void operate(array<T1, S1>& u, array <array <T2, S21>, S22>& p, sep,
const array<T3, S3>& a, const size_array& idx = size_array()) // f only used internally!
{
if (a.empty()) { u.init(); p.init(); return; }
size_array first = find(arr(true).cat(diff(a) != 0));
u = a[first];
if (idx.empty()) (_, p) = partition++(size_array((0, _, a.length() - 1)), first); // TODO: remove size_array copy
else (_, p) = partition++(idx, first);
}
void operate(array<T1, S1>& u, array <T2, S2>& p, sep,
const array<T3, S3>& a, const size_array& idx = size_array()) // f only used internally!
{
if (a.empty()) { u.init(); p.init(); return; }
if (FIRST) (_, p) = find++(arr(true).cat(diff(a) != 0)); // TODO: check speed of cat()
else (_, p) = find++((diff(a) != 0).cat(arr(true)));
u = a[p];
if (!idx.empty()) force(p) = idx[p];
}
void operate(array<T1, S1>& u, array <T2, S2>& p, array <T3, S3>& q, sep,
const array<T4, S4>& a, const size_array& idx = size_array()) // idx only used internally!
{
if (a.empty()) { u.init(); p.init(); q.init(); return; }
q = arr(true).cat(diff(a) != 0);
if (FIRST) (_, p) = find++(q);
else (_, p) = find++(cshift(q, -1));
u = a[p];
if (!idx.empty()) force(p) = idx[p];
q[0] = T3();
q = cumsum(q);
}