From 7a58e3cdc5cbfa731d112d5867240aa89688e84a Mon Sep 17 00:00:00 2001
From: Oliver Jowett <oliver@mutability.co.uk>
Date: Sun, 20 Mar 2016 19:47:27 +0000
Subject: [PATCH] Move Mode C altitude calc into mode A/C message decoding. If
 SPI is set, it cannot be mode C.

---
 mode_ac.c | 43 ++++++++++++++++++++++++++-----------------
 track.c   |  6 +-----
 2 files changed, 27 insertions(+), 22 deletions(-)

diff --git a/mode_ac.c b/mode_ac.c
index 7c376e0..37c20c8 100644
--- a/mode_ac.c
+++ b/mode_ac.c
@@ -75,28 +75,37 @@ int ModeAToModeC(unsigned int ModeA)
 //=========================================================================
 //
 void decodeModeAMessage(struct modesMessage *mm, int ModeA)
-  {
-  mm->msgtype = 32; // Valid Mode S DF's are DF-00 to DF-31.
-                    // so use 32 to indicate Mode A/C
+{
+    mm->msgtype = 32; // Valid Mode S DF's are DF-00 to DF-31.
+    // so use 32 to indicate Mode A/C
 
-  mm->msgbits = 16; // Fudge up a Mode S style data stream
-  mm->msg[0] = (ModeA >> 8);
-  mm->msg[1] = (ModeA);
+    mm->msgbits = 16; // Fudge up a Mode S style data stream
+    mm->msg[0] = (ModeA >> 8);
+    mm->msg[1] = (ModeA);
 
-  // Fudge an address based on Mode A (remove the Ident bit)
-  mm->addr = (ModeA & 0x0000FF7F) | MODES_NON_ICAO_ADDRESS;
+    // Fudge an address based on Mode A (remove the Ident bit)
+    mm->addr = (ModeA & 0x0000FF7F) | MODES_NON_ICAO_ADDRESS;
 
-  // Set the Identity field to ModeA
-  mm->modeA   = ModeA & 0x7777;
-  mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
+    // Set the Identity field to ModeA
+    mm->modeA   = ModeA & 0x7777;
+    mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
 
-  // Flag ident in flight status
-  mm->fs = ModeA & 0x0080;
+    // Flag ident in flight status
+    mm->fs = ModeA & 0x0080;
 
-  // Not much else we can tell from a Mode A/C reply.
-  // Just fudge up a few bits to keep other code happy
-  mm->correctedbits = 0;
-  }
+    // Decode an altitude if this looks like a possible mode C
+    if (!mm->fs) {
+        int modeC = ModeAToModeC(ModeA);
+        if (modeC >= -12) {
+            mm->altitude = modeC * 100;
+            mm->bFlags  |= MODES_ACFLAGS_ALTITUDE_VALID;
+        }
+    }
+
+    // Not much else we can tell from a Mode A/C reply.
+    // Just fudge up a few bits to keep other code happy
+    mm->correctedbits = 0;
+}
 //
 // ===================== Mode A/C detection and decoding  ===================
 //
diff --git a/track.c b/track.c
index 4fc7763..ca51267 100644
--- a/track.c
+++ b/track.c
@@ -73,13 +73,9 @@ struct aircraft *trackCreateAircraft(struct modesMessage *mm) {
     // set them once here during initialisation, and don't bother to set them every 
     // time this ModeA/C is received again in the future
     if (mm->msgtype == 32) {
-        int modeC      = ModeAToModeC(mm->modeA | mm->fs);
         a->modeACflags = MODEAC_MSG_FLAG;
-        if (modeC < -12) {
+        if (!(mm->bFlags & MODES_ACFLAGS_ALTITUDE_VALID)) {
             a->modeACflags |= MODEAC_MSG_MODEA_ONLY;
-        } else {
-            mm->altitude = modeC * 100;
-            mm->bFlags  |= MODES_ACFLAGS_ALTITUDE_VALID;
         }
     }