int c4_GroupByViewer::ScanTransitions(int lo_, int hi_, t4_byte *flags_, const
c4_View &match_)const {
d4_assert(lo_ > 0);
int m = hi_ - lo_;
d4_assert(m >= 0);
// done if nothing left or if entire range is identical
if (m == 0 || match_[lo_ - 1] == match_[hi_ - 1])
return 0;
// range has a transition, done if it is exactly of size one
if (m == 1) {
++(flags_[lo_]);
return 1;
}
// use binary splitting if the range has enough entries
if (m >= 5)
return ScanTransitions(lo_, lo_ + m / 2, flags_, match_) + ScanTransitions
(lo_ + m / 2, hi_, flags_, match_);
// else use a normal linear scan
int n = 0;
for (int i = lo_; i < hi_; ++i)
if (match_[i] != match_[i - 1]) {
++(flags_[i]);
++n;
}
return n;
}
c4_GroupByViewer::c4_GroupByViewer (c4_Sequence& seq_, const c4_View& keys_,
const c4_Property& result_)
: _parent (&seq_), _keys (keys_), _result (result_)
{
_sorted = _parent.SortOn(_keys);
int n = _sorted.GetSize();
c4_Bytes temp;
t4_byte* buf = temp.SetBufferClear(n);
int groups = 0;
if (n > 0)
{
++buf[0]; // the first entry is always a transition
groups = 1 + ScanTransitions(1, n, buf, _sorted.Project(_keys));
}
// set up a map pointing to each transition
_map.SetSize(groups + 1);
int j = 0;
for (int i = 0; i < n; ++i)
if (buf[i])
_map.SetAt(j++, i);
// also append an entry to point just past the end
_map.SetAt(j, n);
d4_assert(_map.GetAt(0) == 0);
d4_assert(j == groups);
}
