From 6b90f8eb2841aeda7087df993d2a3303106e29c8 Mon Sep 17 00:00:00 2001
From: David North <hobnob@users.noreply.github.com>
Date: Mon, 7 Oct 2024 21:19:49 +0100
Subject: [PATCH] Adds unit tests to assure randomness

Tests now run 3 lots of 200,000,000 rolls of dice and check the
distribution of the results in order to test the assertion made in #750
---
 .../main/scala/indigo/shared/dice/Dice.scala  |  4 +-
 .../scala/indigo/shared/dice/DiceTests.scala  | 81 +++++++++++++++++++
 2 files changed, 82 insertions(+), 3 deletions(-)

diff --git a/indigo/indigo/src/main/scala/indigo/shared/dice/Dice.scala b/indigo/indigo/src/main/scala/indigo/shared/dice/Dice.scala
index 3c26a1063..8342c546f 100644
--- a/indigo/indigo/src/main/scala/indigo/shared/dice/Dice.scala
+++ b/indigo/indigo/src/main/scala/indigo/shared/dice/Dice.scala
@@ -168,9 +168,7 @@ object Dice:
 
       val r: Random = new Random(seed)
 
-      def roll: Int =
-        r.nextInt(sanitise(sides)) + 1
-
+      def roll: Int = roll(sides)
       def roll(sides: Int): Int =
         r.nextInt(sanitise(sides)) + 1
 
diff --git a/indigo/indigo/src/test/scala/indigo/shared/dice/DiceTests.scala b/indigo/indigo/src/test/scala/indigo/shared/dice/DiceTests.scala
index d558ae5a0..923c72a4d 100644
--- a/indigo/indigo/src/test/scala/indigo/shared/dice/DiceTests.scala
+++ b/indigo/indigo/src/test/scala/indigo/shared/dice/DiceTests.scala
@@ -2,6 +2,8 @@ package indigo.shared.dice
 
 import indigo.shared.collections.NonEmptyList
 
+import scala.collection.immutable.SortedMap
+
 @SuppressWarnings(Array("scalafix:DisableSyntax.throw"))
 class DiceTests extends munit.FunSuite {
 
@@ -10,6 +12,8 @@ class DiceTests extends munit.FunSuite {
   def checkDice(roll: Int, to: Int): Boolean =
     roll >= 1 && roll <= to
 
+  def almostEquals(d: Double, d2: Double, p: Double) = (d - d2).abs <= p
+
   test("diceSidesN") {
     val roll: Int = Dice.diceSidesN(1, 0).roll(10)
 
@@ -57,4 +61,81 @@ class DiceTests extends munit.FunSuite {
     assertEquals(actual, expected)
   }
 
+  test("all dice rolls have an approximately uniform distribution") {
+    val diceSides = 64
+    val numRuns   = 200_000_000
+    val dice      = Dice.diceSidesN(diceSides, 0)
+    val generatedNums =
+      Array
+        .range(0, numRuns)
+        .foldLeft(SortedMap[Int, Int]()) { (acc, _) =>
+          val roll = dice.roll
+          acc.updated(roll, acc.getOrElse(roll, 0) + 1)
+        }
+
+    assertEquals(generatedNums.size, diceSides)
+    assertEquals(generatedNums.head._1, 1)
+    assertEquals(generatedNums.last._1, diceSides)
+
+    val expectedDistribution = 1.0 / diceSides
+    generatedNums.foreach { case (num, count) =>
+      val distribution = count.toDouble / numRuns
+      assert(
+        almostEquals(distribution, expectedDistribution, 0.01),
+        s"""The distribution for $num was $distribution, but expected $expectedDistribution"""
+      )
+    }
+  }
+
+  test("all dice rolls in rollRange have an approximately uniform distribution") {
+    val diceSides = 64
+    val halfSides = diceSides / 2
+    val numRuns   = 200_000_000
+    val dice      = Dice.diceSidesN(diceSides, 0)
+    val generatedNums =
+      Array
+        .range(0, numRuns)
+        .foldLeft(SortedMap[Int, Int]()) { (acc, _) =>
+          val roll = dice.rollRange(halfSides, diceSides)
+          acc.updated(roll, acc.getOrElse(roll, 0) + 1)
+        }
+
+    assertEquals(generatedNums.size, halfSides + 1)
+    assertEquals(generatedNums.head._1, halfSides)
+    assertEquals(generatedNums.last._1, diceSides)
+
+    val expectedDistribution = 1.0 / halfSides
+    generatedNums.foreach { case (num, count) =>
+      val distribution = count.toDouble / numRuns
+      assert(
+        almostEquals(distribution, expectedDistribution, 0.01),
+        s"""The distribution for $num was $distribution, but expected $expectedDistribution"""
+      )
+    }
+  }
+
+  test("all dice rolls in rollRange(1, 4) have an approximately uniform distribution") {
+    val numRuns = 200_000_000
+    val dice    = Dice.diceSidesN(4, 0)
+    val generatedNums =
+      Array
+        .range(0, numRuns)
+        .foldLeft(SortedMap[Int, Int]()) { (acc, _) =>
+          val roll = dice.rollRange(1, 4)
+          acc.updated(roll, acc.getOrElse(roll, 0) + 1)
+        }
+
+    assertEquals(generatedNums.size, 4)
+    assertEquals(generatedNums.head._1, 1)
+    assertEquals(generatedNums.last._1, 4)
+
+    val expectedDistribution = 0.25
+    generatedNums.foreach { case (num, count) =>
+      val distribution = count.toDouble / numRuns
+      assert(
+        almostEquals(distribution, expectedDistribution, 0.01),
+        s"""The distribution for $num was $distribution, but expected $expectedDistribution"""
+      )
+    }
+  }
 }