construct huffman table from symbol spans

huffman/BUILD.bazel

@@ -8,11 +8,14 @@ cc_library(
         "src/code.hpp",
         "src/decode.hpp",
         "src/detail/element_base_iterator.hpp",
+        "src/detail/flattened_symbol_bitsize_view.hpp",
+        "src/detail/is_specialization_of.hpp",
         "src/detail/iterator_interface.hpp",
         "src/detail/static_vector.hpp",
         "src/detail/table_node.hpp",
         "src/detail/table_storage.hpp",
         "src/encoding.hpp",
+        "src/symbol_span.hpp",
         "src/table.hpp",
         "src/utility.hpp",
     ],

huffman/src/decode.hpp

@@ -2,6 +2,7 @@
 #include "huffman/src/bit_span.hpp"
 #include "huffman/src/code.hpp"
 #include "huffman/src/table.hpp"
+#include "huffman/src/utility.hpp"
 
 #include <iterator>
 #include <span>
@@ -21,7 +22,7 @@ namespace starflate::huffman {
 /// @tparam Extent The extent of the code table.
 /// @tparam O The type of the output iterator.
 template <
-    std::regular Symbol,
+    symbol Symbol,
     std::size_t Extent = std::dynamic_extent,
     std::output_iterator<Symbol> O>
 constexpr auto

huffman/src/detail/flattened_symbol_bitsize_view.hpp

@@ -0,0 +1,197 @@
+#pragma once
+
+#include "huffman/src/code.hpp"
+#include "huffman/src/detail/is_specialization_of.hpp"
+#include "huffman/src/symbol_span.hpp"
+
+#include <concepts>
+#include <cstddef>
+#include <cstdint>
+#include <iterator>
+#include <numeric>
+#include <ranges>
+#include <tuple>
+#include <utility>
+
+namespace starflate::huffman::detail {
+
+/// Range adaptor that flattens a view of symbol_span-bitsize ranges to a single
+///     view
+/// @tparam V random access view of symbol_span-bitsize elements
+///
+/// Flattens a view of symbol_span-bitsize elements. Elements of the flattened
+/// view are symbol-code pairs. Note that code values set by this view are
+/// *unspecified*. It is expected that view is used in construction of a `table`
+/// and that valid codes are set with the `canonicalize()` member.
+///
+/// Example:
+/// ~~~{.cpp}
+/// const auto data = std::vector<std::pair<symbol_span<char>, std::uint8_t>>{
+///   {{'A', 'C'}, 2},
+///   {{'D', 'F'}, 3},
+///   {{'G', 'H'}, 4},
+/// };
+///
+/// const auto elements =
+/// flattened_symbol_bitsize_view{std::ranges::ref_view{data}};
+///
+/// for (auto [symbol, code] : elements) {
+///   std::cout << symbol << ", " code << '\n';
+/// }
+///
+/// // prints
+/// // A, 00
+/// // B, 00
+/// // C, 00
+/// // D, 000
+/// // E, 000
+/// // F, 000
+/// // G, 0000
+/// // H, 0000
+/// ~~~
+///
+template <std::ranges::view V>
+  requires std::ranges::random_access_range<V> and
+           (std::tuple_size_v<std::ranges::range_value_t<V>> == 2) and
+           is_specialization_of_v<
+               std::tuple_element_t<0, std::ranges::range_value_t<V>>,
+               symbol_span> and
+           std::convertible_to<
+               std::tuple_element_t<1, std::ranges::range_value_t<V>>,
+               std::uint8_t>
+class flattened_symbol_bitsize_view
+    : public std::ranges::view_interface<flattened_symbol_bitsize_view<V>>
+{
+  V base_;
+  std::size_t size_;
+
+public:
+  using base_type = V;
+  using symbol_span_type =
+      std::tuple_element_t<0, std::ranges::range_value_t<base_type>>;
+  using symbol_type = typename symbol_span_type::symbol_type;
+
+  /// Iterator for flattened_symbol_bitsize_view
+  ///
+  class iterator
+  {
+  public:
+    using iterator_category = std::forward_iterator_tag;
+    using difference_type = std::ranges::range_difference_t<V>;
+    using reference = std::pair<code, symbol_type>;
+    using value_type = reference;
+    using pointer = void;
+
+  private:
+    const flattened_symbol_bitsize_view* parent_{};
+    difference_type outer_index_{};
+    std::ranges::range_difference_t<symbol_span_type> inner_index_{};
+
+    template <class R, class T>
+    static constexpr auto in_range_cast(T t) -> R
+    {
+      assert(std::in_range<R>(t));
+      return static_cast<R>(t);
+    }
+
+  public:
+    /// Default constructor
+    ///
+    iterator()
+    {
+      // this should never be invoked, but the definition is required
+      // for weakly_incrementable until GCC and Clang are fixed
+      // https://en.cppreference.com/w/cpp/iterator/weakly_incrementable
+      // https://wg21.link/P2325R3
+      assert(false);
+    }
+
+    /// Construct an iterator to a symbol-bitsize element
+    /// @param parent view containing the symbol_span-bitsize sequence
+    /// @param outer_index index specifying the symbol_span-bitsize element
+    /// @param inner_index index within a symbol_span-bitsize element
+    ///
+    constexpr iterator(
+        const flattened_symbol_bitsize_view& parent,
+        // NOLINTBEGIN(bugprone-easily-swappable-parameters)
+        std::size_t outer_index,
+        std::size_t inner_index)
+        // NOLINTEND(bugprone-easily-swappable-parameters)
+        : parent_{&parent},
+          outer_index_{in_range_cast<difference_type>(outer_index)},
+          inner_index_{in_range_cast<
+              std::ranges::range_difference_t<symbol_span_type>>(inner_index)}
+    {}
+
+    [[nodiscard]]
+    constexpr auto
+    operator*() const -> reference
+    {
+      const auto [symbols, bitsize] = parent_->base()[outer_index_];
+      return {code{bitsize, {}}, symbols[inner_index_]};
+    }
+
+    constexpr auto operator++() & -> iterator&
+    {
+      const auto [symbols, _] = parent_->base()[outer_index_];
+
+      // if we've reached the end of a symbol_span
+      if (std::cmp_equal(++inner_index_, symbols.size())) {
+        // advance to the next symbol_span
+        ++outer_index_;
+        // and reset the symbol_span index
+        inner_index_ = {};
+      }
+
+      return *this;
+    }
+
+    constexpr auto operator++(int) -> iterator
+    {
+      auto tmp = *this;
+      ++*this;
+      return tmp;
+    }
+
+    friend constexpr auto
+    operator==(const iterator&, const iterator&) -> bool = default;
+  };
+
+  constexpr explicit flattened_symbol_bitsize_view(V base)
+      : base_{std::move(base)},
+        size_{std::accumulate(
+            std::ranges::cbegin(base_),
+            std::ranges::cend(base_),
+            0UZ,
+            [](auto acc, const auto& symbol_bitsize) {
+              const auto& [symbols, _] = symbol_bitsize;
+              return acc + symbols.size();
+            })}
+  {}
+
+  [[nodiscard]]
+  constexpr auto base() const -> base_type
+  {
+    return base_;
+  }
+
+  [[nodiscard]]
+  constexpr auto size() const -> std::size_t
+  {
+    return size_;
+  }
+
+  [[nodiscard]]
+  constexpr auto begin() const -> iterator
+  {
+    return {*this, 0UZ, 0UZ};
+  }
+
+  [[nodiscard]]
+  constexpr auto end() const -> iterator
+  {
+    return {*this, base().size(), 0UZ};
+  }
+};
+
+}  // namespace starflate::huffman::detail

huffman/src/detail/is_specialization_of.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <type_traits>
+
+namespace starflate::huffman::detail {
+
+/// Checks if a type is a specialization of a class template
+/// @tparam T type
+/// @tparam primary class template
+///
+/// @see https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2098r1.pdf
+///
+/// @{
+
+template <class T, template <class...> class primary>
+struct is_specialization_of : std::false_type
+{};
+
+template <template <class...> class primary, class... Args>
+struct is_specialization_of<primary<Args...>, primary> : std::true_type
+{};
+
+template <class T, template <class...> class primary>
+inline constexpr auto is_specialization_of_v =
+    is_specialization_of<T, primary>::value;
+
+/// @}
+
+}  // namespace starflate::huffman::detail

huffman/src/encoding.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "huffman/src/code.hpp"
+#include "huffman/src/utility.hpp"
 
 #include <concepts>
 #include <ostream>
@@ -12,8 +13,7 @@ namespace starflate::huffman {
 /// This type associates a symbol to a code. It is typically constructed and
 /// updated as part of the construction of a table.
 ///
-template <std::regular Symbol>
-  requires std::totally_ordered<Symbol>
+template <symbol Symbol>
 struct encoding : code
 {
   using symbol_type = Symbol;

huffman/src/symbol_span.hpp

@@ -0,0 +1,57 @@
+#pragma once
+
+#include "huffman/src/utility.hpp"
+
+#include <ranges>
+
+namespace starflate::huffman {
+
+/// An inclusive span of symbols
+///
+template <symbol S>
+class symbol_span : public std::ranges::view_interface<symbol_span<S>>
+{
+  S first_;
+  S last_;
+
+  using range_type = std::ranges::iota_view<S, S>;
+
+  constexpr auto as_range() const -> range_type
+  {
+    return range_type{first_, static_cast<S>(last_ + S{1})};
+  }
+
+public:
+  using symbol_type = S;
+
+  using iterator = std::ranges::iterator_t<range_type>;
+
+  /// Construct a symbol span of a single symbol
+  /// @param first symbol
+  ///
+  constexpr symbol_span(symbol_type first) : symbol_span{first, first} {}
+
+  /// Construct a symbol span from first to last, inclusive
+  /// @param first, last inclusive symbol range
+  /// @pre first <= last
+  ///
+  constexpr symbol_span(symbol_type first, symbol_type last)
+      : first_{first}, last_{last}
+  {
+    assert(first <= last);
+  }
+
+  [[nodiscard]]
+  constexpr auto begin() const -> iterator
+  {
+    return as_range().begin();
+  }
+
+  [[nodiscard]]
+  constexpr auto end() const -> iterator
+  {
+    return as_range().end();
+  }
+};
+
+}  // namespace starflate::huffman