ringbuffer.c

/*
Ringbuffers are surprisingly hard to code because wraparound
can split the readable / writeable region in two. This is the
cutest implementation that I've come up with so far.

NOT ACTUALLY TESTED

- jstimpfle, 2020-09
*/

struct Ringbuffer {
        char *buffer;
        int capacity;
        int start;
        int fill;
};

int ringbuffer_read_bytes(struct Ringbuffer *rb, void *data, int size)
{
        if (size > rb->fill)
                size = rb->fill;
        int start = rb->start;
        int end = rb->start + size;
        int n = (end >= rb->capacity ? rb->capacity : end) - start;
        memcpy(data, rb->buffer + start, n);
        memcpy((char *)data + n, rb->buffer, size - n);
        rb->start = end - (end >= rb->capacity ? rb->capacity : 0);
        rb->fill -= size;
        return size;
}

int ringbuffer_write_bytes(struct Ringbuffer *rb, const void *data, int size)
{
        if (size > rb->capacity - rb->fill)
                size = rb->capacity - rb->fill;
        int start = rb->start + rb->fill;
        if (start >= rb->capacity)
                start -= rb->capacity;
        int end = start + size;
        int n = (end >= rb->capacity ? rb->capacity : end) - start;
        memcpy(rb->buffer + start, data, n);
        memcpy(rb->buffer, (char *)data + n, size - n);
        rb->fill += size;
        return size;
}