From f525628ef13bb6fefd52485c6cdaa3303ea7fb4d Mon Sep 17 00:00:00 2001
From: Sara Damiano <sdamiano@stroudcenter.org>
Date: Fri, 22 Sep 2023 12:42:28 -0400
Subject: [PATCH] Function to verify CRC, #75

Signed-off-by: Sara Damiano <sdamiano@stroudcenter.org>
---
 src/SDI12.cpp | 23 +++++++++++++++++++++++
 src/SDI12.h   | 10 ++++++++++
 2 files changed, 33 insertions(+)

diff --git a/src/SDI12.cpp b/src/SDI12.cpp
index 35abc20..5c73dc6 100644
--- a/src/SDI12.cpp
+++ b/src/SDI12.cpp
@@ -632,6 +632,29 @@ String SDI12::crcToString(uint16_t crc) {
   return (String(crcStr[0]) + String(crcStr[1]) + String(crcStr[2]));
 }
 
+bool SDI12::verifyCRC(String& respWithCRC) {
+  uint16_t nChar = respWithCRC.length() -
+    2;  // number of characters without <CR> and <LF> (readable string composed of
+        // sensor address, values separated by + and -) and the 3 characters
+  String recCRC    = "";  // the CRC portion of the response
+  String recString = "";  // the data portion of the response
+
+  // extract the data portion of the string
+  for (int i = 0; i < (nChar - 3); i++) recString += respWithCRC[i];
+
+  // extract the last 3 characters that are the CRC from the full response string
+  for (int i = (nChar - 3); i < nChar; i++) recCRC += respWithCRC[i];
+
+  // calculate the CRC for the data portion
+  String calcCRC = crcToString(calculateCRC(recString));
+
+  if (recCRC == calcCRC) {
+    return false;
+  } else {
+    return true;
+  }
+}
+
 /* ================ Interrupt Service Routine =======================================*/
 
 // 7.1 - Passes off responsibility for the interrupt to the active object.
diff --git a/src/SDI12.h b/src/SDI12.h
index bbb8ab2..a376031 100644
--- a/src/SDI12.h
+++ b/src/SDI12.h
@@ -934,6 +934,16 @@ class SDI12 : public Stream {
    */
   String crcToString(uint16_t crc);
 
+  /**
+   * @brief Verifies that the CRC returned at the end of an SDI-12 message matches that
+   * of the content of the message.
+   *
+   * @param respWithCRC The full SDI-12 message, including the CRC at the end.
+   * @return *true* The CRC matches and the message is valid
+   * @return *false* The CRC doesn't match; the conversation should be retried.
+   */
+  bool verifyCRC(String& respWithCRC);
+
   /**
    * @brief Send a response out on the data line (for slave use)
    *