use bitsize(), make things private again

Change-Id: Ib720fb2b200a72d54bae4246a42524ec9b1444b7

huffman/src/bit_span.hpp

@@ -17,12 +17,11 @@ 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>
 {
-
-public:
-  // TODO: make private
   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.
   class iterator : public detail::iterator_interface<iterator>
   {

huffman/src/decode.hpp

@@ -50,22 +50,21 @@ struct decode_result
 /// @param bits The bit stream to decode.
 ///
 /// @returns The decoded symbol and how many bits its code was.
+///          If no symbol was found, result.encoded_size == 0.
 /// @tparam Symbol The type of the symbols in the code table.
 /// @tparam Extent The extent of the code table.
 template <symbol Symbol, std::size_t Extent>
 constexpr auto
 decode_one(const table<Symbol, Extent>& code_table, bit_span bits)
     -> decode_result<Symbol>
 {
-  std::uint8_t bits_read{};
   code current_code{};
   auto code_table_pos = code_table.begin();
   for (auto bit : bits) {
     current_code << bit;
-    bits_read++;
     auto found = code_table.find(current_code, code_table_pos);
     if (found) {
-      return {(*found)->symbol, bits_read};
+      return {(*found)->symbol, (*found)->bitsize()};
     }
     if (found.error() == code_table.end()) {
       break;