space locality

#define NUMBER_OF_PARTICLES 1 << 15

typedef struct _PARTICLE {
    float x;
    float y;
    float z;
} PARTICLE;

PARTICLE particles[NUMBER_OF_PARTICLES];

void test1() {
    for (int i = 0; i < NUMBER_OF_PARTICLES; ++i) {
        PARTICLE particle = { 0, 0, 0 };
        for (int j = 0; j < NUMBER_OF_PARTICLES; ++j) {
            particle.x += particles[i].x + particles[j].x;
            particle.y += particles[i].y + particles[j].y;
            particle.z += particles[i].z + particles[j].z;
        }
        particles[i] = particle;
    }
}

void test2() {
    float X[NUMBER_OF_PARTICLES];
    float Y[NUMBER_OF_PARTICLES];
    float Z[NUMBER_OF_PARTICLES];
    for (int i = 0; i < NUMBER_OF_PARTICLES; ++i) {
        X[i] = particles[i].x;
        Y[i] = particles[i].y;
        Z[i] = particles[i].z;
    }
    for (int i = 0; i < NUMBER_OF_PARTICLES; ++i) {
        PARTICLE particle = { 0, 0, 0 };
        for (int j = 0; j < NUMBER_OF_PARTICLES; ++j) {
            particle.x += X[i] + X[j];
            particle.y += Y[i] + Y[j];
            particle.z += Z[i] + Z[j];
        }
        particles[i] = particle;
    }
}

int main(int argc, char** argv) {
    clock_t t1, t2;

    t1 = clock();
    test1();
    t2 = clock();
    printf("test1: %d\n", t2 - t1);

    t1 = clock();
    test2();
    t2 = clock();
    printf("test2: %d\n", t2 - t1);

    return 0;
}