static inline gf8 do_op(char op, gf8 l, gf8 r) {
if (op == '+' || op == '-') return add(l,r);
if (op == '*') return multiply(l,r);
if (op == '/') return multiply(l,inverse(r));
if (op == '^') return pol_pow(l,r);
if (op == 'o') return smallest_nth_primitive_root_of_unity(l);
if (op == 's') {
uint i = 0;
for (i = 0;i < r;++i) {
printf("%u ",pol_pow(l,i));
}
return pol_pow(l,i);
}
printf("unknown operator %c\n",op);
exit(-1);
}
void efftinv(uint P, uint Q, byte * f, byte *res ) {
byte oN = smallest_nth_primitive_root_of_unity(P*Q);
byte oQ = pol_pow(oN,P);
uint pc = 0, qc = 0, k = 0;
byte Qroot = 1;
byte Nroot = 1;
byte rowFFT[256][256] = {{0}}; // P x Q
byte rootK = 0;
oN = inverse(oN);
oQ = inverse(oQ);
for(qc = 0;qc < Q;++qc) {
for(pc = 0;pc < P;++pc) {
rootK = 1;
for(k = 0;k < Q;++k) { // rowFFT[pc,qc] = SUM_{k=0}^{Q-1}
byte term = 0;
// A[k,pc] each term: f_{pc*Q+k}
term = multiply(f[k*P+pc],rootK);
rowFFT[pc][qc] = add(rowFFT[pc][qc],term);
rootK = multiply(rootK, Qroot);
}
}
Qroot = multiply(Qroot,oQ);
}
for(pc = 0;pc < P;++pc) {
for(qc = 0;qc < Q;++qc) {
rootK = 1;
for(k = 0;k < P;++k) {
byte term = 0;
uint res_idx = pc*Q+qc;
term =
multiply(rowFFT[k][qc],rootK);
res[res_idx] = add(res[res_idx],term);
rootK = multiply(rootK, Nroot);
}
Nroot = multiply(Nroot,oN);
}
}
}
