Fix: Checksum algorithm shall force 0x00 value to 0xFF (#28)

.vscode/settings.json

@@ -0,0 +1,7 @@
+{
+    "python.testing.pytestArgs": [
+        "tests"
+    ],
+    "python.testing.unittestEnabled": false,
+    "python.testing.pytestEnabled": true
+}

intex_spa/intex_spa_query.py

@@ -55,13 +55,17 @@ def checksum_as_int(data: str) -> int:
         calculated_checksum = calculated_checksum - (
             int("0x" + data[index : index + 2], 16)
         )
-    return calculated_checksum % 0xFF
+    calculated_checksum = calculated_checksum % 0xFF
+    # Fix: https://github.com/mathieu-mp/intex-spa/issues/27
+    if calculated_checksum == 0x00:
+        calculated_checksum = 0xFF
+    return calculated_checksum
 
 
 def checksum_as_str(data: str) -> str:
     """Return string checksum for the given data, as expected by Intex Spa protocol"""
     # Return checksum as a hex string without 0x prefix
-    return hex(checksum_as_int(data) % 0xFF)[2:].upper()
+    return hex(checksum_as_int(data))[2:].upper()
 
 
 class IntexSpaQuery:

tests/test_intex_spa_query_checksum.py

@@ -0,0 +1,28 @@
+from intex_spa.intex_spa_query import IntexSpaQuery
+
+import pytest
+
+valid_status_responses = [
+    b'{"sid":"12345678901234","data":"FFFF110F010700220000000080808022000012","result":"ok","type":2}\n',
+    b'{"sid":"12345678901234","data":"FFFF110F01070064000000008080806700008A","result":"ok","type":2}\n',  # From https://github.com/mathieu-mp/intex-spa/issues/27
+    b'{"sid":"12345678901234","data":"FFFF110F0107006400000085808085670000FF","result":"ok","type":2}\n',  # From https://github.com/mathieu-mp/intex-spa/issues/27
+]
+invalid_status_responses = [
+    b'{"sid":"12345678901234","data":"FFFF110F010700220000000080808022000044","result":"ok","type":2}\n',  # Arbitrary false checksum
+    b'{"sid":"12345678901234","data":"00000000000000000000000000000000000000","result":"ok","type":2}\n',  # Checksum 0x00 means no checksum calculation
+]
+
+
+@pytest.mark.parametrize("status_response", valid_status_responses)
+def test_valid_intex_spa_checksums(status_response):
+    query = IntexSpaQuery(intent="status")
+    query.intex_timestamp = "12345678901234"
+    query.render_response_data(received_bytes=status_response)
+
+
+@pytest.mark.parametrize("status_response", invalid_status_responses)
+def test_invalid_intex_spa_checksums(status_response):
+    query = IntexSpaQuery(intent="status")
+    query.intex_timestamp = "12345678901234"
+    with pytest.raises(AssertionError):
+        query.render_response_data(received_bytes=status_response)