How to sum values in a sequence of bytes in C - c

Question

I am trying to figure out how to add sequential bytes in a data block starting at a given offset(sequenceOffset) to sequenceLength, by typcasting them to signed 16 bit integers(int16_t). The numbers can be negative and positive. I feel like i am not incrementing the offset properly but cannot figure out how it is meant to be done.
For example:
Summing sequence of 8 bytes at offset 53:
57 AB 2A 2C 4E A4 7A 64
-21673 11306 -23474 25722
You said the sum is: 22848
Should be: -8119
int16_t sumSequence16(const uint8_t* const blockAddress, uint32_t blockLength, uint32_t sequenceOffset,
uint8_t sequenceLength) {
int count = 0;
for (int i = 0; i < blockLength; i++) {
if (*(blockAddress + i) == sequenceOffset) {
count += (int16_t*)(&sequenceOffset);
sequenceOffset++;
}
}
return count;
}

Answer 1

There are some serious problems with your code.
Start by noticing that your code doesn't use sequenceLength at all - that's strange.
Then there is no need to loop over the whole block - you only need to look at the bytes inside the relevant sequence.
This line is very strange:
if (*(blockAddress + i) == sequenceOffset)
^^^^^^^^^^^^^^^^^^^
Reads the data at index i
It compare a data value inside the data block with the sequenceOffset - that doesn't seem correct.
And this part:
(int16_t*)(&sequenceOffset);
is actually a violation of the strict aliasing rule.
Finally, you never mention which endianess the data is stored with. From your example it seems to be little endian so I'll use little endian in the code below:
int16_t sumSequence16(const uint8_t* const blockAddress,
const uint32_t sequenceOffset,
const uint8_t sequenceLength)
{
uint8_t* p = blockAddress + sequenceOffset; // Point to first byte in sequence
int sum = 0;
for (uint8_t i = 0; i < sequenceLength; i += 2)
{
int16_t t = 0;
t = p[i+1]; // Read MSB
t = t << 8; // Shift MSB 8 bits to the left
t = t | p[i]; // Add LSB
sum = sum + t; // Update the running sum
}
return sum;
}