add random access range member functions to bit_span

Change-Id: I765c657b65dfa246b67e20c73c90f05c9b49c5c3

huffman/src/bit_span.hpp

@@ -13,7 +13,7 @@
 
 namespace starflate::huffman {
 /// A non-owning span of bits. Allows for iteration over the individual bits.
-class bit_span
+class bit_span : public std::ranges::view_interface<bit_span>
 {
   const std::byte* data_;
   std::size_t bit_size_;
@@ -53,7 +53,7 @@ class bit_span
     {
       const auto newOffset = static_cast<difference_type>(offset_) + n;
       assert(newOffset >= 0);
-      offset_ = static_cast<size_t>(newOffset);
+      offset_ = static_cast<std::size_t>(newOffset);
       return *this;
     }
 

huffman/src/detail/iterator_interface.hpp

@@ -30,7 +30,6 @@ namespace starflate::huffman::detail {
 template <class D>
 struct iterator_interface
 {
-
   template <class I = D>
   constexpr auto operator->() const -> typename I::pointer
   {
@@ -75,7 +74,7 @@ struct iterator_interface
   constexpr auto
   operator[](typename I::difference_type n) const -> typename I::reference
   {
-    return *(*this + n);
+    return *(static_cast<const I&>(*this) + n);
   }
 
   template <std::same_as<D> I>

huffman/test/bit_span_test.cpp

@@ -28,4 +28,32 @@ auto main() -> int
           return huffman::bit{c};
         })));
   };
+
+  test("indexable") = [] {
+    static constexpr auto data =
+        std::array{std::byte{0b10101010}, std::byte{0xff}};
+    constexpr auto bs = huffman::bit_span{data};
+
+    // NOLINTBEGIN(readability-magic-numbers)
+
+    static_assert(bs[0] == 1_b);
+    static_assert(bs[1] == 0_b);
+    static_assert(bs[2] == 1_b);
+    static_assert(bs[3] == 0_b);
+    static_assert(bs[4] == 1_b);
+    static_assert(bs[5] == 0_b);
+    static_assert(bs[6] == 1_b);
+    static_assert(bs[7] == 0_b);
+
+    expect(bs[8] == 1_b);
+    expect(bs[9] == 1_b);
+    expect(bs[10] == 1_b);
+    expect(bs[11] == 1_b);
+    expect(bs[12] == 1_b);
+    expect(bs[13] == 1_b);
+    expect(bs[14] == 1_b);
+    expect(bs[15] == 1_b);
+
+    // NOLINTEND(readability-magic-numbers)
+  };
 }