sys: bitfield: add bf_get_unset

sys/bitfield/Makefile

@@ -0,0 +1 @@
+include $(RIOTBASE)/Makefile.base

sys/bitfield/bitfield.c

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2015 Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     sys_util
+ * @{
+ *
+ * @file
+ * @brief       Bitfield auxillary functions
+ *
+ * @author      Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * @}
+ */
+
+#include <stdint.h>
+#include "bitfield.h"
+#include "irq.h"
+
+int bf_get_unset(uint8_t field[], int size)
+{
+    int result = -1;
+    int nbytes = (size + 7) / 8;
+    int i = 0;
+
+    unsigned state = disableIRQ();
+
+    /* skip full bytes */
+    for (int j = 0; (j < nbytes) && (field[j] == 255); j++) {
+        i += 8;
+    }
+
+    for (; i < size; i++) {
+        if (!bf_isset(field, i)) {
+            bf_set(field, i);
+            result = i;
+            break;
+        }
+    }
+
+    restoreIRQ(state);
+    return(result);
+}

sys/include/bitfield.h

@@ -85,6 +85,19 @@ static inline bool bf_isset(uint8_t field[], size_t idx)
     return (field[idx / 8] & (1u << (idx % 8)));
 }
 
+/**
+ * @brief  Atomically get the number of an unset bit and set it
+ *
+ * This function can be used to record e.g., empty entries in an array.
+ *
+ * @param[in,out] field The bitfield
+ * @param[in]     size  The size of the bitfield
+ *
+ * @return      number of bit that was set
+ * @return      -1 if no bit was unset
+ */
+int bf_get_unset(uint8_t field[], int size);
+
 #ifdef __cplusplus
 }
 #endif