playback: add shuffle order impl

Add a currently-unused ShuffleOrder implementation with fixes for media
item insertion.

This will be used as the basis of the new gapless playback system,
assuming all else goes well.

app/src/main/java/org/oxycblt/auxio/playback/system/BetterShuffleOrder.kt

@@ -0,0 +1,125 @@
+/*
+ * Copyright (c) 2017 Auxio Project
+ * SaneShuffleOrder.kt is part of Auxio.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.oxycblt.auxio.playback.system
+
+import androidx.media3.common.C
+import androidx.media3.exoplayer.source.ShuffleOrder
+import java.util.*
+
+/**
+ * A ShuffleOrder that fixes the poorly defined default implementation of cloneAndInsert. Whereas
+ * the default implementation will randomly spread out added media items, this implementation will
+ * insert them in the order they are added contiguously.
+ *
+ * @author media3 team, Alexander Capehart (OxygenCobalt)
+ */
+class BetterShuffleOrder
+private constructor(private val shuffled: IntArray, private val random: Random) : ShuffleOrder {
+    private val indexInShuffled: IntArray = IntArray(shuffled.size)
+
+    /**
+     * Creates an instance with a specified length.
+     *
+     * @param length The length of the shuffle order.
+     */
+    constructor(length: Int) : this(length, Random())
+
+    constructor(length: Int, random: Random) : this(createShuffledList(length, random), random)
+
+    init {
+        for (i in shuffled.indices) {
+            indexInShuffled[shuffled[i]] = i
+        }
+    }
+
+    override fun getLength(): Int {
+        return shuffled.size
+    }
+
+    override fun getNextIndex(index: Int): Int {
+        var shuffledIndex = indexInShuffled[index]
+        return if (++shuffledIndex < shuffled.size) shuffled[shuffledIndex] else C.INDEX_UNSET
+    }
+
+    override fun getPreviousIndex(index: Int): Int {
+        var shuffledIndex = indexInShuffled[index]
+        return if (--shuffledIndex >= 0) shuffled[shuffledIndex] else C.INDEX_UNSET
+    }
+
+    override fun getLastIndex(): Int {
+        return if (shuffled.isNotEmpty()) shuffled[shuffled.size - 1] else C.INDEX_UNSET
+    }
+
+    override fun getFirstIndex(): Int {
+        return if (shuffled.isNotEmpty()) shuffled[0] else C.INDEX_UNSET
+    }
+
+    override fun cloneAndInsert(insertionIndex: Int, insertionCount: Int): ShuffleOrder {
+        val newShuffled = IntArray(shuffled.size + insertionCount)
+        val pivot = indexInShuffled[insertionIndex]
+        for (i in shuffled.indices) {
+            var currentIndex = shuffled[i]
+            if (currentIndex > insertionIndex) {
+                currentIndex += insertionCount
+            }
+
+            if (i <= pivot) {
+                newShuffled[i] = currentIndex
+            } else if (i > pivot) {
+                newShuffled[i + insertionCount] = currentIndex
+            }
+        }
+        for (i in 0 until insertionCount) {
+            newShuffled[pivot + i + 1] = insertionIndex + i + 1
+        }
+        return BetterShuffleOrder(newShuffled, Random(random.nextLong()))
+    }
+
+    override fun cloneAndRemove(indexFrom: Int, indexToExclusive: Int): ShuffleOrder {
+        val numberOfElementsToRemove = indexToExclusive - indexFrom
+        val newShuffled = IntArray(shuffled.size - numberOfElementsToRemove)
+        var foundElementsCount = 0
+        for (i in shuffled.indices) {
+            if (shuffled[i] in indexFrom until indexToExclusive) {
+                foundElementsCount++
+            } else {
+                newShuffled[i - foundElementsCount] =
+                    if (shuffled[i] >= indexFrom) shuffled[i] - numberOfElementsToRemove
+                    else shuffled[i]
+            }
+        }
+        return BetterShuffleOrder(newShuffled, Random(random.nextLong()))
+    }
+
+    override fun cloneAndClear(): ShuffleOrder {
+        return BetterShuffleOrder(0, Random(random.nextLong()))
+    }
+
+    companion object {
+        private fun createShuffledList(length: Int, random: Random): IntArray {
+            val shuffled = IntArray(length)
+            for (i in 0 until length) {
+                val swapIndex = random.nextInt(i + 1)
+                shuffled[i] = shuffled[swapIndex]
+                shuffled[swapIndex] = i
+            }
+            return shuffled
+        }
+    }
+}