From c8cb86b8fbb3fcc73e6f38bfb84e9cf115ed6377 Mon Sep 17 00:00:00 2001
From: Moritz Bunkus <moritz@bunkus.org>
Date: Thu, 14 Jun 2018 21:22:49 +0200
Subject: [PATCH] bit_reader_c: fix `get_bit_position/get_remaining_bits` for
 empty buffers

One side affect of the fixed bug was that trying to identify/mux an
empty file (file with a size of 0) resulted in the following error
message:

> Error: buffer_c: num > m_filled. Should not have happened. Please
> file a bug report.
---
 src/common/bit_reader.h          | 4 ++--
 tests/unit/common/bit_reader.cpp | 9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/common/bit_reader.h b/src/common/bit_reader.h
index 7bc4ab391..dd9ddfef9 100644
--- a/src/common/bit_reader.h
+++ b/src/common/bit_reader.h
@@ -194,11 +194,11 @@ public:
   }
 
   int get_bit_position() const {
-    return (m_byte_position - m_start_of_data) * 8 + 8 - m_bits_valid;
+    return (m_byte_position - m_start_of_data) * 8 + (m_bits_valid ? 8 - m_bits_valid : 0);
   }
 
   int get_remaining_bits() const {
-    return (m_end_of_data - m_byte_position) * 8 - 8 + m_bits_valid;
+    return (m_end_of_data - m_byte_position) * 8 - (m_bits_valid ? 8 - m_bits_valid : 0);
   }
 
   void skip_bits(std::size_t num) {
diff --git a/tests/unit/common/bit_reader.cpp b/tests/unit/common/bit_reader.cpp
index 82a6f1a15..e20675f10 100644
--- a/tests/unit/common/bit_reader.cpp
+++ b/tests/unit/common/bit_reader.cpp
@@ -20,6 +20,15 @@ TEST(BitReader, Initialization) {
   EXPECT_FALSE(b.eof());
 }
 
+TEST(BitReader, InitializationEmpty) {
+  unsigned char value{};
+  auto b = mtx::bits::reader_c{&value, 0};
+
+  EXPECT_EQ(0, b.get_bit_position());
+  EXPECT_EQ(0, b.get_remaining_bits());
+  EXPECT_TRUE(b.eof());
+}
+
 TEST(BitReader, GetBit) {
   unsigned char value[4];
   put_uint32_be(value, 0xf7234a81);