From 44af8c908664f4e7c274a6fd0dabc422def2dd68 Mon Sep 17 00:00:00 2001
From: sellensr <sellensr@gmail.com>
Date: Tue, 16 Apr 2019 19:49:01 -0400
Subject: [PATCH] Index checksum explicitly on $ and *

parse() Improves slightly on https://github.com/adafruit/Adafruit_GPS/pull/86 by indexing on the position of the $ and the * in the string instead of the beginning and the end of the string.  Does not require sentence string have a particular termination, e.g. CRLF.
---
 Adafruit_GPS.cpp | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/Adafruit_GPS.cpp b/Adafruit_GPS.cpp
index c53aea4..4f0214a 100755
--- a/Adafruit_GPS.cpp
+++ b/Adafruit_GPS.cpp
@@ -57,14 +57,13 @@ boolean Adafruit_GPS::parse(char *nmea) {
   // do checksum check
 
   // first look if we even have one
-  size_t len = strlen(nmea);
-  if (nmea[len-5] == '*') {
-    uint16_t sum = parseHex(nmea[len-4]) * 16;
-    sum += parseHex(nmea[len-3]);
-
+  char *ast = strchr(nmea,'*');
+  if (ast != NULL) {
+    uint16_t sum = parseHex(*(ast+1)) * 16;
+    sum += parseHex(*(ast+2));
     // check checksum
-    for (uint8_t i=1; i < (len-5); i++) {
-      sum ^= nmea[i];
+    for (char *p = strchr(nmea,'$')+1; p < ast; p++) {
+      sum ^= *p;
     }
     if (sum != 0) {
       // bad checksum :(