string_builder.c

typedef struct StringBuilder StringBuilder;
struct StringBuilder {
	char   *data;
	size_t len;
	size_t cap;
};

static void sbprintf(StringBuilder*, const char*, ...) __attribute__((format(printf, 2, 3)));

int
sbprintf(StringBuilder *b, const char *format, ...)
{
	assert(b->len <= b->cap);

again:;
	char *data = NULL;
	if (b->cap > 0)
		data = b->data + b->len;
	va_list arg;
	va_start(arg, format);
	int r = vsnprintf(data, b->cap-b->len, format, arg);
	va_end(arg);
	if (r <= 0)
		return r;
	size_t n = (size_t)r;
	size_t newcap = b->cap;
	while (newcap-b->len < n+1) {
		newcap = 2*newcap + (newcap == 0);
		assert(newcap >= b->cap); // ensure we don't wrap around
	}
	if (b->cap < newcap) {
		b->data = erealloc(b->data, newcap);
		b->cap = newcap;
		goto again;
	}
	b->len += n;
	b->data[b->len] = '\0';
	return r;
}