diceware: add Die class and tests

passgen/diceware/api/diceware.api

@@ -0,0 +1,10 @@
+public final class dev/msfjarvis/aps/passgen/diceware/Die {
+	public fun <init> (ILdev/msfjarvis/aps/passgen/diceware/RandomIntGenerator;)V
+	public final fun roll ()I
+	public final fun rollMultiple (I)I
+}
+
+public abstract interface class dev/msfjarvis/aps/passgen/diceware/RandomIntGenerator {
+	public abstract fun get (Lkotlin/ranges/IntRange;)I
+}
+

passgen/diceware/src/main/kotlin/dev/msfjarvis/aps/passgen/diceware/Die.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright © 2014-2021 The Android Password Store Authors. All Rights Reserved.
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package dev.msfjarvis.aps.passgen.diceware
+
+import javax.inject.Inject
+
+/** Basic implementation of a die with configurable number of sides. */
+public class Die
+@Inject
+constructor(
+  private val sides: Int,
+  private val random: RandomIntGenerator,
+) {
+
+  /** Roll the die to return a single number. */
+  public fun roll(): Int {
+    return random.get(1..sides)
+  }
+
+  /**
+   * Roll the die multiple times, concatenating each result to obtain a number with [iterations]
+   * digits.
+   */
+  public fun rollMultiple(iterations: Int): Int {
+    return StringBuilder().apply { repeat(iterations) { append(roll()) } }.toString().toInt()
+  }
+}

passgen/diceware/src/main/kotlin/dev/msfjarvis/aps/passgen/diceware/RandomIntGenerator.kt

@@ -0,0 +1,15 @@
+/*
+ * Copyright © 2014-2021 The Android Password Store Authors. All Rights Reserved.
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package dev.msfjarvis.aps.passgen.diceware
+
+/**
+ * SAM interface that takes in an [IntRange] and returns a randomly chosen [Int] within its bounds.
+ * This is used as a replacement for [kotlin.random.Random] since there is no CSPRNG-backed
+ * implementation of it in the Kotlin stdlib.
+ */
+public fun interface RandomIntGenerator {
+  public fun get(range: IntRange): Int
+}

passgen/diceware/src/test/kotlin/dev/msfjarvis/aps/passgen/diceware/DieTest.kt

@@ -0,0 +1,47 @@
+/*
+ * Copyright © 2014-2021 The Android Password Store Authors. All Rights Reserved.
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package dev.msfjarvis.aps.passgen.diceware
+
+import kotlin.random.Random
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class DieTest {
+
+  /** Pre-seeded [Random] instance to ensure tests are deterministic. */
+  private val random = Random(1_00_000)
+
+  private val intGenerator = RandomIntGenerator { it.random(random) }
+
+  @Test
+  fun test_one_roll() {
+    val die = Die(6, intGenerator)
+    assertEquals(5, die.roll())
+  }
+
+  @Test
+  fun test_multiple_rolls() {
+    val die = Die(6, intGenerator)
+    assertEquals(526242, die.rollMultiple(6))
+  }
+
+  @Test
+  fun test_consecutive_rolls() {
+    val die = Die(6, intGenerator)
+    assertEquals(5, die.roll())
+    assertEquals(2, die.roll())
+    assertEquals(6, die.roll())
+    assertEquals(2, die.roll())
+    assertEquals(4, die.roll())
+    assertEquals(2, die.roll())
+  }
+
+  @Test
+  fun test_100_sides() {
+    val die = Die(100, intGenerator)
+    assertEquals(67, die.roll())
+  }
+}