Merge pull request #10228 from bergzand/pr/checksum/fletcher16_split

Fletcher16: extend with multi-part functions

sys/checksum/fletcher16.c

@@ -20,21 +20,41 @@
 
 #include "checksum/fletcher16.h"
 
+static inline void _reduce(fletcher16_ctx_t *ctx)
+{
+    ctx->sum1 = (ctx->sum1 & 0xff) + (ctx->sum1 >> 8);
+    ctx->sum2 = (ctx->sum2 & 0xff) + (ctx->sum2 >> 8);
+}
+
+void fletcher16_init(fletcher16_ctx_t *ctx)
+{
+    ctx->sum1 = 0xff;
+    ctx->sum2 = 0xff;
+}
+
+void fletcher16_update(fletcher16_ctx_t *ctx, const uint8_t *data, size_t len)
+{
+    while (len) {
+        size_t tlen = len > 20 ? 20 : len;
+        len -= tlen;
+        do {
+            ctx->sum2 += ctx->sum1 += *data++;
+        } while (--tlen);
+        _reduce(ctx);
+    }
+}
+
+uint16_t fletcher16_finish(fletcher16_ctx_t *ctx)
+{
+    /* Second reduction step to reduce sums to 8 bits */
+    _reduce(ctx);
+    return (ctx->sum2 << 8) | ctx->sum1;
+}
+
 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;
+    fletcher16_ctx_t ctx;
+    fletcher16_init(&ctx);
+    fletcher16_update(&ctx, data, bytes);
+    return fletcher16_finish(&ctx);
 }

sys/include/checksum/fletcher16.h

@@ -27,8 +27,17 @@
 extern "C" {
 #endif
 
+/**
+ * @brief Fletcher's 16 bit checksum context struct
+ */
+typedef struct {
+    uint16_t sum1;  /**< First sum of the checksum */
+    uint16_t sum2;  /**< Second sum of the checksum */
+} fletcher16_ctx_t;
+
 /**
  * @brief Fletcher's 16 bit checksum
+ * @anchor fletcher16_full
  *
  * found on
  * http://en.wikipedia.org/w/index.php?title=Fletcher%27s_checksum&oldid=661273016#Optimizations
@@ -41,6 +50,34 @@ extern "C" {
  */
 uint16_t fletcher16(const uint8_t *buf, size_t bytes);
 
+/**
+ * @brief Initialize a fletcher16 context
+ *
+ * Multi-part version of @ref fletcher16_full.
+ *
+ * @param[in]   ctx     fletcher16 context to initialize
+ */
+void fletcher16_init(fletcher16_ctx_t *ctx);
+
+/**
+ * @brief Update the fletcher16 context with new data
+ *
+ * @param[in]   ctx     fletcher16 context
+ * @param[in]   data    Data to add to the context
+ * @param[in]   len     Length of the data
+ */
+void fletcher16_update(fletcher16_ctx_t *ctx, const uint8_t *data, size_t len);
+
+/**
+ * @brief Finalize the checksum operation and return the checksum
+ *
+ * @param[in]   ctx     fletcher16 context
+ *
+ * @return              Checksum of the data
+ */
+uint16_t fletcher16_finish(fletcher16_ctx_t *ctx);
+
+
 
 #ifdef __cplusplus
 }

tests/unittests/tests-checksum/tests-checksum-fletcher16.c

@@ -72,6 +72,19 @@ static void test_checksum_fletcher16_wrap_around(void)
                                           sizeof(wrap_around_data) - 1, expect));
 }
 
+static void test_checksum_fletcher16_multipart(void)
+{
+    static const uint16_t expect = 0xf122;
+    static const size_t half_len = sizeof(wrap_around_data)/2;
+    fletcher16_ctx_t ctx;
+
+    fletcher16_init(&ctx);
+    fletcher16_update(&ctx, wrap_around_data, half_len);
+    fletcher16_update(&ctx, wrap_around_data + half_len,
+                      sizeof(wrap_around_data) - half_len - 1);
+    TEST_ASSERT_EQUAL_INT(expect, fletcher16_finish(&ctx));
+}
+
 Test *tests_checksum_fletcher16_tests(void)
 {
     EMB_UNIT_TESTFIXTURES(fixtures) {
@@ -79,6 +92,7 @@ Test *tests_checksum_fletcher16_tests(void)
         new_TestFixture(test_checksum_fletcher16_0to1_undetected),
         new_TestFixture(test_checksum_fletcher16_atoe),
         new_TestFixture(test_checksum_fletcher16_wrap_around),
+        new_TestFixture(test_checksum_fletcher16_multipart),
     };
 
     EMB_UNIT_TESTCALLER(checksum_fletcher16_tests, NULL, NULL, fixtures);