Implement RingBuffer (526b914e) · Commits · e / os / android_frameworks_base

packages/SystemUI/src/com/android/systemui/util/collection/RingBuffer.kt

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		/*
		* Copyright (C) 2022 The Android Open Source Project
		*
		* Licensed under the Apache License, Version 2.0 (the "License");
		* you may not use this file except in compliance with the License.
		* You may obtain a copy of the License at
		*
		* http://www.apache.org/licenses/LICENSE-2.0
		*
		* Unless required by applicable law or agreed to in writing, software
		* distributed under the License is distributed on an "AS IS" BASIS,
		* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
		* See the License for the specific language governing permissions and
		* limitations under the License.
		*/

		package com.android.systemui.util.collection

		import kotlin.math.max

		/**
		* A simple ring buffer implementation
		*
		* Use [advance] to get the least recent item in the buffer (and then presumably fill it with
		* appropriate data). This will cause it to become the most recent item.
		*
		* As the buffer is used, it will grow, allocating new instances of T using [factory] until it
		* reaches [maxSize]. After this point, no new instances will be created. Instead, the "oldest"
		* instances will be recycled from the back of the buffer and placed at the front.
		*
		* @param maxSize The maximum size the buffer can grow to before it begins functioning as a ring.
		* @param factory A function that creates a fresh instance of T. Used by the buffer while it's
		* growing to [maxSize].
		*/
		class RingBuffer<T>(
		private val maxSize: Int,
		private val factory: () -> T
		) : Iterable<T> {

		private val buffer = MutableList<T?>(maxSize) { null }

		/**
		* An abstract representation that points to the "end" of the buffer. Increments every time
		* [advance] is called and never wraps. Use [indexOf] to calculate the associated index into
		* the backing array. Always points to the "next" available slot in the buffer. Before the
		* buffer has completely filled, the value pointed to will be null. Afterward, it will be the
		* value at the "beginning" of the buffer.
		*
		* This value is unlikely to overflow. Assuming [advance] is called at rate of 100 calls/ms,
		* omega will overflow after a little under three million years of continuous operation.
		*/
		private var omega: Long = 0

		/**
		* The number of items currently stored in the buffer. Calls to [advance] will cause this value
		* to increase by one until it reaches [maxSize].
		*/
		val size: Int
		get() = if (omega < maxSize) omega.toInt() else maxSize

		/**
		* Advances the buffer's position by one and returns the value that is now present at the "end"
		* of the buffer. If the buffer is not yet full, uses [factory] to create a new item.
		* Otherwise, reuses the value that was previously at the "beginning" of the buffer.
		*
		* IMPORTANT: The value is returned as-is, without being reset. It will retain any data that
		* was previously stored on it.
		*/
		fun advance(): T {
		val index = indexOf(omega)
		omega += 1
		val entry = buffer[index] ?: factory().also {
		buffer[index] = it
		}
		return entry
		}

		/**
		* Returns the value stored at [index], which can range from 0 (the "start", or oldest element
		* of the buffer) to [size] - 1 (the "end", or newest element of the buffer).
		*/
		operator fun get(index: Int): T {
		if (index < 0 \|\| index >= size) {
		throw IndexOutOfBoundsException("Index $index is out of bounds")
		}

		// If omega is larger than the maxSize, then the buffer is full, and omega is equivalent
		// to the "start" of the buffer. If omega is smaller than the maxSize, then the buffer is
		// not yet full and our start should be 0. However, in modspace, maxSize and 0 are
		// equivalent, so we can get away with using it as the start value instead.
		val start = max(omega, maxSize.toLong())

		return buffer[indexOf(start + index)]!!
		}

		inline fun forEach(action: (T) -> Unit) {
		for (i in 0 until size) {
		action(get(i))
		}
		}

		override fun iterator(): Iterator<T> {
		return object : Iterator<T> {
		private var position: Int = 0

		override fun next(): T {
		if (position >= size) {
		throw NoSuchElementException()
		}
		return get(position).also { position += 1 }
		}

		override fun hasNext(): Boolean {
		return position < size
		}
		}
		}

		private fun indexOf(position: Long): Int {
		return (position % maxSize).toInt()
		}
		}

packages/SystemUI/tests/src/com/android/systemui/util/collection/RingBufferTest.kt

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		/*
		* Copyright (C) 2022 The Android Open Source Project
		*
		* Licensed under the Apache License, Version 2.0 (the "License");
		* you may not use this file except in compliance with the License.
		* You may obtain a copy of the License at
		*
		* http://www.apache.org/licenses/LICENSE-2.0
		*
		* Unless required by applicable law or agreed to in writing, software
		* distributed under the License is distributed on an "AS IS" BASIS,
		* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
		* See the License for the specific language governing permissions and
		* limitations under the License.
		*/

		package com.android.systemui.util.collection

		import android.testing.AndroidTestingRunner
		import androidx.test.filters.SmallTest
		import com.android.systemui.SysuiTestCase
		import org.junit.Assert.assertEquals
		import org.junit.Assert.assertFalse
		import org.junit.Assert.assertSame
		import org.junit.Assert.assertThrows
		import org.junit.Before
		import org.junit.Test
		import org.junit.runner.RunWith
		import org.mockito.MockitoAnnotations

		@SmallTest
		@RunWith(AndroidTestingRunner::class)
		class RingBufferTest : SysuiTestCase() {

		private val buffer = RingBuffer(5) { TestElement() }

		private val history = mutableListOf<TestElement>()

		@Before
		fun setUp() {
		MockitoAnnotations.initMocks(this)
		}

		@Test
		fun testBarelyFillBuffer() {
		fillBuffer(5)

		assertEquals(0, buffer[0].id)
		assertEquals(1, buffer[1].id)
		assertEquals(2, buffer[2].id)
		assertEquals(3, buffer[3].id)
		assertEquals(4, buffer[4].id)
		}

		@Test
		fun testPartiallyFillBuffer() {
		fillBuffer(3)

		assertEquals(3, buffer.size)

		assertEquals(0, buffer[0].id)
		assertEquals(1, buffer[1].id)
		assertEquals(2, buffer[2].id)

		assertThrows(IndexOutOfBoundsException::class.java) { buffer[3] }
		assertThrows(IndexOutOfBoundsException::class.java) { buffer[4] }
		}

		@Test
		fun testSpinBuffer() {
		fillBuffer(277)

		assertEquals(272, buffer[0].id)
		assertEquals(273, buffer[1].id)
		assertEquals(274, buffer[2].id)
		assertEquals(275, buffer[3].id)
		assertEquals(276, buffer[4].id)
		assertThrows(IndexOutOfBoundsException::class.java) { buffer[5] }

		assertEquals(5, buffer.size)
		}

		@Test
		fun testElementsAreRecycled() {
		fillBuffer(23)

		assertSame(history[4], buffer[1])
		assertSame(history[9], buffer[1])
		assertSame(history[14], buffer[1])
		assertSame(history[19], buffer[1])
		}

		@Test
		fun testIterator() {
		fillBuffer(13)

		val iterator = buffer.iterator()

		for (i in 0 until 5) {
		assertEquals(history[8 + i], iterator.next())
		}
		assertFalse(iterator.hasNext())
		assertThrows(NoSuchElementException::class.java) { iterator.next() }
		}

		@Test
		fun testForEach() {
		fillBuffer(13)
		var i = 8

		buffer.forEach {
		assertEquals(history[i], it)
		i++
		}
		assertEquals(13, i)
		}

		private fun fillBuffer(count: Int) {
		for (i in 0 until count) {
		val elem = buffer.advance()
		elem.id = history.size
		history.add(elem)
		}
		}
		}

		private class TestElement(var id: Int = 0) {
		override fun toString(): String {
		return "{TestElement $id}"
		}
		}
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