Fix to_varint

proto.h

@@ -32,12 +32,13 @@ constexpr uint8_t varint_continuation = 0x80;
 // Convert an integer to a varint. May overflow if the result doesn't fit in 64 bits, but can be used constexpr.
 static inline constexpr uint64_t to_varint(uint64_t value) {
   uint64_t result = 0;
+  uint64_t shift = 0;
   while (value > 0x7f) {
-    result |= static_cast<uint8_t>(value | varint_continuation);
-    result <<= 8;
+    result |= (static_cast<uint8_t>(value) | varint_continuation) << shift;
     value >>= 7;
+    shift += 8;
   }
-  result |= static_cast<uint8_t>(value);
+  result |= static_cast<uint8_t>(value) << shift;
   return result;
 }
 

test/proto_test.cc

@@ -17,13 +17,19 @@ uint64_t decode_varint(const std::vector<uint8_t>& bytes) {
   return result;
 }
 
+uint64_t decode_varint(uint64_t bytes) {
+  uint64_t result = 0;
+  read_varint(result, reinterpret_cast<uint8_t*>(&bytes), sizeof(bytes));
+  return result;
+}
+
 TEST(proto, to_varint) {
   for (uint64_t i = 0; i <= 0x7f; ++i) {
     ASSERT_EQ(to_varint(i), i);
   }
-  ASSERT_EQ(to_varint(0x80), 0x8001);
-  ASSERT_EQ(to_varint(0x81), 0x8101);
-  ASSERT_EQ(to_varint(0x82), 0x8201);
+  ASSERT_EQ(to_varint(0x80), 0x0180);
+  ASSERT_EQ(to_varint(0x81), 0x0181);
+  ASSERT_EQ(to_varint(0x82), 0x0182);
 }
 
 // Wrappers that make testing easier.
@@ -59,6 +65,13 @@ TEST(proto, write_varint) {
   ASSERT_EQ(decode_varint(varint), std::numeric_limits<uint64_t>::max());
 }
 
+TEST(proto, to_varint_matches_write_varint) {
+  for (size_t i = 0; i < 1ull << 20; ++i) {
+    uint64_t to = to_varint(i);
+    ASSERT_EQ(decode_varint(to), i) << i;
+  }
+}
+
 TEST(proto, write_padding) {
   std::array<uint8_t, 32> buffer;
   for (uint64_t i = 2; i <= 0x7f; ++i) {