Add span.h from chromium base.chromium/3392

Allows indexing with better bounds-checking to occur. Some small
modifications are required to deal with PDFium being intentionally
held at C++11 compliance, not C++14.

Use in one place as check on correctness.

Change-Id: Id2875cf0a93980112bc536a93c4f9ec5306c0dac
Reviewed-on: https://pdfium-review.googlesource.com/29671
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>

2 files changed, 337 insertions, 0 deletions

diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index 1bff4347df..8b212e1622 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -562,6 +562,7 @@ jumbo_source_set("pdfium_base") {
     "base/numerics/safe_math_impl.h",
     "base/optional.h",
     "base/ptr_util.h",
+    "base/span.h",
     "base/stl_util.h",
     "base/sys_byteorder.h",
     "base/template_util.h",
diff --git a/third_party/base/span.h b/third_party/base/span.h
new file mode 100644
index 0000000000..d8d8f29e7c
--- /dev/null
+++ b/third_party/base/span.h
@@ -0,0 +1,336 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PDFIUM_THIRD_PARTY_BASE_SPAN_H_
+#define PDFIUM_THIRD_PARTY_BASE_SPAN_H_
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <array>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+
+#include "third_party/base/logging.h"
+
+namespace pdfium {
+
+template <typename T>
+class span;
+
+namespace internal {
+
+template <typename T>
+struct IsSpanImpl : std::false_type {};
+
+template <typename T>
+struct IsSpanImpl<span<T>> : std::true_type {};
+
+template <typename T>
+using IsSpan = IsSpanImpl<typename std::decay<T>::type>;
+
+template <typename T>
+struct IsStdArrayImpl : std::false_type {};
+
+template <typename T, size_t N>
+struct IsStdArrayImpl<std::array<T, N>> : std::true_type {};
+
+template <typename T>
+using IsStdArray = IsStdArrayImpl<typename std::decay<T>::type>;
+
+template <typename From, typename To>
+using IsLegalSpanConversion = std::is_convertible<From*, To*>;
+
+template <typename Container, typename T>
+using ContainerHasConvertibleData =
+    IsLegalSpanConversion<typename std::remove_pointer<decltype(
+                              std::declval<Container>().data())>::type,
+                          T>;
+template <typename Container>
+using ContainerHasIntegralSize =
+    std::is_integral<decltype(std::declval<Container>().size())>;
+
+template <typename From, typename To>
+using EnableIfLegalSpanConversion =
+    typename std::enable_if<IsLegalSpanConversion<From, To>::value>::type;
+
+// SFINAE check if Container can be converted to a span<T>. Note that the
+// implementation details of this check differ slightly from the requirements in
+// the working group proposal: in particular, the proposal also requires that
+// the container conversion constructor participate in overload resolution only
+// if two additional conditions are true:
+//
+//   1. Container implements operator[].
+//   2. Container::value_type matches remove_const_t<element_type>.
+//
+// The requirements are relaxed slightly here: in particular, not requiring (2)
+// means that an immutable span can be easily constructed from a mutable
+// container.
+template <typename Container, typename T>
+using EnableIfSpanCompatibleContainer =
+    typename std::enable_if<!internal::IsSpan<Container>::value &&
+                            !internal::IsStdArray<Container>::value &&
+                            ContainerHasConvertibleData<Container, T>::value &&
+                            ContainerHasIntegralSize<Container>::value>::type;
+
+template <typename Container, typename T>
+using EnableIfConstSpanCompatibleContainer =
+    typename std::enable_if<std::is_const<T>::value &&
+                            !internal::IsSpan<Container>::value &&
+                            !internal::IsStdArray<Container>::value &&
+                            ContainerHasConvertibleData<Container, T>::value &&
+                            ContainerHasIntegralSize<Container>::value>::type;
+
+}  // namespace internal
+
+// A span is a value type that represents an array of elements of type T. Since
+// it only consists of a pointer to memory with an associated size, it is very
+// light-weight. It is cheap to construct, copy, move and use spans, so that
+// users are encouraged to use it as a pass-by-value parameter. A span does not
+// own the underlying memory, so care must be taken to ensure that a span does
+// not outlive the backing store.
+//
+// span is somewhat analogous to StringPiece, but with arbitrary element types,
+// allowing mutation if T is non-const.
+//
+// span is implicitly convertible from C++ arrays, as well as most [1]
+// container-like types that provide a data() and size() method (such as
+// std::vector<T>). A mutable span<T> can also be implicitly converted to an
+// immutable span<const T>.
+//
+// Consider using a span for functions that take a data pointer and size
+// parameter: it allows the function to still act on an array-like type, while
+// allowing the caller code to be a bit more concise.
+//
+// For read-only data access pass a span<const T>: the caller can supply either
+// a span<const T> or a span<T>, while the callee will have a read-only view.
+// For read-write access a mutable span<T> is required.
+//
+// Without span:
+//   Read-Only:
+//     // std::string HexEncode(const uint8_t* data, size_t size);
+//     std::vector<uint8_t> data_buffer = GenerateData();
+//     std::string r = HexEncode(data_buffer.data(), data_buffer.size());
+//
+//  Mutable:
+//     // ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt, Args...);
+//     char str_buffer[100];
+//     SafeSNPrintf(str_buffer, sizeof(str_buffer), "Pi ~= %lf", 3.14);
+//
+// With span:
+//   Read-Only:
+//     // std::string HexEncode(base::span<const uint8_t> data);
+//     std::vector<uint8_t> data_buffer = GenerateData();
+//     std::string r = HexEncode(data_buffer);
+//
+//  Mutable:
+//     // ssize_t SafeSNPrintf(base::span<char>, const char* fmt, Args...);
+//     char str_buffer[100];
+//     SafeSNPrintf(str_buffer, "Pi ~= %lf", 3.14);
+//
+// Spans with "const" and pointers
+// -------------------------------
+//
+// Const and pointers can get confusing. Here are vectors of pointers and their
+// corresponding spans (you can always make the span "more const" too):
+//
+//   const std::vector<int*>        =>  base::span<int* const>
+//   std::vector<const int*>        =>  base::span<const int*>
+//   const std::vector<const int*>  =>  base::span<const int* const>
+//
+// Differences from the working group proposal
+// -------------------------------------------
+//
+// https://wg21.link/P0122 is the latest working group proposal, Chromium
+// currently implements R6. The biggest difference is span does not support a
+// static extent template parameter. Other differences are documented in
+// subsections below.
+//
+// Differences from [views.constants]:
+// - no dynamic_extent constant
+//
+// Differences from [span.objectrep]:
+// - no as_bytes()
+// - no as_writeable_bytes()
+//
+// Differences in constants and types:
+// - no element_type type alias
+// - no index_type type alias
+// - no different_type type alias
+// - no extent constant
+//
+// Differences from [span.cons]:
+// - no constructor from a pointer range
+// - no constructor from std::array
+//
+// Differences from [span.sub]:
+// - no templated first()
+// - no templated last()
+// - no templated subspan()
+// - using size_t instead of ptrdiff_t for indexing
+//
+// Differences from [span.obs]:
+// - no size_bytes()
+// - using size_t instead of ptrdiff_t to represent size()
+//
+// Differences from [span.elem]:
+// - no operator ()()
+// - using size_t instead of ptrdiff_t for indexing
+
+// [span], class template span
+template <typename T>
+class span {
+ public:
+  using value_type = typename std::remove_cv<T>::type;
+  using pointer = T*;
+  using reference = 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>;
+
+  // [span.cons], span constructors, copy, assignment, and destructor
+  constexpr span() noexcept : data_(nullptr), size_(0) {}
+  constexpr span(T* data, size_t size) noexcept : data_(data), size_(size) {}
+  // TODO(dcheng): Implement construction from a |begin| and |end| pointer.
+  template <size_t N>
+  constexpr span(T (&array)[N]) noexcept : span(array, N) {}
+  // TODO(dcheng): Implement construction from std::array.
+  // Conversion from a container that provides |T* data()| and |integral_type
+  // size()|.
+  template <typename Container,
+            typename = internal::EnableIfSpanCompatibleContainer<Container, T>>
+  constexpr span(Container& container)
+      : span(container.data(), container.size()) {}
+  template <
+      typename Container,
+      typename = internal::EnableIfConstSpanCompatibleContainer<Container, T>>
+  span(const Container& container) : span(container.data(), container.size()) {}
+  constexpr span(const span& other) noexcept = default;
+  // Conversions from spans of compatible types: this allows a span<T> to be
+  // seamlessly used as a span<const T>, but not the other way around.
+  template <typename U, typename = internal::EnableIfLegalSpanConversion<U, T>>
+  constexpr span(const span<U>& other) : span(other.data(), other.size()) {}
+  span& operator=(const span& other) noexcept = default;
+  ~span() noexcept = default;
+
+  // [span.sub], span subviews
+  const span first(size_t count) const {
+    CHECK(count <= size_);
+    return span(data_, count);
+  }
+
+  const span last(size_t count) const {
+    CHECK(count <= size_);
+    return span(data_ + (size_ - count), count);
+  }
+
+  const span subspan(size_t pos, size_t count = -1) const {
+    const auto npos = static_cast<size_t>(-1);
+    CHECK(pos <= size_);
+    CHECK(count == npos || count <= size_ - pos);
+    return span(data_ + pos, count == npos ? size_ - pos : count);
+  }
+
+  // [span.obs], span observers
+  constexpr size_t size() const noexcept { return size_; }
+  constexpr bool empty() const noexcept { return size_ == 0; }
+
+  // [span.elem], span element access
+  const T& operator[](size_t index) const noexcept {
+    CHECK(index < size_);
+    return data_[index];
+  }
+  constexpr T* data() const noexcept { return data_; }
+
+  // [span.iter], span iterator support
+  constexpr iterator begin() const noexcept { return data_; }
+  constexpr iterator end() const noexcept { return data_ + size_; }
+
+  constexpr const_iterator cbegin() const noexcept { return begin(); }
+  constexpr const_iterator cend() const noexcept { return end(); }
+
+  constexpr reverse_iterator rbegin() const noexcept {
+    return reverse_iterator(end());
+  }
+  constexpr reverse_iterator rend() const noexcept {
+    return reverse_iterator(begin());
+  }
+
+  constexpr const_reverse_iterator crbegin() const noexcept {
+    return const_reverse_iterator(cend());
+  }
+  constexpr const_reverse_iterator crend() const noexcept {
+    return const_reverse_iterator(cbegin());
+  }
+
+ private:
+  T* data_;
+  size_t size_;
+};
+
+// [span.comparison], span comparison operators
+// Relational operators. Equality is a element-wise comparison.
+template <typename T>
+constexpr bool operator==(span<T> lhs, span<T> rhs) noexcept {
+  return lhs.size() == rhs.size() &&
+         std::equal(lhs.cbegin(), lhs.cend(), rhs.cbegin());
+}
+
+template <typename T>
+constexpr bool operator!=(span<T> lhs, span<T> rhs) noexcept {
+  return !(lhs == rhs);
+}
+
+template <typename T>
+constexpr bool operator<(span<T> lhs, span<T> rhs) noexcept {
+  return std::lexicographical_compare(lhs.cbegin(), lhs.cend(), rhs.cbegin(),
+                                      rhs.cend());
+}
+
+template <typename T>
+constexpr bool operator<=(span<T> lhs, span<T> rhs) noexcept {
+  return !(rhs < lhs);
+}
+
+template <typename T>
+constexpr bool operator>(span<T> lhs, span<T> rhs) noexcept {
+  return rhs < lhs;
+}
+
+template <typename T>
+constexpr bool operator>=(span<T> lhs, span<T> rhs) noexcept {
+  return !(lhs < rhs);
+}
+
+// Type-deducing helpers for constructing a span.
+template <typename T>
+constexpr span<T> make_span(T* data, size_t size) noexcept {
+  return span<T>(data, size);
+}
+
+template <typename T, size_t N>
+constexpr span<T> make_span(T (&array)[N]) noexcept {
+  return span<T>(array);
+}
+
+template <typename Container,
+          typename T = typename Container::value_type,
+          typename = internal::EnableIfSpanCompatibleContainer<Container, T>>
+constexpr span<T> make_span(Container& container) {
+  return span<T>(container);
+}
+
+template <
+    typename Container,
+    typename T = typename std::add_const<typename Container::value_type>::type,
+    typename = internal::EnableIfConstSpanCompatibleContainer<Container, T>>
+constexpr span<T> make_span(const Container& container) {
+  return span<T>(container);
+}
+
+}  // namespace pdfium
+
+#endif  // PDFIUM_THIRD_PARTY_BASE_SPAN_H_