From e3ec63e3ff1cd532185af04f7e54541cae4b161c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Asbj=C3=B8rn=20Alexander=20Fellinghaug?=
 <asbjorn@fellinghaug.com>
Date: Mon, 21 Feb 2022 11:54:19 +0100
Subject: [PATCH] Fixes #4 by introducing max value checking as done in
 github.com/canboat/canboat

---
 nmea2k/raw_message.go      | 11 ++++++++++-
 nmea2k/raw_message_test.go | 11 +++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/nmea2k/raw_message.go b/nmea2k/raw_message.go
index 87e0b67..5c88bc3 100644
--- a/nmea2k/raw_message.go
+++ b/nmea2k/raw_message.go
@@ -463,7 +463,16 @@ func (msg *RawMessage) extractNumber(field *Field, start, end, offset, width uin
 		maxValue = 0x7FFFFFFFFFFFFFFF
 	}
 
-	if num > maxValue {
+	var reserved uint64
+	if maxValue >= 15 {
+		reserved = 2
+	} else if maxValue > 1 {
+		reserved = 1
+	} else {
+		reserved = 0
+	}
+
+	if num > maxValue-reserved {
 		e = &DecodeError{data, "Field not present"}
 		return
 	}
diff --git a/nmea2k/raw_message_test.go b/nmea2k/raw_message_test.go
index 73b9ec5..cf6ad81 100644
--- a/nmea2k/raw_message_test.go
+++ b/nmea2k/raw_message_test.go
@@ -233,4 +233,15 @@ func TestPgn127489NumberExtraction(t *testing.T) {
 	if ok {
 		t.Errorf("the value for the field 'total engine hours' is invalid: (%d). Need to check maximum value during extraction. RawMessage: %+v\n", engineHours, pgnParsed.Data[6])
 	}
+
+	// This RawMessage is the 'BAD' one with invalid values. Field 6 should be 'nil' with the proper validation.
+	msg = RawMessage{new(can.RawMessage)}
+	msg.Data = []byte{0x00, 0xb0, 0x09, 0xff, 0xff, 0xab, 0x7c, 0xff, 0x7f, 0x15, 0x00, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x60, 0x02, 0xff, 0x00, 0x00, 0x00, 0x00, 0x0f, 0x05}
+	msg.Pgn = uint32(127489)
+
+	pgnParsed = ParsePacket(msg.RawMessage)
+	alternator, ok := pgnParsed.Data[4].(float64)
+	if ok {
+		t.Errorf("the value for the field 'alternator_potential' is invalid: (%f). Need to check maximum value during extraction. RawMessage: %+v\n", alternator, pgnParsed.Data[4])
+	}
 }