#include "check.h"
#include "timer.h"
#include <cuda_runtime.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

#define SOFTENING 1e-9f

/*
 * Each body contains x, y, and z coordinate positions,
 * as well as velocities in the x, y, and z directions.
 */

typedef struct {
    float x, y, z, vx, vy, vz;
} Body;

/*
 * Do not modify this function. A constraint of this exercise is
 * that it remain a host function.
 */

void randomizeBodies(float* data, int n)
{
    for (int i = 0; i < n; i++) {
        data[i] = 2.0f * (rand() / (float)RAND_MAX) - 1.0f;
    }
}

/*
 * This function calculates the gravitational impact of all bodies in the system
 * on all others, but does not update their positions.
 */

void bodyForce(Body* p, float dt, int n)
{
    for (int i = 0; i < n; ++i) {
        float Fx = 0.0f;
        float Fy = 0.0f;
        float Fz = 0.0f;

        for (int j = 0; j < n; j++) {
            float dx = p[j].x - p[i].x;
            float dy = p[j].y - p[i].y;
            float dz = p[j].z - p[i].z;
            float distSqr = dx * dx + dy * dy + dz * dz + SOFTENING;
            float invDist = rsqrtf(distSqr);
            float invDist3 = invDist * invDist * invDist;

            Fx += dx * invDist3;
            Fy += dy * invDist3;
            Fz += dz * invDist3;
        }

        p[i].vx += dt * Fx;
        p[i].vy += dt * Fy;
        p[i].vz += dt * Fz;
    }
}

int main(const int argc, const char** argv)
{

    /*
     * Do not change the value for `nBodies` here. If you would like to modify it,
     * pass values into the command line.
     */

    int nBodies = 2 << 11;
    int salt = 0;
    if (argc > 1)
        nBodies = 2 << atoi(argv[1]);

    /*
     * This salt is for assessment reasons. Tampering with it will result in automatic failure.
     */

    if (argc > 2)
        salt = atoi(argv[2]);

    const float dt = 0.01f; // time step
    const int nIters = 10; // simulation iterations

    int bytes = nBodies * sizeof(Body);
    float* buf;

    buf = (float*)malloc(bytes);

    Body* p = (Body*)buf;

    /*
     * As a constraint of this exercise, `randomizeBodies` must remain a host function.
     */

    randomizeBodies(buf, 6 * nBodies); // Init pos / vel data

    double totalTime = 0.0;

    /*
     * This simulation will run for 10 cycles of time, calculating gravitational
     * interaction amongst bodies, and adjusting their positions to reflect.
     */

    /*******************************************************************/
    // Do not modify these 2 lines of code.
    for (int iter = 0; iter < nIters; iter++) {
        StartTimer();
        /*******************************************************************/

        /*
         * You will likely wish to refactor the work being done in `bodyForce`,
         * as well as the work to integrate the positions.
         */

        bodyForce(p, dt, nBodies); // compute interbody forces

        /*
         * This position integration cannot occur until this round of `bodyForce` has completed.
         * Also, the next round of `bodyForce` cannot begin until the integration is complete.
         */

        for (int i = 0; i < nBodies; i++) { // integrate position
            p[i].x += p[i].vx * dt;
            p[i].y += p[i].vy * dt;
            p[i].z += p[i].vz * dt;
        }

        /*******************************************************************/
        // Do not modify the code in this section.
        const double tElapsed = GetTimer() / 1000.0;
        totalTime += tElapsed;
    }

    double avgTime = totalTime / (double)(nIters);
    float billionsOfOpsPerSecond = 1e-9 * nBodies * nBodies / avgTime;

#ifdef ASSESS
    checkPerformance(buf, billionsOfOpsPerSecond, salt);
#else
    checkAccuracy(buf, nBodies);
    printf("%d Bodies: average %0.3f Billion Interactions / second\n", nBodies, billionsOfOpsPerSecond);
    salt += 1;
#endif
    /*******************************************************************/

    /*
     * Feel free to modify code below.
     */

    free(buf);
}
