MapToPositiveHalfIndex.h

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#pragma once
 
#include <cassert>
 
constexpr auto MapToPositiveHalfIndex(int index, int fullSize)
{
   assert(fullSize > 0 && fullSize % 2 == 0);
   if (index >= 0)
      index = index % fullSize;
   else
      index = fullSize - (-index % fullSize);
   if (index > fullSize / 2)
      index = fullSize - index;
   return index;
}
 
static_assert(MapToPositiveHalfIndex(-3, 4) == 1);
static_assert(MapToPositiveHalfIndex(-2, 4) == 2);
static_assert(MapToPositiveHalfIndex(-1, 4) == 1);
static_assert(MapToPositiveHalfIndex(0, 4) == 0);
static_assert(MapToPositiveHalfIndex(1, 4) == 1);
static_assert(MapToPositiveHalfIndex(2, 4) == 2);
static_assert(MapToPositiveHalfIndex(3, 4) == 1);
static_assert(MapToPositiveHalfIndex(4, 4) == 0);