From 31df2ba0aa80a19d84b03ae2e333ac512cc6e1d3 Mon Sep 17 00:00:00 2001
From: Koen Zandberg <koen@bergzand.net>
Date: Sat, 17 Sep 2022 22:28:01 +0200
Subject: [PATCH] Fletcher32: Add incremental API

---
 sys/checksum/fletcher32.c                     | 40 ++++++++++++++-----
 sys/include/checksum/fletcher32.h             | 40 +++++++++++++++++++
 .../tests-checksum-fletcher32.c               | 15 +++++++
 3 files changed, 86 insertions(+), 9 deletions(-)

diff --git a/sys/checksum/fletcher32.c b/sys/checksum/fletcher32.c
index 5bbd306f3d..00babde6cc 100644
--- a/sys/checksum/fletcher32.c
+++ b/sys/checksum/fletcher32.c
@@ -21,21 +21,43 @@
 #include "unaligned.h"
 #include "checksum/fletcher32.h"
 
-uint32_t fletcher32(const uint16_t *data, size_t words)
+static inline void _reduce(fletcher32_ctx_t *ctx)
 {
-    uint32_t sum1 = 0xffff, sum2 = 0xffff;
+    ctx->sum1 = (ctx->sum1 & 0xffff) + (ctx->sum1 >> 16);
+    ctx->sum2 = (ctx->sum2 & 0xffff) + (ctx->sum2 >> 16);
+}
 
+void fletcher32_init(fletcher32_ctx_t *ctx)
+{
+    ctx->sum1 = 0xffff;
+    ctx->sum2 = 0xffff;
+}
+
+uint32_t fletcher32_finish(fletcher32_ctx_t *ctx)
+{
+    /* Second reduction step to reduce sums to 8 bits */
+    _reduce(ctx);
+    return (ctx->sum2 << 16) | ctx->sum1;
+}
+
+void fletcher32_update(fletcher32_ctx_t *ctx, const void *data, size_t words)
+{
+    const uint16_t *u16_data = (const uint16_t*)data;
     while (words) {
         unsigned tlen = words > 359 ? 359 : words;
         words -= tlen;
         do {
-            sum2 += sum1 += unaligned_get_u16(data++);
+            ctx->sum1 += unaligned_get_u16(u16_data++);
+            ctx->sum2 += ctx->sum1;
         } while (--tlen);
-        sum1 = (sum1 & 0xffff) + (sum1 >> 16);
-        sum2 = (sum2 & 0xffff) + (sum2 >> 16);
+        _reduce(ctx);
     }
-    /* Second reduction step to reduce sums to 16 bits */
-    sum1 = (sum1 & 0xffff) + (sum1 >> 16);
-    sum2 = (sum2 & 0xffff) + (sum2 >> 16);
-    return (sum2 << 16) | sum1;
+}
+
+uint32_t fletcher32(const uint16_t *data, size_t words)
+{
+    fletcher32_ctx_t ctx;
+    fletcher32_init(&ctx);
+    fletcher32_update(&ctx, data, words);
+    return fletcher32_finish(&ctx);
 }
diff --git a/sys/include/checksum/fletcher32.h b/sys/include/checksum/fletcher32.h
index 8f23851e97..01fc08dfcb 100644
--- a/sys/include/checksum/fletcher32.h
+++ b/sys/include/checksum/fletcher32.h
@@ -15,6 +15,7 @@
  *
  * @file
  * @author      Joakim Nohlgård <joakim.nohlgard@eistec.se>
+ * @author      Koen Zandberg <koen@bergzand.net>
  */
 
 #ifndef CHECKSUM_FLETCHER32_H
@@ -27,6 +28,14 @@
 extern "C" {
 #endif
 
+/**
+ * @brief Fletcher's 32 bit checksum context struct
+ */
+typedef struct {
+    uint32_t sum1;  /**< First sum of the checksum */
+    uint32_t sum2;  /**< Second sum of the checksum */
+} fletcher32_ctx_t;
+
 /**
  * @brief Fletcher's 32 bit checksum
  *
@@ -43,6 +52,37 @@ extern "C" {
  */
 uint32_t fletcher32(const uint16_t *buf, size_t words);
 
+/**
+ * @brief Initialize a fletcher32 context
+ *
+ * Multi-part version of @ref fletcher32.
+ *
+ * @param[in]   ctx     fletcher32 context to initialize
+ */
+void fletcher32_init(fletcher32_ctx_t *ctx);
+
+/**
+ * @brief Incrementally update the fletcher32 context with new data. Can be an arbitrary amount of
+ * times with new data to checksum.
+ *
+ * @note @p words is the number of 16 bit words in the buffer
+ * @note @p data should contain an integer number of 16 bit words
+ *
+ * @param[in]   ctx     fletcher32 context
+ * @param[in]   data    Data to add to the context
+ * @param[in]   words   Length of the data in 16 bit words
+ */
+void fletcher32_update(fletcher32_ctx_t *ctx, const void *data, size_t words);
+
+/**
+ * @brief Finalize the checksum operation and return the checksum
+ *
+ * @param[in]   ctx     fletcher32 context
+ *
+ * @return 32 bit sized hash in the interval [1..2^32]
+ */
+uint32_t fletcher32_finish(fletcher32_ctx_t *ctx);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/tests/unittests/tests-checksum/tests-checksum-fletcher32.c b/tests/unittests/tests-checksum/tests-checksum-fletcher32.c
index 38dcba750b..cd3ed5cd3a 100644
--- a/tests/unittests/tests-checksum/tests-checksum-fletcher32.c
+++ b/tests/unittests/tests-checksum/tests-checksum-fletcher32.c
@@ -85,6 +85,20 @@ static void test_checksum_fletcher32_wrap_around(void)
                                           sizeof(wrap_around_data) - 1, expect));
 }
 
+static void test_checksum_fletcher32_wrap_around_piecewise(void)
+{
+    /* XXX: not verified with external implementation yet */
+    uint32_t expect = 0x5bac8c3d;
+    fletcher32_ctx_t ctx;
+    fletcher32_init(&ctx);
+    size_t full_len = sizeof(wrap_around_data) - 1;
+    size_t initial_len = full_len / 2;
+    fletcher32_update(&ctx, wrap_around_data, initial_len / 2);
+    fletcher32_update(&ctx, (wrap_around_data + initial_len), (full_len - initial_len) / 2);
+    uint32_t result = fletcher32_finish(&ctx);
+    TEST_ASSERT_EQUAL_INT(result, expect);
+}
+
 Test *tests_checksum_fletcher32_tests(void)
 {
     EMB_UNIT_TESTFIXTURES(fixtures) {
@@ -92,6 +106,7 @@ Test *tests_checksum_fletcher32_tests(void)
         new_TestFixture(test_checksum_fletcher32_0to1_undetected),
         new_TestFixture(test_checksum_fletcher32_atof),
         new_TestFixture(test_checksum_fletcher32_wrap_around),
+        new_TestFixture(test_checksum_fletcher32_wrap_around_piecewise),
     };
 
     EMB_UNIT_TESTCALLER(checksum_fletcher32_tests, NULL, NULL, fixtures);