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/*
* Lisp construct implementation.
* This is free and unencumbered software released into the public domain.
*/
#include <stdlib.h>
#include "construct.h"
construct *cons(void *first, construct *rest)
{
construct *list = malloc(sizeof(construct));
list->car = first;
list->cdr = rest;
return list;
}
void *car(construct *list)
{
return list->car;
}
construct *cdr(construct *list)
{
return list->cdr;
}
size_t length(construct *list)
{
if (list == NULL)
return 0;
return length(cdr(list)) + 1;
}
void *nth(construct *list, size_t n)
{
if (list == NULL)
return NULL;
return n ? nth(cdr(list), n - 1) : car(list);
}
/*
* Try to insert x before item of index i and return the new construct.
* Return NULL on invalid index.
*/
construct *insert(void *x, construct *list, size_t i)
{
if (!i)
return cons(x, list);
if (list == NULL)
return NULL;
void *first = car(list);
construct *rest = cdr(list);
free(list);
return cons(first, insert(x, rest, i - 1));
}
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