/*----------------------------------------------------------------------
  File    : stack_l.c
  Contents: general stack management (singly connected list version)
  Author  : Christian Borgelt
  History : 04.06.1998 file created
----------------------------------------------------------------------*/
#include <stdio.h>
#include <stdlib.h>
#include "stack.h"

/*----------------------------------------------------------------------
  Type Definitions
----------------------------------------------------------------------*/
typedef struct _ste {           /* --- a stack element --- */
  struct _ste *succ;            /* pointer to successor element */
  void        *obj;             /* pointer to stored object */
} STE;                          /* (stack element) */

struct stack {                  /* --- a stack --- */
  STE         *top;             /* pointer to top of stack */
};                              /* (stack) */

/*----------------------------------------------------------------------
  Functions
----------------------------------------------------------------------*/

STACK* stk_create (void)
{                               /* --- create an empty stack */
  STACK *stk;                   /* created stack */
  
  stk = (STACK*)malloc(sizeof(STACK));
  if (!stk) return NULL;        /* allocate memory and */
  stk->top = NULL;              /* initialize field */
  return stk;                   /* return the created stack */
}  /* stk_create() */

/*--------------------------------------------------------------------*/

void stk_delete (STACK *stk)
{                               /* --- delete a stack */
  STE *ste, *tmp;               /* to traverse the stack elements */

  ste = stk->top;               /* get the top stack element */
  while (ste) {                 /* while there is another */
    tmp = ste;                  /* stack element, note it, */
    ste = ste->succ;            /* remove it from the list, */
    free(tmp);                  /* and delete it */
  }                             /* (deallocate the memory) */
  free(stk);                    /* delete the stack body */
}  /* stk_delete() */

/*--------------------------------------------------------------------*/

int stk_empty (STACK *stk)
{                               /* --- check whether stack is empty */
  return (stk->top == NULL);    /* check pointer to top element */
}  /* stk_empty() */

/*--------------------------------------------------------------------*/

int stk_push (STACK *stk, void *obj)
{                               /* --- push an object onto a stack */
  STE *ste;                     /* new stack element */

  ste = (STE*)malloc(sizeof(STE));
  if (!ste) return -1;          /* create a new stack element */  
  ste->obj  = obj;              /* enter pointer to object, */
  ste->succ = stk->top;         /* append current stack, */
  stk->top  = ste;              /* and set new top element */
  return 0;                     /* return `ok' */
}  /* stk_push() */

/*--------------------------------------------------------------------*/

void* stk_pop (STACK *stk, int keep)
{                               /* --- pop an object from a stack */
  STE  *ste;                    /* top stack element */
  void *obj;                    /* object in top element */

  if (stk->top == NULL)         /* is the stack is empty, */
    return NULL;                /* abort the function */
  ste = stk->top;               /* get top stack element and */
  obj = ste->obj;               /* the object stored in it */
  if (!keep) {                  /* if not to keep it on the stack, */
    stk->top = ste->succ;       /* remove top element from stack */
    free(ste);                  /* and delete it */
  }                             /* (deallocate the memory) */
  return obj;                   /* return top object */
}  /* stk_pop() */