Add clang-tidy checks for Firestore (#1078)

* Add a .clang-tidy configuration for Firestore C++

* Fix clang-tidy warnings

  * typedef -> using
  * const ref + rvalue ref -> pass by value
  * NULL -> nullptr
  * remove useless default initializations
  * remove useless const value-type parameter declarations (definitions
    can still use them)
  * use auto instead of repeating types in a cast

* Fix typos

* Address use of static method through instance warnings

* Address use after move warnings

3 files changed, 8 insertions, 8 deletions

diff --git a/Firestore/core/src/firebase/firestore/util/bits.h b/Firestore/core/src/firebase/firestore/util/bits.h
index fec1228..94a018f 100644
--- a/Firestore/core/src/firebase/firestore/util/bits.h
+++ b/Firestore/core/src/firebase/firestore/util/bits.h
@@ -139,23 +139,23 @@ inline int Bits::Log2FloorNonZero_Portable(uint32_t n) {
 
 // Log2Floor64() is defined in terms of Log2Floor32(), Log2FloorNonZero32()
 inline int Bits::Log2Floor64_Portable(uint64_t n) {
-  const uint32_t topbits = static_cast<uint32_t>(n >> 32);
-  if (topbits == 0) {
+  const auto top_bits = static_cast<uint32_t>(n >> 32);
+  if (top_bits == 0) {
     // Top bits are zero, so scan in bottom bits
     return Log2Floor(static_cast<uint32_t>(n));
   } else {
-    return 32 + Log2FloorNonZero(topbits);
+    return 32 + Log2FloorNonZero(top_bits);
   }
 }
 
 // Log2FloorNonZero64() is defined in terms of Log2FloorNonZero32()
 inline int Bits::Log2FloorNonZero64_Portable(uint64_t n) {
-  const uint32_t topbits = static_cast<uint32_t>(n >> 32);
-  if (topbits == 0) {
+  const auto top_bits = static_cast<uint32_t>(n >> 32);
+  if (top_bits == 0) {
     // Top bits are zero, so scan in bottom bits
     return Log2FloorNonZero(static_cast<uint32_t>(n));
   } else {
-    return 32 + Log2FloorNonZero(topbits);
+    return 32 + Log2FloorNonZero(top_bits);
   }
 }
 
diff --git a/Firestore/core/src/firebase/firestore/util/comparator_holder.h b/Firestore/core/src/firebase/firestore/util/comparator_holder.h
index 8641b0f..c7f6144 100644
--- a/Firestore/core/src/firebase/firestore/util/comparator_holder.h
+++ b/Firestore/core/src/firebase/firestore/util/comparator_holder.h
@@ -47,7 +47,7 @@ class ComparatorHolder {
 template <typename C>
 class ComparatorHolder<C, true> : private C {
  protected:
-  explicit ComparatorHolder(const C&) noexcept {
+  explicit ComparatorHolder(const C& /* comparator */) noexcept {
   }
 
   const C& comparator() const noexcept {
diff --git a/Firestore/core/src/firebase/firestore/util/comparison.cc b/Firestore/core/src/firebase/firestore/util/comparison.cc
index b346277..5ac4c27 100644
--- a/Firestore/core/src/firebase/firestore/util/comparison.cc
+++ b/Firestore/core/src/firebase/firestore/util/comparison.cc
@@ -78,7 +78,7 @@ ComparisonResult CompareMixedNumber(double double_value, int64_t int64_value) {
 
   // At this point the long representations are equal but this could be due to
   // rounding.
-  double int64_as_double = static_cast<double>(int64_value);
+  auto int64_as_double = static_cast<double>(int64_value);
   return Compare<double>(double_value, int64_as_double);
 }