feat(solarman): add Modbus RTU frame with CRC16

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/main/java/home/gateway/adapters/solarman/ModbusRtuFrame.java

@@ -0,0 +1,54 @@
+package home.gateway.adapters.solarman;
+
+import java.io.IOException;
+
+/** Modbus RTU frame: unit id, PDU, CRC16 with the low byte first. */
+final class ModbusRtuFrame {
+
+    private static final int CRC_LENGTH = 2;
+    private static final int MIN_LENGTH = 1 + 1 + CRC_LENGTH;
+
+    private ModbusRtuFrame() {
+    }
+
+    static byte[] build(int unitId, byte[] pdu) {
+        byte[] frame = new byte[1 + pdu.length + CRC_LENGTH];
+        frame[0] = (byte) unitId;
+        System.arraycopy(pdu, 0, frame, 1, pdu.length);
+        int crc = crc16(frame, frame.length - CRC_LENGTH);
+        frame[frame.length - 2] = (byte) crc;
+        frame[frame.length - 1] = (byte) (crc >> 8);
+        return frame;
+    }
+
+    static boolean hasValidCrc(byte[] frame) {
+        if (frame.length < MIN_LENGTH) {
+            return false;
+        }
+        int crc = crc16(frame, frame.length - CRC_LENGTH);
+        return frame[frame.length - 2] == (byte) crc && frame[frame.length - 1] == (byte) (crc >> 8);
+    }
+
+    static byte[] parse(byte[] frame, int expectedUnitId) throws IOException {
+        if (!hasValidCrc(frame)) {
+            throw new IOException("Modbus RTU frame has a bad CRC or is too short");
+        }
+        if ((frame[0] & 0xFF) != expectedUnitId) {
+            throw new IOException("Modbus RTU frame from unit " + (frame[0] & 0xFF) + ", expected " + expectedUnitId);
+        }
+        byte[] pdu = new byte[frame.length - 1 - CRC_LENGTH];
+        System.arraycopy(frame, 1, pdu, 0, pdu.length);
+        return pdu;
+    }
+
+    static int crc16(byte[] data, int length) {
+        int crc = 0xFFFF;
+        for (int i = 0; i < length; i++) {
+            crc ^= data[i] & 0xFF;
+            for (int bit = 0; bit < 8; bit++) {
+                crc = (crc & 1) != 0 ? (crc >>> 1) ^ 0xA001 : crc >>> 1;
+            }
+        }
+        return crc;
+    }
+}

src/test/java/home/gateway/adapters/solarman/ModbusRtuFrameTest.java

@@ -0,0 +1,47 @@
+package home.gateway.adapters.solarman;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.io.IOException;
+import org.junit.jupiter.api.Test;
+
+class ModbusRtuFrameTest {
+
+    private static final byte[] READ_10_AT_0 = {0x01, 0x03, 0x00, 0x00, 0x00, 0x0A, (byte) 0xC5, (byte) 0xCD};
+
+    @Test
+    void buildsTheStandardExampleWithCrcLowByteFirst() {
+        byte[] pdu = {0x03, 0x00, 0x00, 0x00, 0x0A};
+        assertArrayEquals(READ_10_AT_0, ModbusRtuFrame.build(1, pdu));
+    }
+
+    @Test
+    void crcOfAKnownResponse() {
+        byte[] frame = {0x01, 0x04, 0x02, (byte) 0xFF, (byte) 0xFF};
+        assertEquals(0x80B8, ModbusRtuFrame.crc16(frame, frame.length));
+    }
+
+    @Test
+    void parsesBackThePdu() throws IOException {
+        assertArrayEquals(new byte[] {0x03, 0x00, 0x00, 0x00, 0x0A}, ModbusRtuFrame.parse(READ_10_AT_0, 1));
+    }
+
+    @Test
+    void rejectsBadCrc() {
+        byte[] frame = READ_10_AT_0.clone();
+        frame[7] ^= 1;
+        assertThrows(IOException.class, () -> ModbusRtuFrame.parse(frame, 1));
+    }
+
+    @Test
+    void rejectsOtherUnit() {
+        assertThrows(IOException.class, () -> ModbusRtuFrame.parse(READ_10_AT_0, 2));
+    }
+
+    @Test
+    void rejectsTooShortFrame() {
+        assertThrows(IOException.class, () -> ModbusRtuFrame.parse(new byte[] {0x01, 0x03}, 1));
+    }
+}