// Copyright (c) 2017-2020, The Monero Project
//
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#pragma once
#include <algorithm>
#include <cstdint>
#include <memory>
#include <type_traits>
namespace epee
{
/*!
\brief Non-owning sequence of data. Does not deep copy
Inspired by `gsl::span` and/or `boost::iterator_range`. This class is
intended to be used as a parameter type for functions that need to take a
writable or read-only sequence of data. Most common cases are `span<char>`
and `span<std::uint8_t>`. Using as a class member is only recommended if
clearly documented as not doing a deep-copy. C-arrays are easily convertible
to this type.
\note Conversion from C string literal to `span<const char>` will include
the NULL-terminator.
\note Never allows derived-to-base pointer conversion; an array of derived
types is not an array of base types.
*/
template<typename T>
class span
{
template<typename U>
static constexpr bool safe_conversion() noexcept
{
// Allow exact matches or `T*` -> `const T*`.
using with_const = typename std::add_const<U>::type;
return std::is_same<T, U>() ||
(std::is_const<T>() && std::is_same<T, with_const>());
}
public:
using value_type = T;
using size_type = std::size_t;
using difference_type = std::ptrdiff_t;
using pointer = T*;
using const_pointer = const T*;
using reference = T&;
using const_reference = const T&;
using iterator = pointer;
using const_iterator = const_pointer;
constexpr span() noexcept : ptr(nullptr), len(0) {}
constexpr span(std::nullptr_t) noexcept : span() {}
//! Prevent derived-to-base conversions; invalid in this context.
template<typename U, typename = typename std::enable_if<safe_conversion<U>()>::type>
constexpr span(U* const src_ptr, const std::size_t count) noexcept
: ptr(src_ptr), len(count) {}
//! Conversion from C-array. Prevents common bugs with sizeof + arrays.
template<std::size_t N>
constexpr span(T (&src)[N]) noexcept : span(src, N) {}
constexpr span(const span&) noexcept = default;
span& operator=(const span&) noexcept = default;
/*! Try to remove `amount` elements from beginning of span.
\return Number of elements removed. */
std::size_t remove_prefix(std::size_t amount) noexcept
{
amount = std::min(len, amount);
ptr += amount;
len -= amount;
return amount;
}
constexpr iterator begin() const noexcept { return ptr; }
constexpr const_iterator cbegin() const noexcept { return ptr; }
constexpr iterator end() const noexcept { return begin() + size(); }
constexpr const_iterator cend() const noexcept { return cbegin() + size(); }
constexpr bool empty() const noexcept { return size() == 0; }
constexpr pointer data() const noexcept { return ptr; }
constexpr std::size_t size() const noexcept { return len; }
constexpr std::size_t size_bytes() const noexcept { return size() * sizeof(value_type); }
T &operator[](size_t idx) noexcept { return ptr[idx]; }
const T &operator[](size_t idx) const noexcept { return ptr[idx]; }
private:
T* ptr;
std::size_t len;
};
//! \return `span<const T::value_type>` from a STL compatible `src`.
template<typename T>
constexpr span<const typename T::value_type> to_span(const T& src)
{
// compiler provides diagnostic if size() is not size_t.
return {src.data(), src.size()};
}
//! \return `span<T::value_type>` from a STL compatible `src`.
template<typename T>
constexpr span<typename T::value_type> to_mut_span(T& src)
{
// compiler provides diagnostic if size() is not size_t.
return {src.data(), src.size()};
}
template<typename T>
constexpr bool has_padding() noexcept
{
return !std::is_standard_layout<T>() || alignof(T) != 1;
}
//! \return Cast data from `src` as `span<const std::uint8_t>`.
template<typename T>
span<const std::uint8_t> to_byte_span(const span<const T> src) noexcept
{
static_assert(!has_padding<T>(), "source type may have padding");
return {reinterpret_cast<const std::uint8_t*>(src.data()), src.size_bytes()};
}
//! \return `span<const std::uint8_t>` which represents the bytes at `&src`.
template<typename T>
span<const std::uint8_t> as_byte_span(const T& src) noexcept
{
static_assert(!std::is_empty<T>(), "empty types will not work -> sizeof == 1");
static_assert(!has_padding<T>(), "source type may have padding");
return {reinterpret_cast<const std::uint8_t*>(std::addressof(src)), sizeof(T)};
}
//! \return `span<std::uint8_t>` which represents the bytes at `&src`.
template<typename T>
span<std::uint8_t> as_mut_byte_span(T& src) noexcept
{
static_assert(!std::is_empty<T>(), "empty types will not work -> sizeof == 1");
static_assert(!has_padding<T>(), "source type may have padding");
return {reinterpret_cast<std::uint8_t*>(std::addressof(src)), sizeof(T)};
}
//! make a span from a std::string
template<typename T, typename U>
span<const T> strspan(const U&s) noexcept
{
static_assert(std::is_same<typename U::value_type, char>(), "unexpected source type");
static_assert(std::is_same<T, char>() || std::is_same<T, unsigned char>() || std::is_same<T, int8_t>() || std::is_same<T, uint8_t>(), "Unexpected destination type");
return {reinterpret_cast<const T*>(s.data()), s.size()};
}
}
