Add JagexGzipOutputStream

Equivalent to the built-in GZIPOutputStream, except it produces output
that is bit-for-bit identical to Jagex's implementation.

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

compress/src/main/kotlin/org/openrs2/compress/gzip/JagexGzipOutputStream.kt

@@ -0,0 +1,75 @@
+package org.openrs2.compress.gzip
+
+import com.google.common.io.LittleEndianDataOutputStream
+import java.io.DataOutputStream
+import java.io.OutputStream
+import java.util.zip.CRC32
+import java.util.zip.Deflater
+
+public class JagexGzipOutputStream(
+    private val output: OutputStream
+) : OutputStream() {
+    private val deflater = Deflater(Deflater.DEFAULT_COMPRESSION, true)
+    private val buffer = ByteArray(4096)
+    private val checksum = CRC32()
+    private var size = 0
+    private var closed = false
+
+    init {
+        val dataOutput = DataOutputStream(output)
+        dataOutput.writeShort(HEADER_MAGIC)
+        dataOutput.writeByte(METHOD_DEFLATE)
+        dataOutput.writeByte(0) // FLG
+        dataOutput.writeInt(0) // MTIME
+        dataOutput.writeByte(0) // XFL
+        dataOutput.writeByte(0) // OS
+    }
+
+    override fun write(b: Int) {
+        buffer[0] = b.toByte()
+        write(buffer, 0, 1)
+    }
+
+    override fun write(b: ByteArray, off: Int, len: Int) {
+        checksum.update(b, off, len)
+        size += len
+
+        deflater.setInput(b, off, len)
+
+        while (!deflater.needsInput()) {
+            drain()
+        }
+    }
+
+    private fun drain() {
+        val n = deflater.deflate(buffer)
+        if (n != 0) {
+            output.write(buffer, 0, n)
+        }
+    }
+
+    override fun close() {
+        if (!closed) {
+            deflater.finish()
+
+            while (!deflater.finished()) {
+                drain()
+            }
+
+            val dataOutput = LittleEndianDataOutputStream(output)
+            dataOutput.writeInt(checksum.value.toInt())
+            dataOutput.writeInt(size)
+
+            deflater.end()
+            output.close()
+
+            closed = true
+        }
+    }
+
+    private companion object {
+        private const val HEADER_MAGIC = 0x1F8B
+
+        private const val METHOD_DEFLATE = 8
+    }
+}