locate.c

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int locate(double const *xx, double x, int n) {
  xx -= 1;  // zero-offset to unit-offset
 
  int j;
  int ju, jm, jl;
  int ascnd;
 
  jl = 0;
  ju = n + 1;
  ascnd = (xx[n] >= xx[1]);
  while (ju - jl > 1) {
    jm = (ju + jl) >> 1;
    if (x >= xx[jm] == ascnd)
      jl = jm;
    else
      ju = jm;
  }
  if (x == xx[1])
    j = 1;
  else if (x == xx[n])
    j = n - 1;
  else
    j = jl;
 
  j -= 1;  // unit-offset to zero-offset
  return j;
}