header un/packing

core/src/main/groovy/com/muwire/core/util/DataUtil.groovy

@@ -2,9 +2,9 @@ package com.muwire.core.util
 
 class DataUtil {
 	
-	private static int MAX_SHORT = (0x1 << 16) - 1
+	private final static int MAX_SHORT = (0x1 << 16) - 1
 
-	static writeUnsignedShort(int value, OutputStream os) {
+	static void writeUnsignedShort(int value, OutputStream os) {
 		if (value > MAX_SHORT || value < 0)
 			throw new IllegalArgumentException("$value invalid")
 		
@@ -14,4 +14,26 @@ class DataUtil {
 		os.write(msb)
 		os.write(lsb)	
 	}
+	
+	private final static int MAX_HEADER = 0x7FFFFF
+	
+	static void packHeader(int length, byte [] header) {
+		if (header.length != 3)
+			throw new IllegalArgumentException("header length $header.length")
+		if (length < 0 || length > MAX_HEADER)
+			throw new IllegalArgumentException("length $length")
+		
+		header[2] = (byte) (length & 0xFF)
+		header[1] = (byte) ((length >> 8) & 0xFF)
+		header[0] = (byte) ((length >> 16) & 0x7F)
+	}
+	
+	static int readLength(byte [] header) {
+		if (header.length != 3)
+			throw new IllegalArgumentException("header length $header.length")
+			
+		return (((int)(header[0] & 0x7F)) << 16) |
+				((int)(header[1] & 0xFF << 8)) |
+				((int)header[2] & 0xFF)
+	}
 }

core/src/test/groovy/com/muwire/core/util/DataUtilTest.groovy

@@ -26,4 +26,21 @@ class DataUtilTest {
 			fail()
 		} catch (IllegalArgumentException expected) {}
 	}
+	
+	private static header(int value) {
+		byte [] header = new byte[3]
+		DataUtil.packHeader(value, header)
+		assert value == DataUtil.readLength(header)
+	}
+	
+	@Test
+	void testHeader() {
+		header(0)
+		header(1)
+		header(8 * 1024 * 1024 - 1)
+		try {
+			header(8 * 1024 *  1024)
+			fail()
+		} catch (IllegalArgumentException expected) {}
+	}
 }