Knights tour recursive

Question

I'm trying to write knights tour recursive algorithm:

int mov[8][2] = {{2,1},{2,-1},{-2,1},{-2,-1},{1,2},{-1,2},{1,-2},{-1,-2}};

typedef struct element {
    int x;
    int y;
    struct element *prev, *next;
} node; 

//adding pool to list
void dEnd(node **root, int x,int y)
{
    node *pos;
    pos = *root;
    while(pos->next!= NULL)
    pos = pos->next;
    pos->next = (node *)malloc(sizeof(node));
    pos->next->x=x;
    pos->next->y=y;
    pos->next->prev=pos;
    pos->next->next=NULL;
}

void uEnd(node **root,int x,int y)
{
    node *pos;
    pos = *root;
    while(pos->x!= x && pos->y !=y)
    {
        pos = pos->next;
    }
    pos->prev->next=NULL;
    free(pos);
}

void printAll(node **root)
{
    node *pos = *root;
    while(pos->next)
    {
        printf("%d %d\n", pos->x,pos->y);
        pos = pos->next;
    }
}

int contains(int x,int y)
{
    return(((x >= 0 ) && (x <= 7)) && ((y >= 0) && (y <= 7)));
}
//find move
int searchForMove(int x, int y, int **tab, node **list, int *number)
{

    int i ;
    for(i = 0; i < 8; i++)
    {
        int nx, ny;
        nx = x + mov[i][0];
        ny = y + mov[i][1];
        if(contains(nx, ny) && !tab[nx][ny])
        {
            dEnd(list, nx, ny);
            tab[nx][ny] = 1;
            *number++;
            if(!searchForMove(nx,ny,tab,list,number))
            {
                uEnd(list,nx,ny);
                tab[nx][ny]=0;
                *number--;
            }
        }
    }
    if(i == 7 && *number <64)
        return 0;
    if(*number == 64)
        return 1;
}

Could someone show me where I made a mistake? I've checked step by step what pools algorithm is adding to list. What is big suprise algorithm after adding 4,3 pool and then 6,4 pool should call it self with 6,4 as actual position but I don't know why it's calling itself with 4,3 as actual position.

It's extremely hard to read code with non-English names. I don't know what a 'wesel' or a 'pom' is. — Bo Persson, Dec 03 '12 at 20:00
After translation to English it is still not an easy task to guess how this works. It would be nice to see how searchForMove is called to see what the arguments like tab should look like. — ljedrz, Dec 03 '12 at 20:28
tab is just tab[8][8] and start filled with 0 only pool from which we start is initalized with 1 — whd, Dec 03 '12 at 20:32

score 1 · Accepted Answer · answered Sep 19 '13 at 06:16

OP mostly had it. Aside from some minor code, it was the *number increment/decrement that was wrong in 2 places.

int searchForMove(int x, int y, int **tab, node **list, int *number) {
  ...
  // *number++;  // This increase the pointer to the next pointer.
  (*number)++; // This dereferences number and increases it.
  ...
    // *number--;
    (*number)--; // CD

Working Example. "CD" implies my changes

// CD
#include <assert.h>
#include <inttypes.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

int mov[8][2] = { { 2, 1 }, { 2, -1 }, { -2, 1 }, { -2, -1 }, { 1, 2 },
        { -1, 2 }, { 1, -2 }, { -1, -2 } };

typedef struct element {
  int x;
  int y;
  struct element *prev, *next;
} node;

//adding pool to list
void dEnd(node **root, int x, int y) {
  node *pos;
  pos = *root;
  assert(pos); // CD
  while (pos->next != NULL)
    pos = pos->next;
  pos->next = (node *) malloc(sizeof(node));
  pos->next->x = x;
  pos->next->y = y;
  pos->next->prev = pos;
  pos->next->next = NULL;
}

void uEnd(node **root, int x, int y) {
  node *pos;
  pos = *root;
  while (pos->x != x && pos->y != y) {
    pos = pos->next;
  }
  pos->prev->next = NULL;
  free(pos);
}

void printAll(node **root) {
  node *pos = *root;
  unsigned i = 0; // CD
  uint64_t mask = 0; // CD
  while (pos->next) {
    // printf("%d %d\n", pos->x, pos->y);
    printf("%2u %d %d\n", i++, pos->x, pos->y); // CD prepend visit index.
    mask |= 1llu << (pos->x + 8*pos->y); // CD accumulate visited squares
    pos = pos->next;
  }
  printf("Mask %016" PRIX64 "\n", mask); // CD Show all locations visited
}

int contains(int x, int y) {
  return (((x >= 0) && (x <= 7)) && ((y >= 0) && (y <= 7)));
}

//find move
int searchForMove(int x, int y, int **tab, node **list, int *number) {
  int i;
  for (i = 0; i < 8; i++) {
    int nx, ny;
    nx = x + mov[i][0];
    ny = y + mov[i][1];
    if (contains(nx, ny) && !tab[nx][ny]) {
      dEnd(list, nx, ny);
      tab[nx][ny] = 1;

      // *number++;
      (*number)++; // CD

      if (!searchForMove(nx, ny, tab, list, number)) {
        uEnd(list, nx, ny);
        tab[nx][ny] = 0;

        // *number--;
        (*number)--; // CD

      }
    }
  }
  if (i == 7 && *number < 64)
    return 0;
  if (*number == 64)
    return 1;
  return 2;  // CD added
}

// All added by CD
int main(int argc, char *argv[]) {
  int tab0[8] = {0,0,0,0,0,0,0,0};
  int tab1[8] = {0,0,0,0,0,0,0,0};
  int tab2[8] = {0,0,0,0,0,0,0,0};
  int tab3[8] = {0,0,0,0,0,0,0,0};
  int tab4[8] = {0,0,0,0,0,0,0,0};
  int tab5[8] = {0,0,0,0,0,0,0,0};
  int tab6[8] = {0,0,0,0,0,0,0,0};
  int tab7[8] = {0,0,0,0,0,0,0,0};
  int *tab[8] = {tab0, tab1, tab2, tab3, tab4, tab5, tab6, tab7 };
  node HeadNode = { 0, 0, NULL, NULL };
  node *pHeadNode = &HeadNode;
  int number = 0;
  int result;
  result = searchForMove(0, 0, tab, &pHeadNode, &number);
  printAll(&pHeadNode);
  return result;
}

Knights tour recursive

1 Answers1