diff --git a/cache/src/main/kotlin/org/openrs2/cache/Group.kt b/cache/src/main/kotlin/org/openrs2/cache/Group.kt
new file mode 100644
index 0000000000..7aa923cc36
--- /dev/null
+++ b/cache/src/main/kotlin/org/openrs2/cache/Group.kt
@@ -0,0 +1,99 @@
+package org.openrs2.cache
+
+import io.netty.buffer.ByteBuf
+import it.unimi.dsi.fastutil.ints.Int2ObjectAVLTreeMap
+import it.unimi.dsi.fastutil.ints.Int2ObjectSortedMap
+import it.unimi.dsi.fastutil.ints.Int2ObjectSortedMaps
+import org.openrs2.buffer.use
+
+public object Group {
+    public fun unpack(input: ByteBuf, group: Js5Index.Group): Int2ObjectSortedMap<ByteBuf> {
+        require(group.size >= 1)
+
+        val singleEntry = group.singleOrNull()
+        if (singleEntry != null) {
+            return Int2ObjectSortedMaps.singleton(singleEntry.id, input.retain())
+        }
+
+        require(input.isReadable)
+
+        val stripes = input.getUnsignedByte(input.writerIndex() - 1)
+
+        var dataIndex = input.readerIndex()
+        val trailerIndex = input.writerIndex() - (stripes * group.size * 4) - 1
+        require(trailerIndex >= dataIndex)
+
+        input.readerIndex(trailerIndex)
+
+        val lens = IntArray(group.size)
+        for (i in 0 until stripes) {
+            var prevLen = 0
+            for (j in lens.indices) {
+                prevLen += input.readInt()
+                lens[j] += prevLen
+            }
+        }
+
+        input.readerIndex(trailerIndex)
+
+        val files = Int2ObjectAVLTreeMap<ByteBuf>()
+        try {
+            for ((i, entry) in group.withIndex()) {
+                files[entry.id] = input.alloc().buffer(lens[i], lens[i])
+            }
+
+            for (i in 0 until stripes) {
+                var prevLen = 0
+                for (entry in group) {
+                    prevLen += input.readInt()
+                    input.getBytes(dataIndex, files[entry.id], prevLen)
+                    dataIndex += prevLen
+                }
+            }
+
+            check(dataIndex == trailerIndex)
+
+            // consume stripes byte too
+            input.skipBytes(1)
+            check(!input.isReadable)
+
+            files.values.forEach(ByteBuf::retain)
+            return files
+        } finally {
+            files.values.forEach(ByteBuf::release)
+        }
+    }
+
+    // TODO(gpe): support multiple stripes (tricky, as the best sizes are
+    // probably specific to the format we're packing...)
+    public fun pack(files: Int2ObjectSortedMap<ByteBuf>): ByteBuf {
+        require(files.isNotEmpty())
+
+        val first = files.values.first()
+        if (files.size == 1) {
+            return first
+        }
+
+        first.alloc().buffer().use { output ->
+            if (files.values.all { !it.isReadable }) {
+                output.writeByte(0)
+                return output.retain()
+            }
+
+            for (file in files.values) {
+                output.writeBytes(file, file.readerIndex(), file.readableBytes())
+            }
+
+            var prevLen = 0
+            for (file in files.values) {
+                val len = file.readableBytes()
+                output.writeInt(len - prevLen)
+                prevLen = len
+            }
+
+            output.writeByte(1)
+
+            return output.retain()
+        }
+    }
+}
diff --git a/cache/src/test/kotlin/org/openrs2/cache/GroupTest.kt b/cache/src/test/kotlin/org/openrs2/cache/GroupTest.kt
new file mode 100644
index 0000000000..014a4228a7
--- /dev/null
+++ b/cache/src/test/kotlin/org/openrs2/cache/GroupTest.kt
@@ -0,0 +1,180 @@
+package org.openrs2.cache
+
+import io.netty.buffer.ByteBuf
+import io.netty.buffer.Unpooled
+import it.unimi.dsi.fastutil.ints.Int2ObjectAVLTreeMap
+import it.unimi.dsi.fastutil.ints.Int2ObjectSortedMaps
+import org.openrs2.buffer.use
+import org.openrs2.buffer.wrappedBuffer
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+
+object GroupTest {
+    private val index = Js5Index(Js5Protocol.ORIGINAL)
+    private val zeroFiles = index.createOrGet(0)
+    private val oneFile = index.createOrGet(1).apply {
+        createOrGet(1)
+    }
+    private val multipleFiles = index.createOrGet(2).apply {
+        createOrGet(0)
+        createOrGet(1)
+        createOrGet(3)
+    }
+
+    @Test
+    fun testPackEmpty() {
+        assertFailsWith<IllegalArgumentException> {
+            Group.pack(Int2ObjectSortedMaps.emptyMap()).release()
+        }
+    }
+
+    @Test
+    fun testUnpackEmpty() {
+        assertFailsWith<IllegalArgumentException> {
+            val files = Group.unpack(Unpooled.EMPTY_BUFFER, zeroFiles)
+            files.values.forEach(ByteBuf::release)
+        }
+
+        assertFailsWith<IllegalArgumentException> {
+            val files = Group.unpack(Unpooled.EMPTY_BUFFER, multipleFiles)
+            files.values.forEach(ByteBuf::release)
+        }
+    }
+
+    @Test
+    fun testPackSingle() {
+        wrappedBuffer(0, 1, 2, 3).use { buf ->
+            assertEquals(buf, Group.pack(Int2ObjectSortedMaps.singleton(1, buf)))
+        }
+    }
+
+    @Test
+    fun testUnpackSingle() {
+        wrappedBuffer(0, 1, 2, 3).use { buf ->
+            val expected = Int2ObjectSortedMaps.singleton(1, buf)
+            val actual = Group.unpack(buf.slice(), oneFile)
+            try {
+                assertEquals(expected, actual)
+            } finally {
+                actual.values.forEach(ByteBuf::release)
+            }
+        }
+    }
+
+    @Test
+    fun testPackZeroStripes() {
+        val files = Int2ObjectAVLTreeMap<ByteBuf>()
+        files[0] = Unpooled.EMPTY_BUFFER
+        files[1] = Unpooled.EMPTY_BUFFER
+        files[3] = Unpooled.EMPTY_BUFFER
+
+        wrappedBuffer(0).use { expected ->
+            Group.pack(files).use { actual ->
+                assertEquals(expected, actual)
+            }
+        }
+    }
+
+    @Test
+    fun testUnpackZeroStripes() {
+        val expected = Int2ObjectAVLTreeMap<ByteBuf>()
+        expected[0] = Unpooled.EMPTY_BUFFER
+        expected[1] = Unpooled.EMPTY_BUFFER
+        expected[3] = Unpooled.EMPTY_BUFFER
+
+        wrappedBuffer(0).use { buf ->
+            val actual = Group.unpack(buf, multipleFiles)
+            try {
+                assertEquals(expected, actual)
+            } finally {
+                actual.values.forEach(ByteBuf::release)
+            }
+        }
+    }
+
+    @Test
+    fun testPackOneStripe() {
+        val files = Int2ObjectAVLTreeMap<ByteBuf>()
+        try {
+            files[0] = wrappedBuffer(0, 1, 2)
+            files[1] = wrappedBuffer(3, 4, 5, 6, 7)
+            files[3] = wrappedBuffer(8, 9)
+
+            wrappedBuffer(
+                0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
+                0, 0, 0, 3,
+                0, 0, 0, 2,
+                0xFF.toByte(), 0xFF.toByte(), 0xFF.toByte(), 0xFD.toByte(),
+                1
+            ).use { expected ->
+                Group.pack(files).use { actual ->
+                    assertEquals(expected, actual)
+                }
+            }
+        } finally {
+            files.values.forEach(ByteBuf::release)
+        }
+    }
+
+    @Test
+    fun testUnpackOneStripe() {
+        val expected = Int2ObjectAVLTreeMap<ByteBuf>()
+        try {
+            expected[0] = wrappedBuffer(0, 1, 2)
+            expected[1] = wrappedBuffer(3, 4, 5, 6, 7)
+            expected[3] = wrappedBuffer(8, 9)
+
+            wrappedBuffer(
+                0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
+                0, 0, 0, 3,
+                0, 0, 0, 2,
+                0xFF.toByte(), 0xFF.toByte(), 0xFF.toByte(), 0xFD.toByte(),
+                1
+            ).use { buf ->
+                val actual = Group.unpack(buf, multipleFiles)
+                try {
+                    assertEquals(expected, actual)
+                } finally {
+                    actual.values.forEach(ByteBuf::release)
+                }
+            }
+        } finally {
+            expected.values.forEach(ByteBuf::release)
+        }
+    }
+
+    @Test
+    fun testUnpackMultipleStripes() {
+        val expected = Int2ObjectAVLTreeMap<ByteBuf>()
+        try {
+            expected[0] = wrappedBuffer(0, 1, 2)
+            expected[1] = wrappedBuffer(3, 4, 5, 6, 7)
+            expected[3] = wrappedBuffer(8, 9)
+
+            wrappedBuffer(
+                0, 1,
+                3, 4,
+                8, 9,
+                2,
+                5, 6, 7,
+                0, 0, 0, 2,
+                0, 0, 0, 0,
+                0, 0, 0, 0,
+                0, 0, 0, 1,
+                0, 0, 0, 2,
+                0xFF.toByte(), 0xFF.toByte(), 0xFF.toByte(), 0xFD.toByte(),
+                2
+            ).use { buf ->
+                val actual = Group.unpack(buf, multipleFiles)
+                try {
+                    assertEquals(expected, actual)
+                } finally {
+                    actual.values.forEach(ByteBuf::release)
+                }
+            }
+        } finally {
+            expected.values.forEach(ByteBuf::release)
+        }
+    }
+}