From 7c9c2bc1dd470cd4794d6891b21ee1fc61f401e0 Mon Sep 17 00:00:00 2001
From: naruse <naruse@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Fri, 14 Jul 2017 09:17:55 +0000
Subject: Zlib::GzipReader#pos underflows after calling #ungetbyte or #ungetc
 at start of file [Bug #13616]

patched by Andrew Haines <andrew@haines.org.nz> [ruby-core:81488]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59333 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 ext/zlib/zlib.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

(limited to 'ext/zlib')

diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index bc1b47c221..ce1139dc5d 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -3393,7 +3393,14 @@ static VALUE
 rb_gzfile_total_out(VALUE obj)
 {
     struct gzfile *gz = get_gzfile(obj);
-    return rb_uint2inum(gz->z.stream.total_out - ZSTREAM_BUF_FILLED(&gz->z));
+    uLong total_out = gz->z.stream.total_out;
+    long buf_filled = ZSTREAM_BUF_FILLED(&gz->z);
+
+    if (total_out >= (uLong)buf_filled) {
+        return rb_uint2inum(total_out - buf_filled);
+    } else {
+        return LONG2FIX(-(buf_filled - total_out));
+    }
 }
 
 /*
-- 
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