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Tolerate extra SYN bytes in preamble #29

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2 changes: 1 addition & 1 deletion src/CMRI.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -182,7 +182,7 @@ uint8_t CMRI::_decode(uint8_t c)
case PREAMBLE_3:
if (c == STX)
_mode = DECODE_ADDR;
else
else if (c != 0xFF)
_mode = PREAMBLE_1;
break;

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50 changes: 50 additions & 0 deletions test/test_cmri/test_main.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -176,6 +176,53 @@ void test_transmit_escapes_control_bytes(void)
TEST_ASSERT_EQUAL_UINT8(CMRI::ETX, s.tx[10]);
}

// A third SYN byte does not desync the parser.
void test_triple_syn_accepted(void)
{
Stream s;
CMRI cmri(0, 24, 48, s);

cmri.set_byte(0, 0x42);

s.feed(0xFF); // third SYN
feed_packet(s, 0, CMRI::POLL, nullptr, 0);
TEST_ASSERT_TRUE(cmri.process());
TEST_ASSERT_EQUAL_UINT8(0x42, s.tx[5]);
}

// Four SYN bytes are tolerated.
void test_quad_syn_accepted(void)
{
Stream s;
CMRI cmri(0, 24, 48, s);

cmri.set_byte(0, 0x99);

s.feed(0xFF); // third SYN
s.feed(0xFF); // fourth SYN
feed_packet(s, 0, CMRI::POLL, nullptr, 0);
TEST_ASSERT_TRUE(cmri.process());
TEST_ASSERT_EQUAL_UINT8(0x99, s.tx[5]);
}

// A 0xFF byte inside a SET body is data, not a preamble SYN.
void test_syn_in_body_not_resynced(void)
{
Stream s;
CMRI cmri(0, 24, 48, s);

uint8_t data[6] = {0xFF, 0x00, 0x00, 0x00, 0x00, 0x00};
feed_packet(s, 0, CMRI::SET, data, 6);
TEST_ASSERT_TRUE(cmri.process());

TEST_ASSERT_EQUAL_UINT8(0xFF, cmri.get_byte(0));

cmri.set_byte(0, 0xAA);
feed_packet(s, 0, CMRI::POLL, nullptr, 0);
TEST_ASSERT_TRUE(cmri.process());
TEST_ASSERT_EQUAL_UINT8(0xAA, s.tx[5]);
}

// Garbage before a valid packet is resynced away by the preamble state machine.
void test_preamble_resync_after_garbage(void)
{
Expand Down Expand Up @@ -205,6 +252,9 @@ int main(int, char **)
RUN_TEST(test_set_packet_updates_outputs);
RUN_TEST(test_address_filtering);
RUN_TEST(test_transmit_escapes_control_bytes);
RUN_TEST(test_triple_syn_accepted);
RUN_TEST(test_quad_syn_accepted);
RUN_TEST(test_syn_in_body_not_resynced);
RUN_TEST(test_preamble_resync_after_garbage);
return UNITY_END();
}