From 3538c51efb1738d3d7cabae145f14631350d7a72 Mon Sep 17 00:00:00 2001
From: Bryan Biedenkapp <gatekeep@gmail.com>
Date: Tue, 7 Mar 2023 20:31:34 -0500
Subject: [PATCH] correct CRC-CCITT 162 used for NXDN CAC (inital CRC registers
 are set to 1);

---
 edac/CRC.cpp         | 19 +++++++++----------
 edac/CRC.h           | 14 +++++++-------
 nxdn/channel/CAC.cpp |  2 +-
 3 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/edac/CRC.cpp b/edac/CRC.cpp
index 6883052d..8ebefafc 100644
--- a/edac/CRC.cpp
+++ b/edac/CRC.cpp
@@ -228,7 +228,7 @@ void CRC::encodeFiveBit(const bool* in, uint32_t& tcrc)
 }
 
 /// <summary>
-/// Check 16-bit CRC-CCITT.
+/// Check 16-bit CRC CCITT-162.
 /// </summary>
 /// <remarks>This uses polynomial 0x1021.</remarks>
 /// <param name="in">Input byte array.</param>
@@ -260,7 +260,7 @@ bool CRC::checkCCITT162(const uint8_t *in, uint32_t length)
 }
 
 /// <summary>
-/// Encode 16-bit CRC-CCITT.
+/// Encode 16-bit CRC CCITT-162.
 /// </summary>
 /// <remarks>This uses polynomial 0x1021.</remarks>
 /// <param name="in">Input byte array.</param>
@@ -291,7 +291,7 @@ void CRC::addCCITT162(uint8_t* in, uint32_t length)
 }
 
 /// <summary>
-/// Check 16-bit CRC-CCITT.
+/// Check 16-bit CRC CCITT-161.
 /// </summary>
 /// <remarks>This uses polynomial 0x1189.</remarks>
 /// <param name="in">Input byte array.</param>
@@ -323,7 +323,7 @@ bool CRC::checkCCITT161(const uint8_t *in, uint32_t length)
 }
 
 /// <summary>
-/// Encode 16-bit CRC-CCITT.
+/// Encode 16-bit CRC CCITT-161.
 /// </summary>
 /// <remarks>This uses polynomial 0x1189.</remarks>
 /// <param name="in">Input byte array.</param>
@@ -654,7 +654,7 @@ uint16_t CRC::addCRC15(uint8_t* in, uint32_t bitLength)
 }
 
 /// <summary>
-/// Check 16-bit CRC.
+/// Check 16-bit CRC CCITT-162 w/ initial generator of 1.
 /// </summary>
 /// <param name="in">Input byte array.</param>
 /// <param name="bitLength">Length of byte array in bits.</param>
@@ -686,13 +686,13 @@ bool CRC::checkCRC16(const uint8_t* in, uint32_t bitLength)
 }
 
 /// <summary>
-/// Encode 16-bit CRC.
+/// Encode 16-bit CRC CCITT-162 w/ initial generator of 1.
 /// </summary>
 /// <param name="in">Input byte array.</param>
 /// <param name="bitLength">Length of byte array in bits.</param>
 /// <param name="offset">Offset in bits to write CRC.</param>
 /// <returns>16-bit CRC.</returns>
-uint16_t CRC::addCRC16(uint8_t* in, uint32_t bitLength, uint32_t offset)
+uint16_t CRC::addCRC16(uint8_t* in, uint32_t bitLength)
 {
     assert(in != nullptr);
 
@@ -705,7 +705,7 @@ uint16_t CRC::addCRC16(uint8_t* in, uint32_t bitLength, uint32_t offset)
     uint32_t n = bitLength;
     for (uint32_t i = 0U; i < 16U; i++, n++) {
         bool b = READ_BIT(temp, i);
-        WRITE_BIT(in, n + offset, b);
+        WRITE_BIT(in, n, b);
     }
 
 #if DEBUG_CRC
@@ -795,7 +795,7 @@ uint16_t CRC::createCRC15(const uint8_t* in, uint32_t bitLength)
 /// <returns></returns>
 uint16_t CRC::createCRC16(const uint8_t* in, uint32_t bitLength)
 {
-    uint16_t crc = 0x0000U;
+    uint16_t crc = 0xFFFFU;
 
     for (uint32_t i = 0U; i < bitLength; i++) {
         bool bit1 = READ_BIT(in, i) != 0x00U;
@@ -807,6 +807,5 @@ uint16_t CRC::createCRC16(const uint8_t* in, uint32_t bitLength)
             crc ^= 0x1021U;
     }
 
-    crc = ~crc;
     return crc & 0xFFFFU;
 }
diff --git a/edac/CRC.h b/edac/CRC.h
index 2c598f24..a62062bc 100644
--- a/edac/CRC.h
+++ b/edac/CRC.h
@@ -47,14 +47,14 @@ namespace edac
         /// <summary>Encode 5-bit CRC.</summary>
         static void encodeFiveBit(const bool* in, uint32_t& tcrc);
 
-        /// <summary>Check 16-bit CRC-CCITT.</summary>
+        /// <summary>Check 16-bit CRC CCITT-162.</summary>
         static bool checkCCITT162(const uint8_t* in, uint32_t length);
-        /// <summary>Encode 16-bit CRC-CCITT.</summary>
+        /// <summary>Encode 16-bit CRC CCITT-162.</summary>
         static void addCCITT162(uint8_t* in, uint32_t length);
 
-        /// <summary>Check 16-bit CRC-CCITT.</summary>
+        /// <summary>Check 16-bit CRC CCITT-161.</summary>
         static bool checkCCITT161(const uint8_t* in, uint32_t length);
-        /// <summary>Encode 16-bit CRC-CCITT.</summary>
+        /// <summary>Encode 16-bit CRC CCITT-161.</summary>
         static void addCCITT161(uint8_t* in, uint32_t length);
 
         /// <summary>Check 32-bit CRC.</summary>
@@ -83,10 +83,10 @@ namespace edac
         /// <summary>Encode 15-bit CRC.</summary>
         static uint16_t addCRC15(uint8_t* in, uint32_t bitLength);
 
-        /// <summary>Check 16-bit CRC-CCITT.</summary>
+        /// <summary>Check 16-bit CRC CCITT-162 w/ initial generator of 1.</summary>
         static bool checkCRC16(const uint8_t* in, uint32_t bitLength);
-        /// <summary>Encode 16-bit CRC.</summary>
-        static uint16_t addCRC16(uint8_t* in, uint32_t bitLength, uint32_t offset = 0);
+        /// <summary>Encode 16-bit CRC CCITT-162 w/ initial generator of 1.</summary>
+        static uint16_t addCRC16(uint8_t* in, uint32_t bitLength);
 
     private:
         /// <summary></summary>
diff --git a/nxdn/channel/CAC.cpp b/nxdn/channel/CAC.cpp
index ea8fd348..e611dce1 100644
--- a/nxdn/channel/CAC.cpp
+++ b/nxdn/channel/CAC.cpp
@@ -272,7 +272,7 @@ void CAC::encode(uint8_t* data) const
         WRITE_BIT(buffer, i, b);
     }
 
-    uint16_t crc = CRC::addCRC16(buffer, NXDN_CAC_LENGTH_BITS, 5U);
+    uint16_t crc = CRC::addCRC16(buffer, NXDN_CAC_LENGTH_BITS);
 
 #if DEBUG_NXDN_CAC
     Utils::dump(2U, "Encoded CAC", buffer, NXDN_CAC_FEC_LENGTH_BYTES);