android: CRITICAL fix - commands were byte-reversed (ByteBuffer big-endian); official app sends little-endian, palindrome 66b masked it for 16 rounds

android/app/src/main/kotlin/com/mag160c/thermal/usb/MagProtocol.kt

@@ -1,7 +1,5 @@
 package com.mag160c.thermal.usb
 
-import java.nio.ByteBuffer
-
 /**
  * Vendor command/response protocol — VERBATIM from the official MAG-Cx app's
  * Java source (sdk/UsbCommunication.java + P2DCmd.java/D2PCmd.java, jadx).
@@ -33,17 +31,25 @@ object MagProtocol {
     const val RSP_SEND_CALI_FILE = 0x5BB5B55D
     const val RSP_SEND_CALI_INFO = 0x5BB5B55E
 
-    fun cmd4(magic: Int): ByteArray {
+    /**
-        val b = ByteBuffer.allocate(4)
+     * 4-byte LITTLE-ENDIAN command — byte-identical to the official app's
-        b.putInt(magic)
+     * GlobalFunc.intToByteArray (a&255 first).  CRITICAL: ByteBuffer.putInt
-        return b.array()
+     * defaults to BIG_ENDIAN and once reversed every non-palindrome magic
-    }
+     * (66c/66f/670/672/673/674) — the camera only understood the palindrome
+     * 0x6BB6B66B, which masked this for 16 rounds.
+     */
+    fun cmd4(magic: Int): ByteArray = byteArrayOf(
+        (magic and 0xFF).toByte(),
+        ((magic shr 8) and 0xFF).toByte(),
+        ((magic shr 16) and 0xFF).toByte(),
+        ((magic shr 24) and 0xFF).toByte(),
+    )
 
+    /** 8-byte {u32 magic LE, u32 param LE} (official SetShutterState packet). */
     fun cmd8(magic: Int, param: Int): ByteArray {
-        val b = ByteBuffer.allocate(8)
+        val lo = cmd4(magic)
-        b.putInt(magic)
+        val hi = cmd4(param)
-        b.putInt(param)
+        return byteArrayOf(lo[0], lo[1], lo[2], lo[3], hi[0], hi[1], hi[2], hi[3])
-        return b.array()
     }
 
     /** Camera info block (0x5BB5B55B, 0x38 bytes): +0x00 pid, +0x08 serial,

android/app/src/test/kotlin/com/mag160c/thermal/usb/MagProtocolTest.kt

@@ -0,0 +1,43 @@
+package com.mag160c.thermal.usb
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+/**
+ * Wire-format regression lock: commands MUST be little-endian, byte-identical
+ * to the official MAG-Cx app (GlobalFunc.intToByteArray: a&255 goes first).
+ * ByteBuffer.putInt defaults to BIG_ENDIAN and once reversed every
+ * non-palindrome command magic — the camera answered only 0x6BB6B66B (a
+ * palindrome) which hid the bug for 16 rounds of black-screen debugging.
+ */
+class MagProtocolTest {
+    @Test
+    fun cmd4IsLittleEndian() {
+        // official: intToByteArray(1807136364) == {6C, B6, B6, 6B}
+        val b = MagProtocol.cmd4(0x6BB6B66C)
+        assertEquals(0x6C.toByte(), b[0])
+        assertEquals(0xB6.toByte(), b[1])
+        assertEquals(0xB6.toByte(), b[2])
+        assertEquals(0x6B.toByte(), b[3])
+    }
+
+    @Test
+    fun cmd8MatchesOfficialSetShutterStatePacket() {
+        // official setShutterState_(on): {72, B6, B6, 6B, on, on>>8, on>>16, on>>24}
+        val b = MagProtocol.cmd8(0x6BB6B672, 1)
+        assertEquals(0x72.toByte(), b[0])
+        assertEquals(0xB6.toByte(), b[1])
+        assertEquals(0xB6.toByte(), b[2])
+        assertEquals(0x6B.toByte(), b[3])
+        assertEquals(0x01.toByte(), b[4])
+        assertEquals(0x00.toByte(), b[5])
+        assertEquals(0x00.toByte(), b[6])
+        assertEquals(0x00.toByte(), b[7])
+    }
+
+    @Test
+    fun startTransferImgPacketMatchesOfficial() {
+        val b = MagProtocol.cmd4(0x6BB6B673)
+        assertEquals(listOf<Byte>(0x73, 0xB6.toByte(), 0xB6.toByte(), 0x6B), b.toList())
+    }
+}