Need explanation for the lines of code in the filter function

[patan.gova]

Hi,
I am planning to implement the digital filters with MSP430 microcontroller and want to use this fir_filter function but I don't exactly know how it works

int16_t ir_filter(int16_t sample)  
{     static int16_t buf[32];    
static int offset = 0;   
int32_t z;   
int i;   
buf[offset] = sample;   
z = mul16(coeffs[11], buf[(offset - 11) & 0x1F]);   
for (i = 0;  i < 11;  i++)       
z += mul16(coeffs[i], buf[(offset - i) & 0x1F] + buf[(offset - 22 + i) & 0x1F]);   
offset = (offset + 1) & 0x1F;   
return  z >> 15; 
}

Here the static const int16_t coeffs[12] = {contains 12 cofficient values };
Can someo explain me
1)In z = mul16(coeffs[11], buf[(offset - 11) & 0x1F]); if offset=0; what the value of (offset - 11) & 0x1F will be ?
2)For first iteration(if i=0) what the value of this will be (offset - 22 + i) & 0x1F]?
3)Isn't the (offset - i)='zero' in buf[(offset - i) & 0x1F] for every iteration?
Thanks.

 

[doraemon]

Hello!

The post is old, but since there is no reply...
Judging by your questions, I guess you didn't write this code. Why not writing your own code?
If you can't do it, then read some documentation, some books
Here is a good one:
Digital processing of signals
By Maurice Bellanger

Dora.

NB: I can't solve this problem. Negative index has no meaning.