y
val x = 70
print(z)


// (you cannot reassign values: z = 9 will give an error)
//var z = 9
//z = 10

// Hello World
//=============

// an example of a stand-alone Scala file
// (in the assignments you must submit a plain Scala script)

object Hello extends App { 
  println("hello world")
}

// can then be called with
//
// $> scalac hello-world.scala
// $> scala Hello
//
// $> java -cp /usr/local/src/scala/lib/scala-library.jar:. Hello



// Collections
//=============

List(1,2,3,1)
Set(1,2,3,1)

// picking an element in a list
val lst = List(1, 2, 3, 1)

lst(0)
lst(2)

// head and tail
lst.head
lst.tail

// some alterative syntax for lists

Nil     // empty list

1 :: 2 :: 3 :: Nil
List(1, 2, 3) ::: List(4, 5, 6)

// also
List(1, 2, 3) ++ List(3, 6, 5)
Set(1, 2, 3) ++ Set(3, 6, 5)

// ranges
1 to 10
(1 to 10).toList
(1 to 10).toList.toString

(1 until 10).toList


// Equality in Scala is structural
//=================================
val a = "Dave2"
val b = "Dave"

if (a == b) println("Equal") else println("Unequal")

Set(1,2,3) == Set(3,1,2)
List(1,2,3) == List(3,1,2)


// this applies to "concrete" values...pretty much 
// everything; but for example you cannot compare 
// functions (later), and also not arrays

Array(1) == Array(1)


// Printing/Strings
//==================

println("test")

val tst = "This is a " ++ "test" 

print(tst) 
println(tst) 

val lst = List(1,2,3,1)

println(lst.toString)

println(lst.mkString)
println(lst.mkString(","))

// some methods take more than one argument

println(lst.mkString("{", ",", "}"))

// (in this case .mkString can take no, one, 
// or three arguments...this has to do with
// default arguments)


// Conversion methods
//====================

List(1,2,3,1).toString
List(1,2,3,1).toSet

"hello".toList
"hello".toSet


1.toDouble

1   // an Int
1L  // a Long
1F  // a Float
1D  // a Double

// useful list methods on lists
//==============================

List(1,2,3,4).length
List(1,2,3,4).reverse
List(1,2,3,4).max
List(1,2,3,4).min
List(1,2,3,4).sum
List(1,2,3,4).take(2).sum
List(1,2,3,4).drop(2).sum
List(1,2,3,4,3).indexOf(3)

"1,2,3,4,5".split(",").mkString("\n")
"1,2,3,4,5".split(",").toList
"1,2,3,4,5".split(",3,").mkString("\n")

"abcdefg".startsWith("abc")


// Types (see slide)
//===================

/* Scala is a strongly typed language
 
 * base types

    Int, Long, BigInt, Float, Double
    String, Char
    Boolean...

 * compound types 

    List[Int]
    Set[Double]
    Pairs: (Int, String)        
    List[(BigInt, String)]
    Option[Int]

 * user-defined types (later)

*/


// you can make the type of a value explicit
val name = "bob"


// type errors
math.sqrt("64".toDouble)

// produces
//
// error: type mismatch;
// found   : String("64")
// required: Double
// math.sqrt("64")


// Pairs/Tuples
//==============

val p = (1, "one")
p._1
p._2

val t = (4,1,2,3)
t._4


List(("one", 1), ("two", 2), ("three", 3))


// Function Definitions
//======================


def incr(x: Int) : Int = x + 1
def double(x: Int) : Int = x + x
def square(x: Int) : Int = x * x

def str(x: Int) : String = x.toString


incr(3)
double(4)
square(6)
str(3)


// The general scheme for a function: you have to give a 
// type to each argument and a return type of the function
//
//  def fname(arg1: ty1, arg2: ty2,..., argn: tyn): rty = {
//    
//  }



// If-Conditionals
//=================

// - Scala does not have a then-keyword
// - !!both if-else branches need to be present!!

def fact(n: Int) : Int = 
  if (n == 0) 1 else n * fact(n - 1)

fact(5)
fact(150)

/* boolean operators
 
   ==     equals
   !=     not equals
   !      not
   && ||  and, or
*/



def fib(n: Int) : Int = {
  if (n == 0) 1 else
    if (n == 1) 1 else fib(n - 1) + fib(n - 2)
}

fib(9)




//gcd - Euclid's algorithm

def gcd(a: Int, b: Int) : Int = {
  if (b == 0) a 
  else  gcd(b, a % b)
}

gcd(48, 18)


def power(x: Int, n: Int) : Int =
  if (n == 0) 1 else x * power(x, n - 1) 

power(5, 5)

// BTW: no returns!!
// "last" line (expression) in a function determines the 
// result

def average(xs: List[Int]) : Int = {
  if (xs.length == 0) 0 
  else xs.sum / xs.length
}

average(List())



// For-Comprehensions (not For-Loops)
//====================================

val lst = (1 to 10).toList
for (n <- lst) yield n * n 


for (n <- lst) yield { 
  square(n) + double(n)
}

for (n <- (1 to 10).toList; 
     m <- (1 to 5).toList) yield (n, m, n * m)


// you can assign the result of a for-comprehension
// to a value
val mult_table = 
  for (n <- (1 to 10).toList; 
       m <- (1 to 10).toList) yield n * m

println(mult_table.mkString)
mult_table.sliding(10,10).mkString("\n")

// for-comprehensions also work for other
// collections

for (n <- Set(10,12,4,5,7,8,10)) yield n * n

for (n <- (1 to 10)) yield {
  n * n  
}

// with if-predicates / filters

if (1 == 2) "a" else "b"

for (n <- (1 to 3).toList; 
     m <- (1 to 3).toList;
     if (n + m) % 2 == 0) yield (n, m)


// with patterns

val lst = List((1, 4), (2, 3), (3, 2), (4, 1))

for ((m, n) <- lst) yield m + n 

for (p <- lst) yield p._1 + p._2 


// general pattern of for-yield 
// (yield can be several lines)

for (pat <- ...) yield {
  // potentially complicated
  // calculation of a result
}

// For without yield
//===================

// with only a side-effect (no list is produced),
// has no "yield"

for (n <- (1 to 10).toList) println(n * n)

for (n <- (1 to 10).toList) yield n * n

// BTW: a roundabout way of printing out a list, say
val lst = ('a' to 'm').toList

for (i <- (0 until lst.length)) println(lst(i))

// Why not just? Why making your life so complicated?
for (c <- lst) println(c)



// Functions producing multiple outputs
//======================================

def get_ascii(c: Char) : (Char, Int) = 
  (c, c.toInt)

get_ascii('a')


// .maxBy, sortBy with pairs
def get_length(s: String) : (String, Int) = 
  (s, s.length) 

val lst = List("zero", "one", "two", "three", "four", "ten")
val strs = for (s <- lst) yield get_length(s)

strs.sortBy(_._2)
strs.sortBy(_._1)

strs.maxBy(_._2)
strs.maxBy(_._1)





// Aside: concurrency 
// scala -Yrepl-class-based -cp scala-parallel-collections_2.13-0.2.0.jar 

for (n <- (1 to 10)) println(n)

import scala.collection.parallel.CollectionConverters._

for (n <- (1 to 10).par) println(n)


// for measuring time
def time_needed[T](n: Int, code: => T) = {
  val start = System.nanoTime()
  for (i <- (0 to n)) code
  val end = System.nanoTime()
  (end - start) / 1.0e9
}

val list = (1 to 1000000).toList
time_needed(10, for (n <- list) yield n + 42)
time_needed(10, for (n <- list.par) yield n + 42)

// ...but par does not make everything faster

list.sum
list.par.sum

time_needed(10, list.sum)
time_needed(10, list.par.sum)


// Mutable vs Immutable
//======================
//
// Remember:
// - no vars, no ++i, no +=
// - no mutable data-structures (no Arrays, no ListBuffers)

// But what the heck....lets try to count to 1 Mio in parallel

var cnt = 0

for(i <- (1 to 1000000).par) cnt += 1

println(s"Should be 1 Mio: $cnt")



// Or
// Q: Count how many elements are in the intersections of 
//    two sets?
// A; IMPROPER WAY (mutable counter)

def count_intersection(A: Set[Int], B: Set[Int]) : Int = {
  var count = 0
  for (x <- A.par; if (B contains x)) count += 1 
  count
}

val A = (0 to 999).toSet
val B = (0 to 999 by 4).toSet

count_intersection(A, B)

// but do not try to add .par to the for-loop above


//propper parallel version
def count_intersection2(A: Set[Int], B: Set[Int]) : Int = 
  A.par.count(x => B contains x)

count_intersection2(A, B)


//another bad example
def test() = {
  var cnt = 0
  for(i <- (1 to 1000000).par) cnt += 1
  println(cnt)
}

test()



// Regular Expressions (the built in ones)

val s = """Any so-called "politician" should respect a vote."""
print(s)

print("""foo""")

val reg = """\d+""".r

reg.findAllIn("bbbbaaabbbaaaccc").toList
reg.replaceAllIn("bbbbaaabbbaaaccc", "*")
reg.replaceAllIn("bbbb0aaa1bbba2aac3cc", "_")
reg.replaceAllIn("bbbb00aaa11bbba232aac33cc", "_")


// Further Information
//=====================

// The Scala homepage and general information is at
//
//  http://www.scala-lang.org
//	http://docs.scala-lang.org
//
//
// It should be fairly easy to install the Scala binary and
// run Scala on the commandline. People also use Scala with 
// Vim and Jedit. I currently settled on VS Code
//
//   https://code.visualstudio.com
//
// There are also plugins for Eclipse and IntelliJ - YMMV.
// Finally there are online editors specifically designed for 
// running Scala applications (but do not blame me if you lose 
// all what you typed in):
//
//   https://scalafiddle.io 
//   https://scastie.scala-lang.org
//
//
//
// Scala Library Docs
//====================
//
//  http://www.scala-lang.org/api/current/
//
// Scala Tutorials
//
//  http://docs.scala-lang.org/tutorials/
//
// There are also a massive number of Scala tutorials on youtube
// and there are tons of books and free material. Google is your 
// friend.