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Stuart Longland
44e2d937d6
I left this out because I thought the idea of modulus and division on a
MCU that lacks a hardware multiplier would be too much for it… and
indeed it was too much with my other project.
Thinking about it this afternoon, I had an idea. If I have 2^N samples,
then the modulus can be optimised to:
```
mod = sample & ((2**n)-1)
```
and the segment can be figured out by:
```
segment = sample >> n
```
The segment is two bits. A function that returns the scaled sine value
for a given scaled angle can be given as:
```
int8_t fp_sine(uint8_t angle) {
uint8_t segment = (angle >> POLY_SINE_SZ_BITS) & 3;
uint8_t offset = angle & ((1 << POLY_SINE_SZ_BITS)-1);
switch (segment) {
case 0:
return _poly_sine[offset];
case 1:
return _poly_sine[
POLY_SINE_SZ - offset];
case 2:
return -_poly_sine[offset];
case 3:
return -_poly_sine[
POLY_SINE_SZ - offset];
}
}
```
… or something like that. If `POLY_SINE_SZ_BITS=6`, then `angle=255`
represents 360°.
9 lines
44 B
Plaintext
9 lines
44 B
Plaintext
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