From e438e1e0c54f82c8c3489f4d8058941a1672bd3c Mon Sep 17 00:00:00 2001
From: Oliver Jowett <oliver@mutability.co.uk>
Date: Mon, 17 Aug 2015 18:07:40 +0100
Subject: [PATCH] Fix decoding of even lat=0 + odd lat=0 which can have 3
 results (-90, 0, +90)

---
 cpr.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/cpr.c b/cpr.c
index d641670..3f8187b 100644
--- a/cpr.c
+++ b/cpr.c
@@ -250,8 +250,26 @@ int decodeCPRsurface(double reflat, double reflon,
     //      reflat=-40, rlat=4, use rlat=4
     //      reflat=-40, rlat=6, use rlat=6-90 = -84
 
-    if ( (rlat0 - reflat) > 45 ) rlat0 -= 90;
-    if ( (rlat1 - reflat) > 45 ) rlat1 -= 90;
+    // As a special case, -90, 0 and +90 all encode to zero, so
+    // there's a little extra work to do there.
+
+    if (rlat0 == 0) {
+        if (reflat < -45)
+            rlat0 = -90;
+        else if (reflat > 45)
+            rlat0 = 90;
+    } else if ((rlat0 - reflat) > 45) {
+        rlat0 -= 90;
+    }
+
+    if (rlat1 == 0) {
+        if (reflat < -45)
+            rlat1 = -90;
+        else if (reflat > 45)
+            rlat1 = 90;
+    } else if ((rlat1 - reflat) > 45) {
+        rlat1 -= 90;
+    }
 
     // Check to see that the latitude is in range: -90 .. +90
     if (rlat0 < -90 || rlat0 > 90 || rlat1 < -90 || rlat1 > 90)