ch03 equality for all

Chapter03.scala

package fp_tdd

import org.scalacheck.Prop.forAll
import org.scalacheck.{Gen, Properties}

object Chapter03 extends Properties("Ch03") {
// ======== TODO ========
// $5 + 10 CHF = $10 if rate is 2:1
// Make "amount" private
// Money rounding?
// hashCode
// Equal null
// Equal object
// ======== DONE ========
// 1. $5 * 2 = $10
// 2. Dollar side effect
// 2. times -> `*`
// 3. Equality

// product code -------------------------------------------------------------------
  class Dollar(val amount: Int) {
    override def equals(a: Any): Boolean =
      a.asInstanceOf[Dollar].amount == this.amount

    def *(multiplier: Int): Dollar = Dollar(amount * multiplier)
  }
  object Dollar { def apply(amount: Int) = new Dollar(amount) }

// test code --------------------------------------------------------------------
  val intGen:  Gen[Int]             = Gen.chooseNum(-1000000, 1000000)
  implicit val dollars: Gen[Dollar] = intGen.map(Dollar.apply)

  private def dollar2 = for {
    a <- dollars
    b <- dollars
  } yield (a, b)
  private def dollar3 = for {
    a <- dollars
    b <- dollars
    c <- dollars
  } yield (a, b, c)

  implicit val unique3: Gen[(Dollar, Dollar, Dollar)] =
    dollar3.suchThat { case (a, b, c) => a != b && b != c && c != a }

// properties -------------------------------------------------------------------
  property("$0 * x = $0") = forAll { (x: Int) =>
    Dollar(0) * x == Dollar(0)
  }
  property("$1 * x = $x") = forAll { (x: Int) =>
    Dollar(1) * x == Dollar(x)
  }
  property("$a * b = $b * a") = forAll { (a: Int, b: Int) =>
    Dollar(a) * b == Dollar(b) * a
  }
  property("($a * b) * c = $a * (b * c)") = forAll { (a: Int, b: Int, c: Int) =>
    (Dollar(a) * b) * c == Dollar(a) * (b * c)
  }

  property("$a == $b <=> a == b") = forAll { (a: Int, b: Int) =>
    (Dollar(a) == Dollar(b)) == (a == b)
  }
  property("$a == $b <=> $a.equals($b)") = forAll(dollar2) { case (a, b) =>
    (a equals b) == (a == b)
  }
  property("reflexive law") = forAll(dollars) { (a) =>
    a equals a
  }
  property("symmetric law") = forAll(dollar2) { case (a, b) =>
    (a equals b) == (b equals a)
  }
  property("transitive law") = forAll( for {
    oneOf3 <- unique3.map { case (a, b, c)=> Gen.oneOf(a, b, c) }
    x      <- oneOf3
    y      <- oneOf3.suchThat(_ equals x)
    z      <- oneOf3.retryUntil(_ equals y)
  } yield (x, z)) { case ((x, z)) => x equals z }
}