Add method for stripping the version trailer from a ByteBuf

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

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

@@ -0,0 +1,16 @@
+package dev.openrs2.cache
+
+import io.netty.buffer.ByteBuf
+
+public object VersionTrailer {
+    public fun strip(buf: ByteBuf): Int? {
+        return if (buf.readableBytes() >= 2) {
+            val index = buf.writerIndex() - 2
+            val version = buf.getUnsignedShort(index)
+            buf.writerIndex(index)
+            version
+        } else {
+            null
+        }
+    }
+}

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

@@ -0,0 +1,35 @@
+package dev.openrs2.cache
+
+import dev.openrs2.buffer.use
+import io.netty.buffer.Unpooled
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+
+object VersionTrailerTest {
+    @Test
+    public fun testStrip() {
+        assertNull(VersionTrailer.strip(Unpooled.EMPTY_BUFFER))
+
+        Unpooled.wrappedBuffer(byteArrayOf(0)).use { buf ->
+            assertNull(VersionTrailer.strip(buf))
+
+            Unpooled.wrappedBuffer(byteArrayOf(0)).use { expected ->
+                assertEquals(expected, buf)
+            }
+        }
+
+        Unpooled.wrappedBuffer(byteArrayOf(0x12, 0x34)).use { buf ->
+            assertEquals(0x1234, VersionTrailer.strip(buf))
+            assertEquals(Unpooled.EMPTY_BUFFER, buf)
+        }
+
+        Unpooled.wrappedBuffer(byteArrayOf(0x12, 0x34, 0x56)).use { buf ->
+            assertEquals(0x3456, VersionTrailer.strip(buf))
+
+            Unpooled.wrappedBuffer(byteArrayOf(0x012)).use { expected ->
+                assertEquals(expected, buf)
+            }
+        }
+    }
+}