nlohmannjson/develop/detail/parsing/input_adapters.hpp

#pragma once

#include <algorithm> // min
#include <array> // array
#include <cassert> // assert
#include <cstddef> // size_t
#include <cstring> // strlen
#include <ios> // streamsize, streamoff, streampos
#include <istream> // istream
#include <iterator> // begin, end, iterator_traits, random_access_iterator_tag, distance, next
#include <memory> // shared_ptr, make_shared, addressof
#include <numeric> // accumulate
#include <string> // string, char_traits
#include <type_traits> // enable_if, is_base_of, is_pointer, is_integral, remove_pointer
#include <utility> // pair, declval

#include "detail/macro_scope.hpp"

namespace nlohmann
{
namespace detail
{
////////////////////
// input adapters //
////////////////////

/*!
@brief abstract input adapter interface

Produces a stream of std::char_traits<char>::int_type characters from a
std::istream, a buffer, or some other input type.  Accepts the return of exactly
one non-EOF character for future input.  The int_type characters returned
consist of all valid char values as positive values (typically unsigned char),
plus an EOF value outside that range, specified by the value of the function
std::char_traits<char>::eof().  This value is typically -1, but could be any
arbitrary value which is not a valid char value.
*/
struct input_adapter_protocol
{
    /// get a character [0,255] or std::char_traits<char>::eof().
    virtual std::char_traits<char>::int_type get_character() = 0;
    /// restore the last non-eof() character to input
    virtual void unget_character() = 0;
    virtual ~input_adapter_protocol() = default;
};

/// a type to simplify interfaces
using input_adapter_t = std::shared_ptr<input_adapter_protocol>;

/*!
Input adapter for a (caching) istream. Ignores a UFT Byte Order Mark at
beginning of input. Does not support changing the underlying std::streambuf
in mid-input. Maintains underlying std::istream and std::streambuf to support
subsequent use of standard std::istream operations to process any input
characters following those used in parsing the JSON input.  Clears the
std::istream flags; any input errors (e.g., EOF) will be detected by the first
subsequent call for input from the std::istream.
*/
class input_stream_adapter : public input_adapter_protocol
{
  public:
    ~input_stream_adapter() override
    {
        // clear stream flags; we use underlying streambuf I/O, do not
        // maintain ifstream flags
        is.clear();
    }

    explicit input_stream_adapter(std::istream& i)
        : is(i), sb(*i.rdbuf())
    {
        // skip byte order mark
        std::char_traits<char>::int_type c;
        if ((c = get_character()) == 0xEF)
        {
            if ((c = get_character()) == 0xBB)
            {
                if ((c = get_character()) == 0xBF)
                {
                    return; // Ignore BOM
                }
                else if (c != std::char_traits<char>::eof())
                {
                    is.unget();
                }
                is.putback('\xBB');
            }
            else if (c != std::char_traits<char>::eof())
            {
                is.unget();
            }
            is.putback('\xEF');
        }
        else if (c != std::char_traits<char>::eof())
        {
            is.unget(); // no byte order mark; process as usual
        }
    }

    // delete because of pointer members
    input_stream_adapter(const input_stream_adapter&) = delete;
    input_stream_adapter& operator=(input_stream_adapter&) = delete;

    // std::istream/std::streambuf use std::char_traits<char>::to_int_type, to
    // ensure that std::char_traits<char>::eof() and the character 0xFF do not
    // end up as the same value, eg. 0xFFFFFFFF.
    std::char_traits<char>::int_type get_character() override
    {
        return sb.sbumpc();
    }

    void unget_character() override
    {
        sb.sungetc();  // is.unget() avoided for performance
    }

  private:
    /// the associated input stream
    std::istream& is;
    std::streambuf& sb;
};

/// input adapter for buffer input
class input_buffer_adapter : public input_adapter_protocol
{
  public:
    input_buffer_adapter(const char* b, const std::size_t l)
        : cursor(b), limit(b + l), start(b)
    {
        // skip byte order mark
        if (l >= 3 and b[0] == '\xEF' and b[1] == '\xBB' and b[2] == '\xBF')
        {
            cursor += 3;
        }
    }

    // delete because of pointer members
    input_buffer_adapter(const input_buffer_adapter&) = delete;
    input_buffer_adapter& operator=(input_buffer_adapter&) = delete;

    std::char_traits<char>::int_type get_character() noexcept override
    {
        if (JSON_LIKELY(cursor < limit))
        {
            return std::char_traits<char>::to_int_type(*(cursor++));
        }

        return std::char_traits<char>::eof();
    }

    void unget_character() noexcept override
    {
        if (JSON_LIKELY(cursor > start))
        {
            --cursor;
        }
    }

  private:
    /// pointer to the current character
    const char* cursor;
    /// pointer past the last character
    const char* limit;
    /// pointer to the first character
    const char* start;
};

class input_adapter
{
  public:
    // native support

    /// input adapter for input stream
    input_adapter(std::istream& i)
        : ia(std::make_shared<input_stream_adapter>(i)) {}

    /// input adapter for input stream
    input_adapter(std::istream&& i)
        : ia(std::make_shared<input_stream_adapter>(i)) {}

    /// input adapter for buffer
    template<typename CharT,
             typename std::enable_if<
                 std::is_pointer<CharT>::value and
                 std::is_integral<typename std::remove_pointer<CharT>::type>::value and
                 sizeof(typename std::remove_pointer<CharT>::type) == 1,
                 int>::type = 0>
    input_adapter(CharT b, std::size_t l)
        : ia(std::make_shared<input_buffer_adapter>(reinterpret_cast<const char*>(b), l)) {}

    // derived support

    /// input adapter for string literal
    template<typename CharT,
             typename std::enable_if<
                 std::is_pointer<CharT>::value and
                 std::is_integral<typename std::remove_pointer<CharT>::type>::value and
                 sizeof(typename std::remove_pointer<CharT>::type) == 1,
                 int>::type = 0>
    input_adapter(CharT b)
        : input_adapter(reinterpret_cast<const char*>(b),
                        std::strlen(reinterpret_cast<const char*>(b))) {}

    /// input adapter for iterator range with contiguous storage
    template<class IteratorType,
             typename std::enable_if<
                 std::is_same<typename std::iterator_traits<IteratorType>::iterator_category, std::random_access_iterator_tag>::value,
                 int>::type = 0>
    input_adapter(IteratorType first, IteratorType last)
    {
        // assertion to check that the iterator range is indeed contiguous,
        // see http://stackoverflow.com/a/35008842/266378 for more discussion
        assert(std::accumulate(
                   first, last, std::pair<bool, int>(true, 0),
                   [&first](std::pair<bool, int> res, decltype(*first) val)
        {
            res.first &= (val == *(std::next(std::addressof(*first), res.second++)));
            return res;
        }).first);

        // assertion to check that each element is 1 byte long
        static_assert(
            sizeof(typename std::iterator_traits<IteratorType>::value_type) == 1,
            "each element in the iterator range must have the size of 1 byte");

        const auto len = static_cast<size_t>(std::distance(first, last));
        if (JSON_LIKELY(len > 0))
        {
            // there is at least one element: use the address of first
            ia = std::make_shared<input_buffer_adapter>(reinterpret_cast<const char*>(&(*first)), len);
        }
        else
        {
            // the address of first cannot be used: use nullptr
            ia = std::make_shared<input_buffer_adapter>(nullptr, len);
        }
    }

    /// input adapter for array
    template<class T, std::size_t N>
    input_adapter(T (&array)[N])
        : input_adapter(std::begin(array), std::end(array)) {}

    /// input adapter for contiguous container
    template<class ContiguousContainer, typename
             std::enable_if<not std::is_pointer<ContiguousContainer>::value and
                            std::is_base_of<std::random_access_iterator_tag, typename std::iterator_traits<decltype(std::begin(std::declval<ContiguousContainer const>()))>::iterator_category>::value,
                            int>::type = 0>
    input_adapter(const ContiguousContainer& c)
        : input_adapter(std::begin(c), std::end(c)) {}

    operator input_adapter_t()
    {
        return ia;
    }

  private:
    /// the actual adapter
    input_adapter_t ia = nullptr;
};
}
}