SEQ_Poisson.c

/*
 * SEQ_Poisson.c
 * 2D Poison equation solver
 */

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

#define DEBUG 0

#define max(a,b) ((a)>(b)?a:b)

enum
{
  X_DIR, Y_DIR
};

/* global variables */
int gridsize[2];
double precision_goal;		/* precision_goal of solution */
int max_iter;			/* maximum number of iterations alowed */

/* benchmark related variables */
clock_t ticks;			/* number of systemticks */
int timer_on = 0;		/* is timer running? */

/* local grid related variables */
double **phi;			/* grid */
int **source;			/* TRUE if subgrid element is a source */
int dim[2];			/* grid dimensions */

void Setup_Grid();
double Do_Step(int parity);
void Solve();
void Write_Grid();
void Clean_Up();
void Debug(char *mesg, int terminate);
void start_timer();
void resume_timer();
void stop_timer();
void print_timer();

void start_timer()
{
  if (!timer_on)
  {
    ticks = clock();
    timer_on = 1;
  }
}

void resume_timer()
{
  if (!timer_on)
  {
    ticks = clock() - ticks;
    timer_on = 1;
  }
}

void stop_timer()
{
  if (timer_on)
  {
    ticks = clock() - ticks;
    timer_on = 0;
  }
}

void print_timer()
{
  if (timer_on)
  {
    stop_timer();
    printf("Elapsed processortime: %14.6f s\n", ticks * (1.0 / CLOCKS_PER_SEC));
    resume_timer();
  }
  else
    printf("Elapsed processortime: %14.6f s\n", ticks * (1.0 / CLOCKS_PER_SEC));
}

void Debug(char *mesg, int terminate)
{
  if (DEBUG || terminate)
    printf("%s\n", mesg);
  if (terminate)
    exit(1);
}

void Setup_Grid()
{
  int x, y, s;
  double source_x, source_y, source_val;
  FILE *f;

  Debug("Setup_Subgrid", 0);

  f = fopen("input.dat", "r");
  if (f == NULL)
    Debug("Error opening input.dat", 1);
  fscanf(f, "nx: %i\n", &gridsize[X_DIR]);
  fscanf(f, "ny: %i\n", &gridsize[Y_DIR]);
  fscanf(f, "precision goal: %lf\n", &precision_goal);
  fscanf(f, "max iterations: %i\n", &max_iter);

  /* Calculate dimensions of local subgrid */
  dim[X_DIR] = gridsize[X_DIR] + 2;
  dim[Y_DIR] = gridsize[Y_DIR] + 2;

  /* allocate memory */
  if ((phi = malloc(dim[X_DIR] * sizeof(*phi))) == NULL)
    Debug("Setup_Subgrid : malloc(phi) failed", 1);
  if ((source = malloc(dim[X_DIR] * sizeof(*source))) == NULL)
    Debug("Setup_Subgrid : malloc(source) failed", 1);
  if ((phi[0] = malloc(dim[Y_DIR] * dim[X_DIR] * sizeof(**phi))) == NULL)
    Debug("Setup_Subgrid : malloc(*phi) failed", 1);
  if ((source[0] = malloc(dim[Y_DIR] * dim[X_DIR] * sizeof(**source))) == NULL)
    Debug("Setup_Subgrid : malloc(*source) failed", 1);
  for (x = 1; x < dim[X_DIR]; x++)
  {
    phi[x] = phi[0] + x * dim[Y_DIR];
    source[x] = source[0] + x * dim[Y_DIR];
  }

  /* set all values to '0' */
  for (x = 0; x < dim[X_DIR]; x++)
    for (y = 0; y < dim[Y_DIR]; y++)
    {
      phi[x][y] = 0.0;
      source[x][y] = 0;
    }

  /* put sources in field */
  do
  {
    s = fscanf(f, "source: %lf %lf %lf\n", &source_x, &source_y, &source_val);
    if (s==3)
    {
      x = source_x * gridsize[X_DIR];
      y = source_y * gridsize[Y_DIR];
      x += 1;
      y += 1;
      phi[x][y] = source_val;
      source[x][y] = 1;
    }
  }
  while (s==3);

  fclose(f);
}

double Do_Step(int parity)
{
  int x, y;
  double old_phi;
  double max_err = 0.0;

  /* calculate interior of grid */
  for (x = 1; x < dim[X_DIR] - 1; x++)
    for (y = 1; y < dim[Y_DIR] - 1; y++)
      if ((x + y) % 2 == parity && source[x][y] != 1)
      {
	old_phi = phi[x][y];
	phi[x][y] = (phi[x + 1][y] + phi[x - 1][y] +
		     phi[x][y + 1] + phi[x][y - 1]) * 0.25;
	if (max_err < fabs(old_phi - phi[x][y]))
	  max_err = fabs(old_phi - phi[x][y]);
      }

  return max_err;
}

void Solve()
{
  int count = 0;
  double delta;
  double delta1, delta2;

  Debug("Solve", 0);

  /* give global_delta a higher value then precision_goal */
  delta = 2 * precision_goal;

  while (delta > precision_goal && count < max_iter)
  {
    Debug("Do_Step 0", 0);
    delta1 = Do_Step(0);

    Debug("Do_Step 1", 0);
    delta2 = Do_Step(1);

    delta = max(delta1, delta2);
    count++;
  }

  printf("Number of iterations : %i\n", count);
}

void Write_Grid()
{
  int x, y;
  FILE *f;

  if ((f = fopen("output.dat", "w")) == NULL)
    Debug("Write_Grid : fopen failed", 1);

  Debug("Write_Grid", 0);

  for (x = 1; x < dim[X_DIR] - 1; x++)
    for (y = 1; y < dim[Y_DIR] - 1; y++)
      fprintf(f, "%i %i %f\n", x, y, phi[x][y]);

  fclose(f);
}

void Clean_Up()
{
  Debug("Clean_Up", 0);

  free(phi[0]);
  free(phi);
  free(source[0]);
  free(source);
}

int main(int argc, char **argv)
{
  start_timer();

  Setup_Grid();

  Solve();

  Write_Grid();

  print_timer();

  Clean_Up();

  return 0;
}