/* listp.c  using more pointers for the functions of list.asm            */

#include <stdio.h>       /* extern printf   in list.asm                  */

static char heap[20000]; /* space to store strings, do not reuse or free */
static char *hp = heap;  /* pointer to next available heap space         */
static int  list[1000];  /* space to store list block (3 pointers)       */
static int  *lp= list;   /* pointer to next available list space         */
static char *q;          /* a variable char pointer                      */
static int  *i;          /* a variable int pointer                       */
/*   +-----------------+   +-----------------+   +-----------------+
*L-> |pointer to next----->|pointer to next----->|  0              |
     |  0              |<----pointer to prev |<----pointer to prev |<-*(L+1)
     | ptr to heap str |   | ptr to heap str |   | ptr to heap str |
     +-----------------+   +-----------------+   +-----------------+
The pointers to heap strings are character pointers to terminated strings.
*/

void clear(int *L)       /* initialize front and back pointers to zero */
{
  *L=0;          /* later, will be pointer into "front" of list */
  *(L+1)=0;      /* later, will be pointer into "back" of list */
}

void print(int *L)
{
  i=(int *) *L;  /* pointer into "front" of list */
  while(i)      /* keep looping until i==0, meaning end of list */
  {
    q=(char *) *(i+2);   /* get this list items pointer to string */
    printf("%s\n",q);    /* print this list item's string */
    i=(int *) *i;        /* move to next list item */
  }
  printf("\n");          /* blank line after items */
}

void push_front(int *L, char *s)
{
  if(*L==0)              /* list is empty */
  {
    *L=(int)lp;          /* front pointer */
    *(L+1)=(int)lp;      /* back pointer */
    *lp=0;               /* new next pointer is end */
    *(lp+1)=0;           /* new prev pointer is end */
  }
  else
  {
    i=(int *) *L;        /* save pointer to old front */
    *L=(int)lp;          /* new front pointer */
    *lp=(int)i;          /* new next pointer is old front */
    *(lp+1)=0;           /* new prev pointer is end */
                         /* old front next is unchanged */
    *(i+1)=(int)lp;      /* old front prev is now new front */
  }
  *(lp+2)=(int)hp;       /* list pointer to string on heap*/
  lp=lp+3;               /* update to next free space in list */
  q=s;
  while(*q)              /* copy string s to heap */
  {
    *hp++=*q++;
  }
  *hp=0;                 /* save the final null and update heap pointer */
  hp++;                  /* we should do range checking, but won't */
}

void push_back(int *L, char *s)
{
  if(*L==0)              /* list is empty */
  {
    *L=(int)lp;          /* front pointer */
    *(L+1)=(int)lp;      /* back pointer */
    *lp=0;               /* new next pointer is end */
    *(lp+1)=0;           /* new prev pointer is end */
  }
  else
  {
    i=(int *) *(L+1);    /* save pointer to old back */
    *(L+1)=(int)lp;      /* new back pointer */
    *lp=0;               /* new next is end */
    *(lp+1)=(int)i;      /* new prev pointer is old back */
    *i=(int)lp;          /* old back next is new back */
                         /* old back prev is unchanged */
  }
  *(lp+2)=(int)hp;       /* list pointer to string on heap*/
  lp=lp+3;               /* update to next free space in list */
  q=s;
  while(*q)              /* copy string s to heap */
  {
    *hp++=*q++;
  }
  *hp=0;                 /* save the final null and update heap pointer */
  hp++;                  /* we should do range checking, but won't */
}

void pop_front(int *L)
{
  if(*L==0) return;      /* list is already empty */
  if(*(L+1)==*L)         /* only one item on list */
  {
    *L=0;                /* delete one item, same as clear */
    *(L+1)=0;
    return;
  }
  i=(int *) *L;          /* get pointer of old front */
  i=(int *) *i;          /* get next from old front */
  *L=(int)i;             /* L is new front */
                         /* new next is unchanged */
  *(i+1)=0;              /* new prev is now end */         
}

void pop_back(int *L)
{
  if(*(L+1)==0) return;  /* list already empty */
  if(*(L+1)==*L)         /* only one item on list */
  {
    *L=0;                /* delete one item, same as clear */
    *(L+1)=0;
    return;
  }
  i=(int *) *(L+1);      /* get pointer to back */
  i=(int *) *(i+1);      /* get pointer of prev to back */
  *(L+1)=(int)i;         /* L+1 is new back */
  *i=0;                  /* new next is now end */
                         /* new prev is unchanged */
}
/* end list.c file */