Add digi implementation notes.

TNC/Digipeater.cpp

@@ -5,6 +5,23 @@
 #include "Digipeater.h"
 #include "IOEventTask.h"
 
+/*
+ * APRS Digipeater implementation.
+ *
+ * Note that these rules are for digipeating only.  We have other rules for
+ * packets sent to this station directly.
+ *
+ * 0. Ignore all packets that are not UI packets.
+ * 1. Ignore all packets with destination address == MYCALL or any ALIASES.
+ * 2. Compute CRC32 on source and destination address, and information fields.
+ * 3. Ignore all packets with same CRC32 heard in past 30 seconds.
+ * 4. Ignore all digipeated VIA entries.
+ * 5. Find VIA entries matching preempted aliases.
+ * 5.1. http://www.aprs.org/aprs12/preemptive-digipeating.txt
+ * 6. Find first non-digipeated VIA entry.
+ * 6.1. If it matches one of our set and used aliases, relay.
+ */
+
 void startDigipeaterTask(void* arg)
 {
   using mobilinkd::tnc::Digipeater;

TNC/Digipeater.hpp

@@ -57,6 +57,7 @@ struct Digipeater
   /**
    * Can the frame be digipeated?
    *
+   *  - Is it a UI frame
    *  - Does it match an active digi alias?
    *    - set = true
    *    - use = true
@@ -65,11 +66,12 @@ struct Digipeater
    *      matches any frame via.
    *  - Does it not exist in history?
    * @param frame
-   * @return
+   * @return nullptr if the frame cannot be digipeated, otherwise a pointer
+   *    to the alias that it matched.
    */
-  bool can_repeat(hdlc::IoFrame* frame)
+  const kiss::Alias* can_repeat(hdlc::IoFrame* frame)
   {
-    return false;
+    return nullptr;
   }
 
   hdlc::IoFrame* rewrite_frame(hdlc::IoFrame* frame)