#include <sys/param.h>

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

struct strdata {
	char **ptr;
	size_t lim;
	size_t cur;
	size_t max;
};

static ssize_t
fixptr(struct strdata *d, size_t len)
{
	char *ptr = *d->ptr;

	len++;	/* for NUL */
	if (ptr != NULL && d->lim - d->cur > len)
		return 0;

	char *nptr = realloc(ptr, d->lim + len);
	if (nptr == NULL)
		return -1;

	*d->ptr = nptr;
	d->lim += len;

	return 0;
}

static ssize_t
strwrite(void *p, const void *buf, size_t len)
{
	struct strdata *d = p;

	if (fixptr(d, len) == -1)
		return -1;

	memcpy(*d->ptr + d->cur, buf, len);
	d->cur += len;
	(*d->ptr)[d->cur] = '\0';
	d->max = MAX(d->cur, d->max);

	return len;
}

static ssize_t
strread(void *p, void *buf, size_t len)
{
	struct strdata *d = p;

	if (len > d->max - d->cur)
		len = d->max - d->cur;

	memcpy(buf, *d->ptr + d->cur, len);
	d->cur += len;

	return len;
}

static off_t
strseek(void *p, off_t pos, int whence)
{
	struct strdata *d = p;
	off_t len;

	switch (whence) {
	case SEEK_END:
		len = pos + d->max;
		break;
	case SEEK_CUR:
		len = pos + d->cur;
		break;
	case SEEK_SET:
		len = pos;
		break;
	default:
		errno = EINVAL;
		return -1;
	}

	if (len < 0) {
		errno = EINVAL;
		return -1;
	}

	if (fixptr(d, (size_t)len) == -1)
		return -1;

	d->cur = (size_t)len;
	d->max = MAX(d->cur, d->max);
	return d->cur;
}

static int
fstrclose(void *p)
{
	free(p);
}

FILE *
fstropen(char **str)
{
		
	struct strdata *d = calloc(1, sizeof(*d));

	if (d == NULL)
		return NULL;

	*str = NULL;
	d->ptr = str;

	return funopen2(d, strread, strwrite, strseek, NULL, fstrclose);
}

#ifdef TEST
int
main(int argc, char *argv[])
{
	char *str;
	FILE *fp = fstropen(&str);

	if (fp == NULL)
		err(EXIT_FAILURE, "Can't open stream");

	for (int i = 1; i < argc; i++)
		fprintf(fp, "%d: %s\n", i, argv[i]);

	fflush(fp);
	printf("str=%s", str);

	if (fseek(fp, 0, SEEK_SET) == -1)
		err(EXIT_FAILURE, "seek");

	size_t size = strlen(str) + 1;
	char *buf = malloc(size);
	size_t len = fread(buf, 1, size, fp);
	if (len != size - 1)
		errx(EXIT_FAILURE, "%zu != %zu", len, size - 1);
	buf[len] = '\0';
	printf("buf=%s", buf);
	fclose(fp);

	return EXIT_SUCCESS;
}
#endif