Rotate a 2D vector by i turns of 90 degrees.

rotate.c

#include <math.h>
#include <stdio.h>

typedef union Float2
{
  struct 
  {
    float x;
    float y;
  };
  float e[2];
} Float2;

// Rotate a 2D vector by i turns of 90 degrees.
Float2 rotate(Float2 v, int i)
{
  Float2 result;
  const float ones[2] = {1.0f, -1.0f};

  int j = i + 1;
  int x_index = i & 1;
  int y_index = j & 1;
  float sign_x = ones[(i & 2) >> 1];
  float sign_y = ones[(j & 2) >> 1];
  result.x = copysignf(v.e[x_index], sign_x);
  result.y = copysignf(v.e[y_index], sign_y);

  return result;
}

int main(int argc, const char** argv)
{
  Float2 arm = {0.2f, 0.5f};

  for(int i = -4; i < 4; i += 1)
  {
    Float2 spun = rotate(arm, i);
    printf("<%f, %f>\n", spun.x, spun.y);
  }

  return 0;
}