Add BufferedFileChannel

Similar to the equivalent class in the client.

Signed-off-by: Graham <gpe@openrs2.dev>

cache/src/main/java/dev/openrs2/cache/BufferedFileChannel.kt

@@ -1,53 +1,300 @@
 package dev.openrs2.cache
 
 import io.netty.buffer.ByteBuf
+import io.netty.buffer.ByteBufAllocator
 import java.io.Closeable
 import java.io.EOFException
 import java.io.Flushable
 import java.nio.channels.FileChannel
+import kotlin.math.max
+import kotlin.math.min
 
-// TODO(gpe): actually implement buffering
 class BufferedFileChannel(
-    private val channel: FileChannel
+    private val channel: FileChannel,
+    readBufferSize: Int,
+    writeBufferSize: Int,
+    alloc: ByteBufAllocator = ByteBufAllocator.DEFAULT
 ) : Flushable, Closeable {
+    private var size = channel.size()
+
+    private val readBuffer = alloc.buffer(readBufferSize, readBufferSize)
+    private var readPos = -1L
+
+    private val writeBuffer = alloc.buffer(writeBufferSize, writeBufferSize)
+    private var writePos = -1L
+
     fun read(pos: Long, dest: ByteBuf, len: Int) {
+        require(pos >= 0)
         require(len <= dest.writableBytes())
 
+        val originalDestIndex = dest.writerIndex()
+
         var off = pos
         var remaining = len
 
+        /*
+         * Service the whole read from the write buffer, if we can. This code
+         * isn't necessary, but it is more optimal than following the whole
+         * sequence of reads below.
+         */
+        val writeLen = writeBuffer.readableBytes()
+        if (writePos != -1L && off >= writePos && off + remaining <= writePos + writeLen) {
+            val copyOff = (off - writePos).toInt()
+            dest.writeBytes(writeBuffer, copyOff, remaining)
+            return
+        }
+
+        // Service the first part of the read from the read buffer.
+        if (readPos != -1L && off >= readPos && off < readPos + readBuffer.readableBytes()) {
+            val copyOff = (off - readPos).toInt()
+            val copyLen = min(readBuffer.readableBytes() - copyOff, remaining)
+
+            dest.writeBytes(readBuffer, copyOff, copyLen)
+
+            off += copyLen
+            remaining -= copyLen
+        }
+
+        if (remaining > readBuffer.capacity()) {
+            /*
+             * If the remaining part of the read is larger than the read
+             * buffer, read directly from the file into the destination buffer.
+             */
             while (remaining > 0) {
                 val n = dest.writeBytes(channel, off, remaining)
                 if (n == -1) {
-                throw EOFException()
+                    break
                 }
                 off += n
                 remaining -= n
             }
+        } else if (remaining > 0) {
+            /*
+             * Otherwise clear and repopulate the entire read buffer from the
+             * current position, then copy into the destination buffer.
+             */
+            fill(off)
+
+            val copyLen = min(readBuffer.readableBytes(), remaining)
+
+            dest.writeBytes(readBuffer, 0, copyLen)
+
+            off += copyLen
+            remaining -= copyLen
+        }
+
+        if (writePos != -1L) {
+            /*
+             * If an unflushed write extended the length of the file, fill in
+             * the gap between the current position and the write position with
+             * zeroes to reflect what the filesystem would do.
+             */
+            if (off < writePos && remaining > 0) {
+                val zeroLen = min((writePos - off).toInt(), remaining)
+
+                dest.writeZero(zeroLen)
+
+                off += zeroLen
+                remaining -= zeroLen
+            }
+
+            /*
+             * If a subset of the write buffer overlaps with a subset of the
+             * destination buffer, overwrite that subset of the destination
+             * buffer with the write buffer as the write buffer must take
+             * precedence over the read buffer.
+             */
+            val start = if (writePos >= pos && writePos < pos + len) {
+                writePos
+            } else if (pos >= writePos && pos < writePos + writeLen) {
+                pos
+            } else {
+                -1L
+            }
+
+            val end = if (writePos + writeLen > pos && writePos + writeLen <= pos + len) {
+                writePos + writeLen
+            } else if (pos + len > writePos && pos + len <= writePos + writeLen) {
+                pos + len
+            } else {
+                -1L
+            }
+
+            if (start != -1L && end != -1L && start < end) {
+                val destIndex = originalDestIndex + (start - pos).toInt()
+
+                val copyOff = (start - writePos).toInt()
+                val copyLen = (end - start).toInt()
+
+                dest.setBytes(destIndex, writeBuffer, copyOff, copyLen)
+
+                /*
+                 * If we filled in any remaining bytes in the destination
+                 * buffer from the write buffer then adjust the indexes to take
+                 * that into account.
+                 */
+                if (end > off) {
+                    val n = (end - off).toInt()
+
+                    dest.writerIndex(dest.writerIndex() + n)
+
+                    off += n
+                    remaining -= n
+                }
+            }
+        }
+
+        if (remaining > 0) {
+            throw EOFException()
+        }
+    }
+
+    private fun fill(pos: Long) {
+        require(pos >= 0)
+
+        readBuffer.clear()
+        readPos = pos
+
+        var off = pos
+        while (readBuffer.isWritable) {
+            val n = readBuffer.writeBytes(channel, off, readBuffer.writableBytes())
+            if (n == -1) {
+                break
+            }
+            off += n
+        }
     }
 
     fun write(pos: Long, src: ByteBuf, len: Int) {
+        require(pos >= 0)
+        require(len <= src.readableBytes())
+
+        size = max(size, pos + len.toLong())
+
+        var off = pos
+        var remaining = len
+
+        /*
+         * If the start of the write doesn't overlap with the write buffer,
+         * flush the existing write buffer.
+         */
+        if (writePos != -1L && (off < writePos || off > writePos + writeBuffer.readableBytes())) {
+            flush()
+        }
+
+        /*
+         * If the start of the write does overlap with the write buffer
+         * (implicit due to the if condition and flush() call above) and the
+         * end of the write runs beyond the end of the write buffer, overwrite
+         * the relevant part of the write buffer with the start of the source
+         * buffer and then flush the whole write buffer.
+         */
+        if (writePos != -1L && off + remaining > writePos + writeBuffer.capacity()) {
+            val copyOff = (off - writePos).toInt()
+            val copyLen = writeBuffer.capacity() - copyOff
+
+            src.readBytes(writeBuffer, copyOff, copyLen)
+
+            off += copyLen
+            remaining -= copyLen
+
+            writeBuffer.writerIndex(writeBuffer.capacity())
+
+            flush()
+        }
+
+        if (remaining > writeBuffer.capacity()) {
+            /*
+             * If the remaining part of the write is longer than the write
+             * buffer, write directly to the underlying file.
+             */
+            val originalSrcIndex = src.readerIndex()
+            writeFully(off, src, remaining)
+
+            /*
+             * If the write overlaps with the read buffer, update the relevant
+             * portion of the read buffer. (As we bypassed the write buffer, we
+             * can't rely on the write buffer taking precedence over the read
+             * buffer.)
+             */
+            val readLen = readBuffer.readableBytes()
+
+            val start = if (off >= readPos && off < readPos + readLen) {
+                off
+            } else if (readPos >= off && readPos < off + remaining) {
+                readPos
+            } else {
+                -1L
+            }
+
+            val end = if (off + remaining > readPos && off + remaining <= readPos + readLen) {
+                off + remaining
+            } else if (readPos + readLen > off && readPos + readLen <= off + remaining) {
+                readPos + readLen
+            } else {
+                -1L
+            }
+
+            if (start != -1L && end != -1L && start < end) {
+                val srcIndex = originalSrcIndex + (start - off).toInt()
+
+                val copyOff = (start - readPos).toInt()
+                val copyLen = (end - start).toInt()
+
+                src.getBytes(srcIndex, readBuffer, copyOff, copyLen)
+            }
+        } else if (remaining > 0) {
+            // Otherwise write to the write buffer.
+            if (writePos == -1L) {
+                writePos = off
+            }
+
+            val copyOff = (off - writePos).toInt()
+
+            src.readBytes(writeBuffer, copyOff, remaining)
+
+            off += remaining
+
+            // Increase write buffer length if necessary.
+            val newWriteLen = (off - writePos).toInt()
+            if (newWriteLen > writeBuffer.readableBytes()) {
+                writeBuffer.writerIndex(newWriteLen)
+            }
+        }
+    }
+
+    private fun writeFully(pos: Long, src: ByteBuf, len: Int) {
+        require(pos >= 0)
         require(len <= src.readableBytes())
 
         var off = pos
         var remaining = len
 
         while (remaining > 0) {
-            val n = src.readBytes(channel, off, remaining)
+            val n = src.readBytes(channel, off, len)
             off += n
             remaining -= n
         }
     }
 
     fun size(): Long {
-        return channel.size()
+        return size
     }
 
     override fun flush() {
-        // empty
+        if (writePos != -1L) {
+            writeFully(writePos, writeBuffer, writeBuffer.readableBytes())
+            writeBuffer.clear()
+            writePos = -1L
+        }
     }
 
     override fun close() {
+        flush()
+
         channel.close()
+
+        readBuffer.release()
+        writeBuffer.release()
     }
 }

cache/src/main/java/dev/openrs2/cache/DiskStore.kt

@@ -109,7 +109,12 @@ class DiskStore private constructor(
             return index
         }
 
-        val newIndex = BufferedFileChannel(FileChannel.open(indexPath(root, archive), CREATE, READ, WRITE))
+        val newIndex = BufferedFileChannel(
+            FileChannel.open(indexPath(root, archive), CREATE, READ, WRITE),
+            INDEX_BUFFER_SIZE,
+            INDEX_BUFFER_SIZE,
+            alloc
+        )
         indexes[archive] = newIndex
         return newIndex
     }
@@ -444,6 +449,8 @@ class DiskStore private constructor(
         private const val MAX_BLOCK = (1 shl 24) - 1
 
         private val TEMP_BUFFER_SIZE = max(INDEX_ENTRY_SIZE, max(BLOCK_HEADER_SIZE, EXTENDED_BLOCK_HEADER_SIZE))
+        private const val INDEX_BUFFER_SIZE = INDEX_ENTRY_SIZE * 1000
+        private const val DATA_BUFFER_SIZE = BLOCK_SIZE * 10
 
         private fun dataPath(root: Path): Path {
             return root.resolve("main_file_cache.dat2")
@@ -454,22 +461,42 @@ class DiskStore private constructor(
         }
 
         fun open(root: Path, alloc: ByteBufAllocator = ByteBufAllocator.DEFAULT): Store {
-            val data = BufferedFileChannel(FileChannel.open(dataPath(root), READ, WRITE))
+            val data = BufferedFileChannel(
+                FileChannel.open(dataPath(root), READ, WRITE),
+                DATA_BUFFER_SIZE,
+                DATA_BUFFER_SIZE,
+                alloc
+            )
+
             val archives = Array(Store.MAX_ARCHIVE + 1) { archive ->
                 val path = indexPath(root, archive)
                 if (Files.exists(path)) {
-                    BufferedFileChannel(FileChannel.open(path, READ, WRITE))
+                    BufferedFileChannel(
+                        FileChannel.open(path, READ, WRITE),
+                        INDEX_BUFFER_SIZE,
+                        INDEX_BUFFER_SIZE,
+                        alloc
+                    )
                 } else {
                     null
                 }
             }
+
             return DiskStore(root, data, archives, alloc)
         }
 
         fun create(root: Path, alloc: ByteBufAllocator = ByteBufAllocator.DEFAULT): Store {
             Files.createDirectories(root)
-            val data = BufferedFileChannel(FileChannel.open(dataPath(root), CREATE, READ, WRITE))
+
+            val data = BufferedFileChannel(
+                FileChannel.open(dataPath(root), CREATE, READ, WRITE),
+                DATA_BUFFER_SIZE,
+                DATA_BUFFER_SIZE,
+                alloc
+            )
+
             val archives = Array<BufferedFileChannel?>(Store.MAX_ARCHIVE + 1) { null }
+
             return DiskStore(root, data, archives, alloc)
         }
     }

cache/src/test/java/dev/openrs2/cache/BufferedFileChannelTest.kt

@@ -0,0 +1,559 @@
+package dev.openrs2.cache
+
+import com.google.common.jimfs.Configuration
+import com.google.common.jimfs.Jimfs
+import dev.openrs2.buffer.use
+import io.netty.buffer.Unpooled
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+import java.io.EOFException
+import java.nio.channels.FileChannel
+import java.nio.file.Files
+import java.nio.file.StandardOpenOption.CREATE
+import java.nio.file.StandardOpenOption.READ
+import java.nio.file.StandardOpenOption.WRITE
+import kotlin.test.assertEquals
+
+object BufferedFileChannelTest {
+    @Test
+    fun testEmpty() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                assertEquals(0, channel.size())
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWrite() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(0, buf, buf.readableBytes())
+                }
+
+                assertEquals(7, channel.size())
+            }
+
+            Unpooled.wrappedBuffer(Files.readAllBytes(path)).use { actual ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { expected ->
+                    assertEquals(expected, actual)
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteOverlapStart() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(1, buf.slice(), buf.readableBytes())
+                    channel.write(0, buf.slice(), buf.readableBytes())
+                }
+            }
+
+            Unpooled.wrappedBuffer(Files.readAllBytes(path)).use { actual ->
+                Unpooled.wrappedBuffer("OpenRS22".toByteArray()).use { expected ->
+                    assertEquals(expected, actual)
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteOverlapEnd() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(0, buf.slice(), buf.readableBytes())
+                    channel.write(1, buf.slice(), buf.readableBytes())
+                }
+            }
+
+            Unpooled.wrappedBuffer(Files.readAllBytes(path)).use { actual ->
+                Unpooled.wrappedBuffer("OOpenRS2".toByteArray()).use { expected ->
+                    assertEquals(expected, actual)
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteAdjacent() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(0, buf.slice(), buf.readableBytes())
+                    channel.write(7, buf.slice(), buf.readableBytes())
+                }
+            }
+
+            Unpooled.wrappedBuffer(Files.readAllBytes(path)).use { actual ->
+                Unpooled.wrappedBuffer("OpenRS2OpenRS2".toByteArray()).use { expected ->
+                    assertEquals(expected, actual)
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteNoOverlap() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(0, buf.slice(), buf.readableBytes())
+                    channel.write(8, buf.slice(), buf.readableBytes())
+                }
+            }
+
+            Unpooled.wrappedBuffer(Files.readAllBytes(path)).use { actual ->
+                Unpooled.wrappedBuffer("OpenRS2\u0000OpenRS2".toByteArray()).use { expected ->
+                    assertEquals(expected, actual)
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testUnbufferedWrite() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("Hello, world!".toByteArray()).use { buf ->
+                    channel.write(0, buf, buf.readableBytes())
+                }
+
+                assertEquals(13, channel.size())
+            }
+
+            Unpooled.wrappedBuffer(Files.readAllBytes(path)).use { actual ->
+                Unpooled.wrappedBuffer("Hello, world!".toByteArray()).use { expected ->
+                    assertEquals(expected, actual)
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedRead() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ), 8, 8).use { channel ->
+                Unpooled.buffer(7, 7).use { actual ->
+                    Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { expected ->
+                        channel.read(0, actual, actual.writableBytes())
+                        assertEquals(expected, actual)
+
+                        actual.clear()
+
+                        channel.read(7, actual, actual.writableBytes())
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testUnbufferedRead() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ), 8, 8).use { channel ->
+                Unpooled.buffer(14, 14).use { actual ->
+                    Unpooled.wrappedBuffer("OpenRS2OpenRS2".toByteArray()).use { expected ->
+                        channel.read(0, actual, actual.writableBytes())
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedEof() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.buffer(1, 1).use { buf ->
+                    assertThrows<EOFException> {
+                        channel.read(0, buf, buf.writableBytes())
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testUnbufferedEof() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ), 8, 8).use { channel ->
+                Unpooled.buffer(15, 15).use { buf ->
+                    assertThrows<EOFException> {
+                        channel.read(0, buf, buf.writableBytes())
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testZeroExtension() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer(byteArrayOf(1)).use { buf ->
+                    channel.write(7, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(8, 8).use { actual ->
+                    channel.read(0, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer(byteArrayOf(0, 0, 0, 0, 0, 0, 0, 1)).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteThenRead() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(0, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(7, 7).use { actual ->
+                    channel.read(0, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteThenReadSubsetStart() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(0, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(6, 6).use { actual ->
+                    channel.read(0, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("OpenRS".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteThenReadSubsetEnd() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("OpenRS2".toByteArray()).use { buf ->
+                    channel.write(0, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(6, 6).use { actual ->
+                    channel.read(1, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("penRS2".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteThenReadSuperset() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("Hello".toByteArray()).use { buf ->
+                    channel.write(4, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(14, 14).use { actual ->
+                    channel.read(0, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("OpenHelloenRS2".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteThenReadSupersetStart() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("Hello".toByteArray()).use { buf ->
+                    channel.write(4, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(7, 7).use { actual ->
+                    channel.read(0, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("OpenHel".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteThenReadSupersetEnd() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, CREATE, READ, WRITE), 8, 8).use { channel ->
+                Unpooled.wrappedBuffer("Hello".toByteArray()).use { buf ->
+                    channel.write(4, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(7, 7).use { actual ->
+                    channel.read(7, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("loenRS2".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedWriteThenReadNoOverlap() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ, WRITE), 4, 8).use { channel ->
+                Unpooled.wrappedBuffer("Hello".toByteArray()).use { buf ->
+                    channel.write(4, buf, buf.readableBytes())
+                }
+
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(0, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("Open".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+
+                Unpooled.buffer(5, 5).use { actual ->
+                    channel.read(9, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("enRS2".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedReadThenWrite() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ, WRITE), 4, 0).use { channel ->
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("RS2O".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+
+                Unpooled.wrappedBuffer("ABCD".toByteArray()).use { buf ->
+                    channel.write(4, buf.slice(), buf.readableBytes())
+                }
+
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("ABCD".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedReadThenWriteSubsetStart() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ, WRITE), 4, 0).use { channel ->
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("RS2O".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+
+                Unpooled.wrappedBuffer("ABC".toByteArray()).use { buf ->
+                    channel.write(4, buf.slice(), buf.readableBytes())
+                }
+
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("ABCO".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedReadThenWriteSubsetEnd() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ, WRITE), 4, 0).use { channel ->
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("RS2O".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+
+                Unpooled.wrappedBuffer("BCD".toByteArray()).use { buf ->
+                    channel.write(5, buf.slice(), buf.readableBytes())
+                }
+
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("RBCD".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedReadThenWriteSupersetStart() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ, WRITE), 4, 0).use { channel ->
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("RS2O".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+
+                Unpooled.wrappedBuffer("ZABCD".toByteArray()).use { buf ->
+                    channel.write(3, buf.slice(), buf.readableBytes())
+                }
+
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("ABCD".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testBufferedReadThenWriteSupersetEnd() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val path = fs.getPath("/test.dat")
+
+            Files.write(path, "OpenRS2OpenRS2".toByteArray())
+
+            BufferedFileChannel(FileChannel.open(path, READ, WRITE), 4, 0).use { channel ->
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("RS2O".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+
+                Unpooled.wrappedBuffer("ABCDZ".toByteArray()).use { buf ->
+                    channel.write(4, buf.slice(), buf.readableBytes())
+                }
+
+                Unpooled.buffer(4, 4).use { actual ->
+                    channel.read(4, actual, actual.writableBytes())
+
+                    Unpooled.wrappedBuffer("ABCD".toByteArray()).use { expected ->
+                        assertEquals(expected, actual)
+                    }
+                }
+            }
+        }
+    }
+}