libcamera: Add a C++20-compliant std::span<> implementation

C++20 will contain a std::span<> class that provides view over a
contiguous sequence of objects, the storage of which is owned by some
other object.

Add a compatible implementation to the utils namespace. This will be
used to implement array controls.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

3 files changed, 420 insertions, 1 deletions

diff --git a/Documentation/Doxyfile.in b/Documentation/Doxyfile.in
index beeaf6d3..d5ba5c4e 100644
--- a/Documentation/Doxyfile.in
+++ b/Documentation/Doxyfile.in
@@ -840,7 +840,8 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                = @TOP_SRCDIR@/src/libcamera/device_enumerator_sysfs.cpp \
+EXCLUDE                = @TOP_SRCDIR@/include/libcamera/span.h \
+			 @TOP_SRCDIR@/src/libcamera/device_enumerator_sysfs.cpp \
 			 @TOP_SRCDIR@/src/libcamera/device_enumerator_udev.cpp \
 			 @TOP_SRCDIR@/src/libcamera/include/device_enumerator_sysfs.h \
 			 @TOP_SRCDIR@/src/libcamera/include/device_enumerator_udev.h \
diff --git a/include/libcamera/meson.build b/include/libcamera/meson.build
index f58c02d2..f47c583c 100644
--- a/include/libcamera/meson.build
+++ b/include/libcamera/meson.build
@@ -14,6 +14,7 @@ libcamera_api = files([
     'pixelformats.h',
     'request.h',
     'signal.h',
+    'span.h',
     'stream.h',
     'timer.h',
 ])
diff --git a/include/libcamera/span.h b/include/libcamera/span.h
new file mode 100644
index 00000000..513ddb43
--- /dev/null
+++ b/include/libcamera/span.h
@@ -0,0 +1,417 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2020, Google Inc.
+ *
+ * span.h - C++20 std::span<> implementation for C++11
+ */
+
+#ifndef __LIBCAMERA_SPAN_H__
+#define __LIBCAMERA_SPAN_H__
+
+#include <array>
+#include <iterator>
+#include <limits>
+#include <stddef.h>
+#include <type_traits>
+
+namespace libcamera {
+
+static constexpr std::size_t dynamic_extent = std::numeric_limits<std::size_t>::max();
+
+template<typename T, std::size_t Extent = dynamic_extent>
+class Span;
+
+namespace details {
+
+template<typename U>
+struct is_array : public std::false_type {
+};
+
+template<typename U, std::size_t N>
+struct is_array<std::array<U, N>> : public std::true_type {
+};
+
+template<typename U>
+struct is_span : public std::false_type {
+};
+
+template<typename U, std::size_t Extent>
+struct is_span<Span<U, Extent>> : public std::true_type {
+};
+
+} /* namespace details */
+
+namespace utils {
+
+template<typename C>
+constexpr auto size(const C &c) -> decltype(c.size())
+{
+	return c.size();
+}
+
+template<typename C>
+constexpr auto data(const C &c) -> decltype(c.data())
+{
+	return c.data();
+}
+
+template<typename C>
+constexpr auto data(C &c) -> decltype(c.data())
+{
+	return c.data();
+}
+
+template<class T, std::size_t N>
+constexpr T *data(T (&array)[N]) noexcept
+{
+	return array;
+}
+
+template<std::size_t I, typename T>
+struct tuple_element;
+
+template<std::size_t I, typename T, std::size_t N>
+struct tuple_element<I, Span<T, N>> {
+	using type = T;
+};
+
+template<typename T>
+struct tuple_size;
+
+template<typename T, std::size_t N>
+struct tuple_size<Span<T, N>> : public std::integral_constant<std::size_t, N> {
+};
+
+template<typename T>
+struct tuple_size<Span<T, dynamic_extent>>;
+
+} /* namespace utils */
+
+template<typename T, std::size_t Extent>
+class Span
+{
+public:
+	using element_type = T;
+	using value_type = typename std::remove_cv_t<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;
+	using reverse_iterator = std::reverse_iterator<iterator>;
+	using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
+	static constexpr std::size_t extent = Extent;
+
+	template<bool Dependent = false,
+		 typename = std::enable_if_t<Dependent || Extent == 0>>
+	constexpr Span() noexcept
+		: data_(nullptr)
+	{
+	}
+
+	constexpr Span(pointer ptr, size_type count)
+		: data_(ptr)
+	{
+	}
+
+	constexpr Span(pointer first, pointer last)
+		: data_(first)
+	{
+	}
+
+	template<std::size_t N>
+	constexpr Span(element_type (&arr)[N],
+		       std::enable_if_t<std::is_convertible<std::remove_pointer_t<decltype(utils::data(arr))> (*)[],
+							    element_type (*)[]>::value &&
+					N == Extent,
+					std::nullptr_t> = nullptr) noexcept
+		: data_(arr)
+	{
+	}
+
+	template<std::size_t N>
+	constexpr Span(std::array<value_type, N> &arr,
+		       std::enable_if_t<std::is_convertible<std::remove_pointer_t<decltype(utils::data(arr))> (*)[],
+							    element_type (*)[]>::value &&
+					N == Extent,
+					std::nullptr_t> = nullptr) noexcept
+		: data_(arr.data())
+	{
+	}
+
+	template<std::size_t N>
+	constexpr Span(const std::array<value_type, N> &arr,
+		       std::enable_if_t<std::is_convertible<std::remove_pointer_t<decltype(utils::data(arr))> (*)[],
+							    element_type (*)[]>::value &&
+					N == Extent,
+					std::nullptr_t> = nullptr) noexcept
+		: data_(arr.data())
+	{
+	}
+
+	template<class Container>
+	constexpr Span(Container &cont,
+		       std::enable_if_t<!details::is_span<Container>::value &&
+					!details::is_array<Container>::value &&
+					!std::is_array<Container>::value &&
+					std::is_convertible<std::remove_pointer_t<decltype(utils::data(cont))> (*)[],
+							    element_type (*)[]>::value,
+					std::nullptr_t> = nullptr)
+		: data_(utils::data(cont))
+	{
+	}
+
+	template<class Container>
+	constexpr Span(const Container &cont,
+		       std::enable_if_t<!details::is_span<Container>::value &&
+					!details::is_array<Container>::value &&
+					!std::is_array<Container>::value &&
+					std::is_convertible<std::remove_pointer_t<decltype(utils::data(cont))> (*)[],
+							    element_type (*)[]>::value,
+					std::nullptr_t> = nullptr)
+		: data_(utils::data(cont))
+	{
+		static_assert(utils::size(cont) == Extent, "Size mismatch");
+	}
+
+	template<class U, std::size_t N>
+	constexpr Span(const Span<U, N> &s,
+		       std::enable_if_t<std::is_convertible<U (*)[], element_type (*)[]>::value &&
+					N == Extent,
+					std::nullptr_t> = nullptr) noexcept
+		: data_(s.data())
+	{
+	}
+
+	constexpr Span(const Span &other) noexcept = default;
+
+	constexpr Span &operator=(const Span &other) noexcept
+	{
+		data_ = other.data_;
+		return *this;
+	}
+
+	constexpr iterator begin() const { return data(); }
+	constexpr const_iterator cbegin() const { return begin(); }
+	constexpr iterator end() const { return data() + size(); }
+	constexpr const_iterator cend() const { return end(); }
+	constexpr reverse_iterator rbegin() const { return reverse_iterator(data() + size() - 1); }
+	constexpr const_reverse_iterator crbegin() const { return rbegin(); }
+	constexpr reverse_iterator rend() const { return reverse_iterator(data() - 1); }
+	constexpr const_reverse_iterator crend() const { return rend(); }
+
+	constexpr reference front() const { return *data(); }
+	constexpr reference back() const { return *(data() + size() - 1); }
+	constexpr reference operator[](size_type idx) const { return data()[idx]; }
+	constexpr pointer data() const noexcept { return data_; }
+
+	constexpr size_type size() const noexcept { return Extent; }
+	constexpr size_type size_bytes() const noexcept { return size() * sizeof(element_type); }
+	constexpr bool empty() const noexcept { return size() == 0; }
+
+	template<std::size_t Count>
+	constexpr Span<element_type, Count> first() const
+	{
+		static_assert(Count <= Extent, "Count larger than size");
+		return { data(), Count };
+	}
+
+	constexpr Span<element_type, dynamic_extent> first(std::size_t Count) const
+	{
+		return { data(), Count };
+	}
+
+	template<std::size_t Count>
+	constexpr Span<element_type, Count> last() const
+	{
+		static_assert(Count <= Extent, "Count larger than size");
+		return { data() + size() - Count, Count };
+	}
+
+	constexpr Span<element_type, dynamic_extent> last(std::size_t Count) const
+	{
+		return { data() + size() - Count, Count };
+	}
+
+	template<std::size_t Offset, std::size_t Count = dynamic_extent>
+	constexpr Span<element_type, Count != dynamic_extent ? Count : Extent - Offset> subspan() const
+	{
+		static_assert(Offset <= Extent, "Offset larger than size");
+		static_assert(Count == dynamic_extent || Count + Offset <= Extent,
+			      "Offset + Count larger than size");
+		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+	}
+
+	constexpr Span<element_type, dynamic_extent>
+	subspan(std::size_t Offset, std::size_t Count = dynamic_extent) const
+	{
+		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+	}
+
+private:
+	pointer data_;
+};
+
+template<typename T>
+class Span<T, dynamic_extent>
+{
+public:
+	using element_type = T;
+	using value_type = typename std::remove_cv_t<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 = T *;
+	using const_iterator = const T *;
+	using reverse_iterator = std::reverse_iterator<iterator>;
+	using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
+	static constexpr std::size_t extent = dynamic_extent;
+
+	constexpr Span() noexcept
+		: data_(nullptr), size_(0)
+	{
+	}
+
+	constexpr Span(pointer ptr, size_type count)
+		: data_(ptr), size_(count)
+	{
+	}
+
+	constexpr Span(pointer first, pointer last)
+		: data_(first), size_(last - first)
+	{
+	}
+
+	template<std::size_t N>
+	constexpr Span(element_type (&arr)[N],
+		       std::enable_if_t<std::is_convertible<std::remove_pointer_t<decltype(utils::data(arr))> (*)[],
+							    element_type (*)[]>::value,
+					std::nullptr_t> = nullptr) noexcept
+		: data_(arr), size_(N)
+	{
+	}
+
+	template<std::size_t N>
+	constexpr Span(std::array<value_type, N> &arr,
+		       std::enable_if_t<std::is_convertible<std::remove_pointer_t<decltype(utils::data(arr))> (*)[],
+							    element_type (*)[]>::value,
+					std::nullptr_t> = nullptr) noexcept
+		: data_(utils::data(arr)), size_(N)
+	{
+	}
+
+	template<std::size_t N>
+	constexpr Span(const std::array<value_type, N> &arr) noexcept
+		: data_(utils::data(arr)), size_(N)
+	{
+	}
+
+	template<class Container>
+	constexpr Span(Container &cont,
+		       std::enable_if_t<!details::is_span<Container>::value &&
+					!details::is_array<Container>::value &&
+					!std::is_array<Container>::value &&
+					std::is_convertible<std::remove_pointer_t<decltype(utils::data(cont))> (*)[],
+							    element_type (*)[]>::value,
+					std::nullptr_t> = nullptr)
+		: data_(utils::data(cont)), size_(utils::size(cont))
+	{
+	}
+
+	template<class Container>
+	constexpr Span(const Container &cont,
+		       std::enable_if_t<!details::is_span<Container>::value &&
+					!details::is_array<Container>::value &&
+					!std::is_array<Container>::value &&
+					std::is_convertible<std::remove_pointer_t<decltype(utils::data(cont))> (*)[],
+							    element_type (*)[]>::value,
+					std::nullptr_t> = nullptr)
+		: data_(utils::data(cont)), size_(utils::size(cont))
+	{
+	}
+
+	template<class U, std::size_t N>
+	constexpr Span(const Span<U, N> &s,
+		       std::enable_if_t<std::is_convertible<U (*)[], element_type (*)[]>::value,
+					std::nullptr_t> = nullptr) noexcept
+		: data_(s.data()), size_(s.size())
+	{
+	}
+
+	constexpr Span(const Span &other) noexcept = default;
+
+	constexpr Span &operator=(const Span &other) noexcept
+	{
+		data_ = other.data_;
+		size_ = other.size_;
+		return *this;
+	}
+
+	constexpr iterator begin() const { return data(); }
+	constexpr const_iterator cbegin() const { return begin(); }
+	constexpr iterator end() const { return data() + size(); }
+	constexpr const_iterator cend() const { return end(); }
+	constexpr reverse_iterator rbegin() const { return reverse_iterator(data() + size() - 1); }
+	constexpr const_reverse_iterator crbegin() const { return rbegin(); }
+	constexpr reverse_iterator rend() const { return reverse_iterator(data() - 1); }
+	constexpr const_reverse_iterator crend() const { return rend(); }
+
+	constexpr reference front() const { return *data(); }
+	constexpr reference back() const { return *(data() + size() - 1); }
+	constexpr reference operator[](size_type idx) const { return data()[idx]; }
+	constexpr pointer data() const noexcept { return data_; }
+
+	constexpr size_type size() const noexcept { return size_; }
+	constexpr size_type size_bytes() const noexcept { return size() * sizeof(element_type); }
+	constexpr bool empty() const noexcept { return size() == 0; }
+
+	template<std::size_t Count>
+	constexpr Span<element_type, Count> first() const
+	{
+		return { data(), Count };
+	}
+
+	constexpr Span<element_type, dynamic_extent> first(std::size_t Count) const
+	{
+		return { data(), Count };
+	}
+
+	template<std::size_t Count>
+	constexpr Span<element_type, Count> last() const
+	{
+		return { data() + size() - Count, Count };
+	}
+
+	constexpr Span<element_type, dynamic_extent> last(std::size_t Count) const
+	{
+		return { data() + size() - Count, Count };
+	}
+
+	template<std::size_t Offset, std::size_t Count = dynamic_extent>
+	constexpr Span<element_type, Count> subspan() const
+	{
+		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+	}
+
+	constexpr Span<element_type, dynamic_extent>
+	subspan(std::size_t Offset, std::size_t Count = dynamic_extent) const
+	{
+		return { data() + Offset, Count == dynamic_extent ? size() - Offset : Count };
+	}
+
+private:
+	pointer data_;
+	size_type size_;
+};
+
+}; /* namespace libcamera */
+
+#endif /* __LIBCAMERA_SPAN_H__ */