Triangular Index Helpers

triangleindex.cpp

// Triangle indexes
// if you have a set of N items with indexes.
// 0,1,2,3, ... , N-1
// You want a linear index for every pair i,j, i != j.

// N Choose 2.
inline int triangleSize(int N) {
  return N*(N-1)/2;
}

// You need
// i < j < N.
inline int triangleIndex(int N, int i, int j) {
  return i*(2*N - i - 1)/2 + j - i - 1;
}