update bit_span constructors and consume member

Define a default constructor for `bit_span`. Update `consume` to return
a reference.

Change-Id: I08edcc3e240a44a04b09a8ba787030665134f3a8

huffman/src/bit_span.hpp

@@ -17,10 +17,9 @@ namespace starflate::huffman {
 /// A non-owning span of bits. Allows for iteration over the individual bits.
 class bit_span : public std::ranges::view_interface<bit_span>
 {
-  const std::byte* data_;
-  std::size_t bit_size_;
-  std::size_t init_bit_size_;  // initial value of bit_size_
-  std::uint8_t bit_offset_;    // always less than CHAR_BIT
+  const std::byte* data_{nullptr};
+  std::size_t bit_size_{};
+  std::uint8_t bit_offset_{};  // always less than CHAR_BIT
 
 public:
   /// An iterator over the bits in a bit_span.
@@ -39,7 +38,7 @@ class bit_span : public std::ranges::view_interface<bit_span>
 
     iterator() = default;
     constexpr iterator(const bit_span& parent, std::size_t offset)
-        : parent_(&parent), offset_(offset)
+        : parent_{&parent}, offset_{offset}
     {
       assert(offset_ <= std::numeric_limits<difference_type>::max());
     }
@@ -74,6 +73,10 @@ class bit_span : public std::ranges::view_interface<bit_span>
     operator<=>(const iterator&, const iterator&) = default;
   };
 
+  /// Constructs an empty bit_span
+  ///
+  bit_span() = default;
+
   /// Constructs a bit_span from the given data.
   ///
   /// @param data a pointer to the first byte of the data.
@@ -85,10 +88,7 @@ class bit_span : public std::ranges::view_interface<bit_span>
   // NOLINTBEGIN(bugprone-easily-swappable-parameters)
   constexpr bit_span(
       const std::byte* data, std::size_t bit_size, std::uint8_t bit_offset = {})
-      : data_{data},
-        bit_size_{bit_size},
-        init_bit_size_{bit_size},
-        bit_offset_{bit_offset}
+      : data_{data}, bit_size_{bit_size}, bit_offset_{bit_offset}
   // NOLINTEND(bugprone-easily-swappable-parameters)
   {
     assert(
@@ -145,16 +145,23 @@ class bit_span : public std::ranges::view_interface<bit_span>
   /// Consumes the given number of bits. Advances the start of the view.
   ///
   /// @pre n <= std::ranges::size(*this)
-  constexpr auto consume(std::size_t n) -> void
+  ///
+  constexpr auto consume(std::size_t n) & -> bit_span&
   {
-    assert(n <= bit_size_);
+    assert(n <= bit_size_);  // pre
     bit_size_ -= n;
-    // invariant
-    assert(bit_offset_ < CHAR_BIT);
+
     const auto distance = bit_offset_ + n;
     std::advance(data_, distance / CHAR_BIT);
-
     bit_offset_ = static_cast<std::uint8_t>(distance % CHAR_BIT);
+
+    assert(bit_offset_ < CHAR_BIT);  // post
+    return *this;
+  }
+  constexpr auto consume(std::size_t n) && -> bit_span&&
+  {
+    consume(n);
+    return std::move(*this);
   }
 
   /// Consumes bits until the start is aligned to a byte boundary.

huffman/test/bit_span_test.cpp

@@ -13,6 +13,7 @@
 auto main() -> int
 {
   using ::boost::ut::aborts;
+  using ::boost::ut::eq;
   using ::boost::ut::expect;
   using ::boost::ut::test;
 
@@ -31,6 +32,12 @@ auto main() -> int
         })));
   };
 
+  test("default constructible") = [] {
+    constexpr auto bits = huffman::bit_span{};
+
+    expect(eq(0, bits.size()));
+  };
+
   test("indexable") = [] {
     static constexpr auto data = huffman::byte_array(0b10101010, 0xff);
     constexpr auto bs = huffman::bit_span{data};
@@ -114,6 +121,21 @@ auto main() -> int
     }
   } | std::views::iota(0UZ, 2UZ * (CHAR_BIT + 1UZ));
 
+  test("consume returns reference") = [] {
+    static constexpr auto data = std::byte{};
+
+    constexpr auto consumed_size = [] {
+      auto bits = huffman::bit_span{&data, CHAR_BIT};
+      return bits.consume(1).size();
+    }();
+
+    expect(eq(CHAR_BIT - 1, consumed_size));
+
+    constexpr auto consumed_bits =
+        huffman::bit_span{&data, CHAR_BIT}.consume(1);
+    expect(eq(CHAR_BIT - 1, consumed_bits.size()));
+  };
+
   test("consume_to_byte_boundary") = [] {
     static constexpr auto data = huffman::byte_array(0b10101010, 0b01010101);
     huffman::bit_span span{data};