Improve bitmap::_calculateArea performance

src/inc/LibraryIncludes.h

@@ -84,7 +84,10 @@
 
 // Dynamic Bitset (optional dependency on LibPopCnt for perf at bit counting)
 // Variable-size compressed-storage header-only bit flag storage library.
+#pragma warning(push)
+#pragma warning(disable:4702) // unreachable code
 #include <dynamic_bitset.hpp>
+#pragma warning(pop)
 
 // {fmt}, a C++20-compatible formatting library
 #include <fmt/format.h>

src/inc/til/bitmap.h

@@ -81,16 +81,20 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
             const ptrdiff_t _end;
             til::rectangle _run;
 
+            // Update _run to contain the next rectangle of consecutively set bits within this bitmap.
+            // _calculateArea may be called repeatedly to yield all those rectangles.
             void _calculateArea()
             {
-                // Backup the position as the next one.
-                _nextPos = _pos;
+                // The following logic first finds the next set bit in this bitmap and the next unset bit past that.
+                // The area in between those positions are thus all set bits and will end up being the next _run.
 
-                // Seek forward until we find an on bit.
-                while (_nextPos < _end && !_values[_nextPos])
-                {
-                    ++_nextPos;
-                }
+                // dynamic_bitset allows you to quickly find the next set bit using find_next(prev),
+                // where "prev" is the position _past_ which should be searched (i.e. excluding position "prev").
+                // If _pos is still 0, we thus need to use the counterpart find_first().
+                const auto nextPos = _pos == 0 ? _values.find_first() : _values.find_next(_pos - 1);
+                // If no next set bit can be found, npos is returned, which is SIZE_T_MAX.
+                // saturated_cast can ensure that this will be converted to PTRDIFF_T_MAX (which is greater than _end).
+                _nextPos = base::saturated_cast<ptrdiff_t>(nextPos);
 
                 // If we haven't reached the end yet...
                 if (_nextPos < _end)
@@ -118,8 +122,10 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
                 }
                 else
                 {
-                    // If we reached the end, set the pos because the run is empty.
-                    _pos = _nextPos;
+                    // If we reached the end _nextPos may be >= _end (potentially even PTRDIFF_T_MAX).
+                    // ---> Mark the end of the iterator by updating the state with _end.
+                    _pos = _end;
+                    _nextPos = _end;
                     _run = til::rectangle{};
                 }
             }