sys: lib: introduce ringbuffer_remove()

This commit adds a function that deletes n elements from the ringbuffer
without actually writing them somewhere.

sys/include/ringbuffer.h

@@ -86,6 +86,14 @@ int ringbuffer_get_one(ringbuffer_t *restrict rb);
  */
 unsigned ringbuffer_get(ringbuffer_t *restrict rb, char *buf, unsigned n);
 
+/**
+ * @brief           Remove a number of elements from the ringbuffer.
+ * @param[in,out]   rb    Ringbuffer to operate on.
+ * @param[in]       n     Read at most n elements.
+ * @returns         Number of elements actually removed.
+ */
+unsigned ringbuffer_remove(ringbuffer_t *restrict rb, unsigned n);
+
 /**
  * @brief           Test if the ringbuffer is empty.
  * @param[in,out]   rb    Ringbuffer to operate on.

sys/lib/ringbuffer.c

@@ -102,6 +102,25 @@ unsigned ringbuffer_get(ringbuffer_t *restrict rb, char *buf, unsigned n)
     return n;
 }
 
+unsigned ringbuffer_remove(ringbuffer_t *restrict rb, unsigned n)
+{
+    if (n > rb->avail) {
+        n = rb->avail;
+        rb->start = rb->avail = 0;
+    }
+    else {
+        rb->start -= n;
+        rb->avail -= n;
+
+        /* compensate underflow */
+        if (rb->start > rb->size) {
+            rb->start += rb->size;
+        }
+    }
+
+    return n;
+}
+
 int ringbuffer_peek_one(const ringbuffer_t *restrict rb_)
 {
     ringbuffer_t rb = *rb_;