From 1f94a93fb8f2e4a67c7ff1477d8e8c7ea60085e8 Mon Sep 17 00:00:00 2001
From: Joakim Gebart <joakim.gebart@eistec.se>
Date: Sun, 24 May 2015 07:31:43 +0200
Subject: [PATCH] sys/hashes: Add Fletcher's checksums

---
 sys/hashes/hashes.c  | 39 +++++++++++++++++++++++++++++++++++++++
 sys/include/hashes.h | 31 +++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+)

diff --git a/sys/hashes/hashes.c b/sys/hashes/hashes.c
index 5086130f62..8b85642d1f 100644
--- a/sys/hashes/hashes.c
+++ b/sys/hashes/hashes.c
@@ -12,6 +12,7 @@
  * @file
  * @author      Jason Linehan <patientulysses@gmail.com>
  * @author      Christian Mehlis <mehlis@inf.fu-berlin.de>
+ * @author      Joakim Gebart <joakim.gebart@eistec.se>
  */
 
 #include "hashes.h"
@@ -109,3 +110,41 @@ uint32_t one_at_a_time_hash(const uint8_t *buf, size_t len)
     hash += hash << 15;
     return hash;
 }
+
+uint16_t fletcher16(const uint8_t *data, size_t bytes)
+{
+    uint16_t sum1 = 0xff, sum2 = 0xff;
+
+    while (bytes) {
+        size_t tlen = bytes > 20 ? 20 : bytes;
+        bytes -= tlen;
+        do {
+            sum2 += sum1 += *data++;
+        } while (--tlen);
+        sum1 = (sum1 & 0xff) + (sum1 >> 8);
+        sum2 = (sum2 & 0xff) + (sum2 >> 8);
+    }
+    /* Second reduction step to reduce sums to 8 bits */
+    sum1 = (sum1 & 0xff) + (sum1 >> 8);
+    sum2 = (sum2 & 0xff) + (sum2 >> 8);
+    return (sum2 << 8) | sum1;
+}
+
+uint32_t fletcher32(const uint16_t *data, size_t words)
+{
+    uint32_t sum1 = 0xffff, sum2 = 0xffff;
+
+    while (words) {
+        unsigned tlen = words > 359 ? 359 : words;
+        words -= tlen;
+        do {
+            sum2 += sum1 += *data++;
+        } while (--tlen);
+        sum1 = (sum1 & 0xffff) + (sum1 >> 16);
+        sum2 = (sum2 & 0xffff) + (sum2 >> 16);
+    }
+    /* Second reduction step to reduce sums to 16 bits */
+    sum1 = (sum1 & 0xffff) + (sum1 >> 16);
+    sum2 = (sum2 & 0xffff) + (sum2 >> 16);
+    return (sum2 << 16) | sum1;
+}
diff --git a/sys/include/hashes.h b/sys/include/hashes.h
index d81e2dfe39..c03848b12c 100644
--- a/sys/include/hashes.h
+++ b/sys/include/hashes.h
@@ -17,6 +17,7 @@
  *
  * @author      Jason Linehan <patientulysses@gmail.com>
  * @author      Christian Mehlis <mehlis@inf.fu-berlin.de>
+ * @author      Joakim Gebart <joakim.gebart@eistec.se>
  */
 
 #ifndef HASHES_H_
@@ -159,6 +160,36 @@ uint32_t rotating_hash(const uint8_t *buf, size_t len);
  */
 uint32_t one_at_a_time_hash(const uint8_t *buf, size_t len);
 
+/**
+ * @brief Fletcher's 16 bit checksum
+ *
+ * found on
+ * http://en.wikipedia.org/w/index.php?title=Fletcher%27s_checksum&oldid=661273016#Optimizations
+ *
+ * @note the returned sum is never 0
+ *
+ * @param buf input buffer to hash
+ * @param bytes length of buffer, in bytes
+ * @return 16 bit sized hash in the interval [1..65535]
+ */
+uint16_t fletcher16(const uint8_t *buf, size_t bytes);
+
+/**
+ * @brief Fletcher's 32 bit checksum
+ *
+ * found on
+ * http://en.wikipedia.org/w/index.php?title=Fletcher%27s_checksum&oldid=661273016#Optimizations
+ *
+ * @note the returned sum is never 0
+ * @note pay attention to alignment issues since this operates on an input
+ *       buffer containing 16 bit words, not bytes.
+ *
+ * @param buf input buffer to hash
+ * @param words length of buffer, in 16 bit words
+ * @return 32 bit sized hash in the interval [1..2^32]
+ */
+uint32_t fletcher32(const uint16_t *buf, size_t words);
+
 #ifdef __cplusplus
 }
 #endif