/*
Sample input:
   A partridge in a pear tree.
   Two turtle doves.
   Three french hens.
   Four calling birds.
   Five golden rings.
Some standard library functions used here:
   printf("%s",...)
   realloc()
   strlen()
   strcpy()
   strcat()
   fprintf(stderr,...)
Make sure you understand how ds_getline() works.
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void ds_print(char **p,int *q)
{
  if (*p) printf("%s",*p);
}

int ds_copy(char **p,int *q,const char *s)
{
  int len;
  char *y;

  len = strlen(s) + 1;

  if (*q < len) {
    len += len / 8;
    y = realloc(*p,len);
    if (!y) return 0;
    *p = y;
    *q = len;
  }
  strcpy(*p,s);
  return 1;
}

int ds_concat(char **p,int *q,const char *s)
{
  int len;
  char *y;

  if (!*p) return ds_copy(p,q,s);
  len = strlen(*p) + strlen(s) + 1;

  if (*q < len) {
    len += len / 8;
    y = realloc(*p,len);
    if (!y) return 0;
    *p = y;
    *q = len;
  }
  strcat(*p,s);
  return 1;
}

int ds_getline(char **p,int *q)
{
  char s[2];

  if (!ds_copy(p,q,"")) return 0;

  for (;;) {
    if (scanf("%c",s) < 1) return 1;
    if (!*s) *s = '\n'; /* treat 0 in input as if it were \n */
    s[1] = 0;
    if (!ds_concat(p,q,s)) return 0;
    if (*s == '\n') return 1;
  }
}

void die_nomem(void)
{
  fprintf(stderr,"dsrev: fatal: out of memory\n");
  exit(111);
}

int main(void)
{
  char *x = 0; /* current line */
  int xspace = 0;
  char *y = 0; /* all previous lines, in reverse order */
  int yspace = 0;

  if (!ds_copy(&y,&yspace,"")) die_nomem();
  for (;;) {
    if (!ds_getline(&x,&xspace)) die_nomem();
    if (!*x) return 0;  /* !*x from ds_getline() means end of input */
    if (!ds_concat(&x,&xspace,y)) die_nomem();
    ds_print(&x,&xspace);
    if (!ds_copy(&y,&yspace,x)) die_nomem();
  }
}