ex pseries::coeff(const symbol &s, int n) const
{
if (var.is_equal(s)) {
- epvector::const_iterator it = seq.begin(), itend = seq.end();
- while (it != itend) {
- int pow = ex_to_numeric(it->coeff).to_int();
- if (pow == n)
- return it->rest;
- if (pow > n)
- return _ex0();
- it++;
- }
- return _ex0();
+ if (seq.size() == 0)
+ return _ex0();
+
+ // Binary search in sequence for given power
+ numeric looking_for = numeric(n);
+ int lo = 0, hi = seq.size() - 1;
+ while (lo <= hi) {
+ int mid = (lo + hi) / 2;
+ GINAC_ASSERT(is_ex_exactly_of_type(seq[mid].coeff, numeric));
+ int cmp = ex_to_numeric(seq[mid].coeff).compare(looking_for);
+ switch (cmp) {
+ case -1:
+ lo = mid + 1;
+ break;
+ case 0:
+ return seq[mid].rest;
+ case 1:
+ hi = mid - 1;
+ break;
+ default:
+ throw(std::logic_error("pseries::coeff: compare() didn't return -1, 0 or 1"));
+ }
+ }
+ return _ex0();
} else
return convert_to_poly().coeff(s, n);
}