We have: // sorted values size_t lines = [20, 1755, 1756, 1757, 1798, 1824, 1825, 1839, 1840]; size_t search = 21; Goal: // Fast find index of the '21' in ordered array 'lines' auto found = lines.binarySearch( 20 ); // 0 - index auto low = lines.binarySearchLow( 21 ); // 0 - near lowest index Where is the implementation of "binary search", please?