From a6623f834f3d7af12f1557aba0745a40597f29df Mon Sep 17 00:00:00 2001
From: Martine Lenders <mail@martine-lenders.eu>
Date: Wed, 20 Nov 2019 18:45:58 +0100
Subject: [PATCH] bitfield: unify order

Currently the bitfield type mixes up the order of bits: While the byte
order is big-endian (most significant byte first), the bit order of each
single byte within the bitfield is little-endian (most significant bit
last). While this isn't a problem for most applications of the bitfield
type it becomes one when sending the bitfield over the network (as done
e.g. in the [ACKs of Selective Fragment Recovery][SFR-ACKs]).

This change unifies byte order and bit order to both be most
significant bX first.

[SFR-ACKs]: https://tools.ietf.org/html/draft-ietf-6lo-fragment-recovery-07
---
 sys/include/bitfield.h | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/sys/include/bitfield.h b/sys/include/bitfield.h
index a0e89acd62..73225ef14f 100644
--- a/sys/include/bitfield.h
+++ b/sys/include/bitfield.h
@@ -10,6 +10,10 @@
  * @defgroup    sys_bitfield Bitfields
  * @ingroup     sys
  * @brief       Bitfields of arbitrary length
+ *
+ * The bitfields in this module have their most significant bytes first and
+ * their most significant bits within each byte of the bitfield also first.
+ *
  * @file
  * @{
  *
@@ -49,7 +53,7 @@ extern "C" {
  */
 static inline void bf_set(uint8_t field[], size_t idx)
 {
-    field[idx / 8] |= (1u << (idx % 8));
+    field[idx / 8] |= (1u << (7 - (idx % 8)));
 }
 
 /**
@@ -60,7 +64,7 @@ static inline void bf_set(uint8_t field[], size_t idx)
  */
 static inline void bf_unset(uint8_t field[], size_t idx)
 {
-    field[idx / 8] &= ~(1u << (idx % 8));
+    field[idx / 8] &= ~(1u << (7 - (idx % 8)));
 }
 
 /**
@@ -71,7 +75,7 @@ static inline void bf_unset(uint8_t field[], size_t idx)
  */
 static inline void bf_toggle(uint8_t field[], size_t idx)
 {
-    field[idx / 8] ^= (1u << (idx % 8));
+    field[idx / 8] ^= (1u << (7 - (idx % 8)));
 }
 
 /**
@@ -82,7 +86,7 @@ static inline void bf_toggle(uint8_t field[], size_t idx)
  */
 static inline bool bf_isset(uint8_t field[], size_t idx)
 {
-    return (field[idx / 8] & (1u << (idx % 8)));
+    return (field[idx / 8] & (1u << (7 - (idx % 8))));
 }
 
 /**