From 549edd01e98366d1702d4dc93061f7205d248814 Mon Sep 17 00:00:00 2001
From: Scott Bender <scott@scottbender.net>
Date: Thu, 25 Jul 2024 09:45:09 -0400
Subject: [PATCH] feature: support little endian $MXPGN (#277)

---
 bin/analyzerjs   |  5 ++++-
 lib/fromPgn.js   |  2 +-
 lib/stringMsg.js | 21 +++++++++++++++++++--
 3 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/bin/analyzerjs b/bin/analyzerjs
index 7a02d47..b7e2a6f 100755
--- a/bin/analyzerjs
+++ b/bin/analyzerjs
@@ -2,7 +2,8 @@
 
 const argv = require('minimist')(process.argv.slice(2), {
   alias: { h: 'help' },
-  boolean: ['n']
+  boolean: ['n'],
+  boolean: ['r']
 })
 
 if ( argv['help'] ) {
@@ -11,6 +12,7 @@ if ( argv['help'] ) {
 Options:
   -c                  don't check for invalid values
   -n                  output null values
+  -r                  parse $MXPGN as little endian
   -h, --help          output usage information`)
   process.exit(1)
 }
@@ -18,6 +20,7 @@ Options:
 const Parser = require('../index').FromPgn
 var parser = new Parser( {
   returnNulls: argv['n'] === true,
+  littleEndianMXPGN: argv['r'] === true,
   checkForInvalidFields: argv['c'] !== true
 })
 
diff --git a/lib/fromPgn.js b/lib/fromPgn.js
index 5ccd15a..79e416b 100644
--- a/lib/fromPgn.js
+++ b/lib/fromPgn.js
@@ -454,7 +454,7 @@ class Parser extends EventEmitter {
 
   parseString (pgn_data, cb) {
     try {
-      const { coalesced, data, error, len, ...pgn } = parseN2kString(pgn_data)
+      const { coalesced, data, error, len, ...pgn } = parseN2kString(pgn_data, this.options)
       if (error) {
         cb && cb(error)
         this.emit('error', pgn, error)
diff --git a/lib/stringMsg.js b/lib/stringMsg.js
index 3d19727..cef0e76 100644
--- a/lib/stringMsg.js
+++ b/lib/stringMsg.js
@@ -156,12 +156,22 @@ exports.encodePCDIN = ({ prefix = '$PCDIN', pgn, data, dst = 255}) => {
   return sentence + compute0183Checksum(sentence)
 }
 
+const changeEndianness = (string) => {
+  const result = [];
+  let len = string.length - 2;
+  while (len >= 0) {
+    result.push(string.substr(len, 2));
+    len -= 2;
+  }
+  return result.join('');
+}
+
 // $MXPGN,01F801,2801,C1308AC40C5DE343*19
 exports.isMXPGN = (msg) => {
   const sentence = get0183Sentence(msg)
   return sentence.startsWith('$MXPGN,')
 }
-exports.parseMXPGN = (input) => {
+exports.parseMXPGN = (input, options) => {
   const sentence = get0183Sentence(input)
   const [ prefix, pgn, attr_word, data ] = sentence.split(',')
 
@@ -172,11 +182,18 @@ exports.parseMXPGN = (input) => {
   const len = parseInt(send_prio_len.substr(4,4), 2);
   let src, dst;
   send ? dst = addr: src = addr;
+
+  let reversed
+
+  if ( options && options.littleEndianMXPGN )
+    reversed = changeEndianness(rmChecksum(data))
+  else
+    reversed = data
   
   return buildMsg(
     buildCanId(0, parseInt(pgn, 16), 255, parseInt(src, 16)),
     'MXPGN',
-    Buffer.from(rmChecksum(data), 'hex'),
+    Buffer.from(reversed, 'hex'),
     { coalesced: true, prefix },
   )
 }