concat_util.h

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/*
* (C) 2023-2024 René Meusel - Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#ifndef BOTAN_CONCAT_UTIL_H_
#define BOTAN_CONCAT_UTIL_H_
 
#include <botan/assert.h>
#include <botan/concepts.h>
#include <botan/range_concepts.h>
#include <botan/strong_type.h>
#include <array>
#include <iterator>
#include <ranges>
#include <span>
#include <tuple>
 
namespace Botan {
 
namespace detail {
 
/**
 * Helper function that performs range size-checks as required given the
 * selected output and input range types. If all lengths are known at compile
 * time, this check will be performed at compile time as well. It will then
 * instantiate an output range and concatenate the input ranges' contents.
 */
template <ranges::spanable_range OutR, ranges::spanable_range... Rs>
constexpr OutR concatenate(Rs&&... ranges)
   requires(concepts::reservable_container<OutR> || ranges::statically_spanable_range<OutR>)
{
   OutR result{};
 
   // Prepare and validate the output range and construct a lambda that does the
   // actual filling of the result buffer.
   // (if no input ranges are given, GCC claims that fill_fn is unused)
   [[maybe_unused]] auto fill_fn = [&] {
      if constexpr(concepts::reservable_container<OutR>) {
         // dynamically allocate the correct result byte length
         const size_t total_size = (ranges.size() + ... + 0);
         result.reserve(total_size);
 
         // fill the result buffer using a back-inserter
         return [&result](auto&& range) {
            std::copy(
               std::ranges::begin(range), std::ranges::end(range), std::back_inserter(unwrap_strong_type(result)));
         };
      } else {
         if constexpr((ranges::statically_spanable_range<Rs> && ... && true)) {
            // all input ranges have a static extent, so check the total size at compile time
            // (work around an issue in MSVC that warns `total_size` is unused)
            [[maybe_unused]] constexpr size_t total_size = (decltype(std::span{ranges})::extent + ... + 0);
            static_assert(result.size() == total_size, "size of result buffer does not match the sum of input buffers");
         } else {
            // at least one input range has a dynamic extent, so check the total size at runtime
            const size_t total_size = (ranges.size() + ... + 0);
            BOTAN_ARG_CHECK(result.size() == total_size,
                            "result buffer has static extent that does not match the sum of input buffers");
         }
 
         // fill the result buffer and hold the current output-iterator position
         return [itr = std::ranges::begin(result)](auto&& range) mutable {
            std::copy(std::ranges::begin(range), std::ranges::end(range), itr);
            std::advance(itr, std::ranges::size(range));
         };
      }
   }();
 
   // perform the actual concatenation
   (fill_fn(std::forward<Rs>(ranges)), ...);
 
   return result;
}
 
}  // namespace detail
 
/**
 * Concatenate an arbitrary number of buffers. Performs range-checks as needed.
 *
 * The output type can be auto-detected based on the input ranges, or explicitly
 * specified by the caller. If all input ranges have a static extent, the total
 * size is calculated at compile time and a statically sized std::array<> is used.
 * Otherwise this tries to use the type of the first input range as output type.
 *
 * Alternatively, the output container type can be specified explicitly.
 */
template <typename OutR = detail::AutoDetect, ranges::spanable_range... Rs>
constexpr auto concat(Rs&&... ranges)
   requires(all_same_v<std::ranges::range_value_t<Rs>...>)
{
   if constexpr(std::same_as<detail::AutoDetect, OutR>) {
      // Try to auto-detect a reasonable output type given the input ranges
      static_assert(sizeof...(Rs) > 0, "Cannot auto-detect the output type if not a single input range is provided.");
      using candidate_result_t = std::remove_cvref_t<std::tuple_element_t<0, std::tuple<Rs...>>>;
      using result_range_value_t = std::remove_cvref_t<std::ranges::range_value_t<candidate_result_t>>;
 
      if constexpr((ranges::statically_spanable_range<Rs> && ...)) {
         // If all input ranges have a static extent, we can calculate the total size at compile time
         // and therefore can use a statically sized output container. This is constexpr.
         constexpr size_t total_size = (decltype(std::span{ranges})::extent + ... + 0);
         using out_array_t = std::array<result_range_value_t, total_size>;
         return detail::concatenate<out_array_t>(std::forward<Rs>(ranges)...);
      } else {
         // If at least one input range has a dynamic extent, we must use a dynamically allocated output container.
         // We assume that the user wants to use the first input range's container type as output type.
         static_assert(
            concepts::reservable_container<candidate_result_t>,
            "First input range has static extent, but a dynamically allocated output range is required. Please explicitly specify a dynamically allocatable output type.");
         return detail::concatenate<candidate_result_t>(std::forward<Rs>(ranges)...);
      }
   } else {
      // The caller has explicitly specified the output type
      return detail::concatenate<OutR>(std::forward<Rs>(ranges)...);
   }
}
 
}  // namespace Botan
 
#endif