Add first characterization tests

src/test/java/codechicken/multipart/IDWriterCharacterizationTest.java

@@ -0,0 +1,74 @@
+package codechicken.multipart;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.lang.reflect.Proxy;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+import codechicken.lib.data.MCDataInput;
+import codechicken.lib.data.MCDataOutput;
+import codechicken.lib.data.MCDataOutputWrapper;
+
+class IDWriterCharacterizationTest {
+
+    @ParameterizedTest(name = "max {0} encodes {1} as {2}")
+    @CsvSource({ "0,           305419896, '78',          120", "255,         305419896, '78',          120",
+            "256,         305419896, '56 78',       22136", "65535,       305419896, '56 78',       22136",
+            "65536,       305419896, '12 34 56 78', 305419896", "2147483647,  305419896, '12 34 56 78', 305419896",
+            "-2147483648, 305419896, '78',          120", "-1,          305419896, '78',          120", })
+    void selectsCarrierFromMaximum(int maximum, int value, String encodedHex, int decodedValue) throws Exception {
+        IDWriter writer = new IDWriter();
+        writer.setMax(maximum);
+
+        byte[] encoded = encode(writer, value);
+
+        assertArrayEquals(hex(encodedHex), encoded);
+        assertEquals(decodedValue, decode(writer, encoded));
+    }
+
+    private static byte[] encode(IDWriter writer, int value) {
+        ByteArrayOutputStream bytes = new ByteArrayOutputStream();
+        MCDataOutput output = new MCDataOutputWrapper(new DataOutputStream(bytes));
+        writer.write().apply(output, value);
+        return bytes.toByteArray();
+    }
+
+    private static int decode(IDWriter writer, byte[] bytes) throws Exception {
+        DataInputStream data = new DataInputStream(new ByteArrayInputStream(bytes));
+        MCDataInput input = (MCDataInput) Proxy.newProxyInstance(
+                MCDataInput.class.getClassLoader(),
+                new Class<?>[] { MCDataInput.class },
+                (proxy, method, arguments) -> {
+                    switch (method.getName()) {
+                        case "readUByte":
+                            return (short) data.readUnsignedByte();
+                        case "readUShort":
+                            return data.readUnsignedShort();
+                        case "readInt":
+                            return data.readInt();
+                        default:
+                            throw new AssertionError("Unexpected read method: " + method.getName());
+                    }
+                });
+
+        int value = (Integer) writer.read().apply(input);
+        assertEquals(0, data.available(), "The selected reader must consume its whole carrier");
+        return value;
+    }
+
+    private static byte[] hex(String value) {
+        String[] octets = value.split(" ");
+        byte[] bytes = new byte[octets.length];
+        for (int i = 0; i < octets.length; i++) {
+            bytes[i] = (byte) Integer.parseInt(octets[i], 16);
+        }
+        return bytes;
+    }
+}