From ff2cfbaafbdf1b09b21a12ea0d8c7a951b7ed0b5 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Sun, 27 Oct 2019 22:50:01 +1100
Subject: [PATCH] AP_Math: added smaller footprint crc32

---
 libraries/AP_Math/crc.cpp | 16 ++++++++++++++++
 libraries/AP_Math/crc.h   |  1 +
 2 files changed, 17 insertions(+)

diff --git a/libraries/AP_Math/crc.cpp b/libraries/AP_Math/crc.cpp
index 7c5a017c50..3f0b31a87d 100644
--- a/libraries/AP_Math/crc.cpp
+++ b/libraries/AP_Math/crc.cpp
@@ -167,6 +167,7 @@ static const uint32_t crc32_tab[] = {
     0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
 };
 
+
 uint32_t crc_crc32(uint32_t crc, const uint8_t *buf, uint32_t size)
 {
 	for (uint32_t i=0; i<size; i++) {
@@ -176,6 +177,21 @@ uint32_t crc_crc32(uint32_t crc, const uint8_t *buf, uint32_t size)
 	return crc;
 }
 
+// smaller (and slower) crc32 for bootloader
+uint32_t crc32_small(uint32_t crc, const uint8_t *buf, uint32_t size)
+{
+    while (size--) {
+        const uint8_t byte = *buf++;
+        crc ^= byte;
+        for (uint8_t i=0; i<8; i++) {
+            const uint32_t mask = -(crc & 1);
+            crc >>= 1;
+            crc ^= (0xEDB88320 & mask);
+        }
+    }
+    return crc;
+}
+
 /*
  * Copyright (C) 2010 Swift Navigation Inc.
  * Contact: Fergus Noble <fergus@swift-nav.com>
diff --git a/libraries/AP_Math/crc.h b/libraries/AP_Math/crc.h
index 2a307734ca..a970a99c90 100644
--- a/libraries/AP_Math/crc.h
+++ b/libraries/AP_Math/crc.h
@@ -22,6 +22,7 @@ uint8_t crc_crc8(const uint8_t *p, uint8_t len);
 uint16_t crc_xmodem_update(uint16_t crc, uint8_t data);
 uint16_t crc_xmodem(const uint8_t *data, uint16_t len);
 uint32_t crc_crc32(uint32_t crc, const uint8_t *buf, uint32_t size);
+uint32_t crc32_small(uint32_t crc, const uint8_t *buf, uint32_t size);
 
 // Copyright (C) 2010 Swift Navigation Inc.
 // Contact: Fergus Noble <fergus@swift-nav.com>