From 0ab4afbf4236e6823be6b8845b7e9fa77f7c61f9 Mon Sep 17 00:00:00 2001
From: Isaac Hier <isaachier@gmail.com>
Date: Thu, 21 Jun 2018 08:14:26 -0400
Subject: [PATCH] Fix increment operation for bigint -1

---
 src/bigint.cpp | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/bigint.cpp b/src/bigint.cpp
index 367ae79b6c..088703402d 100644
--- a/src/bigint.cpp
+++ b/src/bigint.cpp
@@ -1683,10 +1683,20 @@ void bigint_incr(BigInt *x) {
         bigint_init_unsigned(x, 1);
         return;
     }
-    
-    if (x->digit_count == 1 && x->data.digit != UINT64_MAX) {
-        x->data.digit += 1;
-        return;
+
+    if (x->digit_count == 1) {
+        if (x->is_negative) {
+            if (x->data.digit != 0) {
+                x->data.digit -= 1;
+            }
+            return;
+        }
+        else {
+            if (x->data.digit != UINT64_MAX) {
+                x->data.digit += 1;
+            }
+            return;
+        }
     }
 
     BigInt copy;