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

/* The C Programming Language: 2nd Edition
 *
 * Exercise 8-6: The standard library function calloc(n, size) returns a
 * pointer to `n` objects of size `size`, with the storage initialized to zero.
 * Write `calloc`, by calling malloc or by modifying it.
 *
 * Notes: Replacing the standard library functions here can be problematic if
 * you also intend to use the stdlib to output text or use other functions that
 * interact with *alloc, morecore, and free.
 *
 * I prefixed the functions with 'z' so I could hack on them *and* use the
 * standard library as needed.
 *
 * Ideally, one would use a debugger or other tool to show that memory is
 * assigned in the free list, and verify the size of the block of memory. We'll
 * revisit this when I'm good at debuggers. :)
 */

typedef long Align;

union header {
	struct {
		union header *ptr;
		unsigned size;
	} s;
	/* unused, just for alignment */
	Align x;
} hdr;

typedef union header Header;

static Header base;
static Header *freep = NULL;
static Header *zmorecore(unsigned);
void *zmalloc(unsigned);
void zfree(void *);

void *zmalloc(unsigned nbytes) {
	/* TODO: get the size field updated correctly */
	Header *p, *prevp;
	unsigned nunits;
	nunits = (nbytes + sizeof (Header) - 1) / sizeof (Header) + 1;

	if ((prevp = freep) == NULL) {
		base.s.ptr = freep = prevp = &base;
		base.s.size = 0;
	}
	for (p = prevp->s.ptr; ; prevp = p, p = p->s.ptr) {
		/* larger than requested units */
		if (p->s.size >= nunits) {
			if (p->s.size == nunits) {
				prevp->s.ptr = p->s.ptr;
			} else {
				p->s.size -= nunits;
				p += p->s.size;
				p->s.size = nunits;
			}
			freep = prevp;
			return (void *)(p+1);
		}
		if (p == freep) {
			if ((p = zmorecore(nunits)) == NULL) {
				return NULL;
			}
		}
	}
}

#define NALLOC 1024

static Header *zmorecore(unsigned nu) {
	char *cp, *sbrk(int);
	Header *up;
	if (nu < NALLOC) {
		nu = NALLOC;
	}
	cp = sbrk(nu * sizeof (Header));
	if (cp == (char*) -1) {
		return NULL;
	}
	up = (Header *) cp;
	up->s.size = nu;
	zfree((void *)(up+1));
	return freep;
}

void zfree(void *ap) {
	Header *bp, *p;
	bp = (Header *)ap - 1;
	for (p = freep; !(bp > p && bp < p->s.ptr); p = p->s.ptr) {
		if (p >= p->s.ptr && (bp > p || bp < p->s.ptr)) {
			break;
		}
	}
	if (bp + bp->s.size == p->s.ptr) {
		bp->s.size += p->s.ptr->s.size;
		bp->s.ptr = p->s.ptr->s.ptr;
	} else {
		bp->s.ptr = p->s.ptr;
	}
	if (p + p->s.size == bp) {
		p->s.size += bp->s.size;
		p->s.ptr = bp->s.ptr;
	} else {
		p->s.ptr = bp;
	}
	freep = p;
}

void * zcalloc(int count, unsigned size) {
	char *p;
	int i;
	p = zmalloc(count * size);
	if (p != NULL) {
		for (i = 0; i < (count * size); i++) {
			p[i] = '\0';
		}
		return p;
	} else {
		return (int *)(-1);
	}
}

int main(int argc, char *argv[]) {
	char *foo = "hello there!";
	char *buf = zcalloc(strlen(foo) + 1, sizeof (char));
	strncpy(buf, foo, 5);
	printf("%s\n", buf);
	printf("buf is %d long\n", strlen(buf));
	printf("foo is %d long\n", strlen(foo));
	printf("the header says buf's max size is %d\n", freep->s.size);
	return 0;
}