Add pdfium::span::as_bytes() and as_writable_bytes().

Picks up some enhancements from base/span.h.  In turn, also adds
the size_bytes() helper.  Differs from base version in that it
works around C++14 enable_if_t<>, and avoids the dynamic_extent
template specialization tricks.

Use it in a few places where appropriate.

Change-Id: I86f72cf0023f2d4317a7afa351fddee601c8f86c
Reviewed-on: https://pdfium-review.googlesource.com/38251
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

3 files changed, 18 insertions, 12 deletions

diff --git a/core/fpdfapi/parser/fpdf_parser_decode.cpp b/core/fpdfapi/parser/fpdf_parser_decode.cpp
index 82d84ab705..9ccca121ad 100644
--- a/core/fpdfapi/parser/fpdf_parser_decode.cpp
+++ b/core/fpdfapi/parser/fpdf_parser_decode.cpp
@@ -500,9 +500,8 @@ ByteString PDF_EncodeText(const wchar_t* pString, int len) {
   size_t dest_index = 0;
   size_t encLen = len * 2 + 2;
   {
-    pdfium::span<char> cspan = result.GetBuffer(encLen);
-    auto dest_buf = pdfium::make_span(reinterpret_cast<uint8_t*>(cspan.data()),
-                                      cspan.size());
+    pdfium::span<uint8_t> dest_buf =
+        pdfium::as_writable_bytes(result.GetBuffer(encLen));
     dest_buf[dest_index++] = 0xfe;
     dest_buf[dest_index++] = 0xff;
     for (int j = 0; j < len; ++j) {
diff --git a/core/fxcrt/bytestring_unittest.cpp b/core/fxcrt/bytestring_unittest.cpp
index fb5fc0d996..7095a1b799 100644
--- a/core/fxcrt/bytestring_unittest.cpp
+++ b/core/fxcrt/bytestring_unittest.cpp
@@ -1060,8 +1060,7 @@ TEST(ByteStringView, NotNull) {
   ByteStringView string3("abc");
   ByteStringView string6("abcdef");
   ByteStringView alternate_string3("abcdef", 3);
-  ByteStringView span_string4(
-      pdfium::span<const uint8_t>(reinterpret_cast<const uint8_t*>("abcd"), 4));
+  ByteStringView span_string4(pdfium::as_bytes(pdfium::make_span("abcd", 4)));
   ByteStringView embedded_nul_string7("abc\0def", 7);
   ByteStringView illegal_string7("abcdef", 7);
 
@@ -1355,8 +1354,8 @@ TEST(ByteStringView, OperatorEQ) {
   EXPECT_FALSE(c_string2 == byte_string_c);
   EXPECT_FALSE(c_string3 == byte_string_c);
 
-  pdfium::span<const uint8_t> span5(reinterpret_cast<const uint8_t*>("hello"),
-                                    5);
+  pdfium::span<const uint8_t> span5(
+      pdfium::as_bytes(pdfium::make_span("hello", 5)));
   EXPECT_EQ(byte_string_c.span(), span5);
 }
 
diff --git a/third_party/base/span.h b/third_party/base/span.h
index 0fb627ba8c..ccffbc1e78 100644
--- a/third_party/base/span.h
+++ b/third_party/base/span.h
@@ -152,10 +152,6 @@ using EnableIfConstSpanCompatibleContainer =
 // 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
@@ -173,7 +169,6 @@ using EnableIfConstSpanCompatibleContainer =
 // - 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]:
@@ -244,6 +239,7 @@ class span {
 
   // [span.obs], span observers
   constexpr size_t size() const noexcept { return size_; }
+  constexpr size_t size_bytes() const noexcept { return size() * sizeof(T); }
   constexpr bool empty() const noexcept { return size_ == 0; }
 
   // [span.elem], span element access
@@ -313,6 +309,18 @@ constexpr bool operator>=(span<T> lhs, span<T> rhs) noexcept {
   return !(lhs < rhs);
 }
 
+// [span.objectrep], views of object representation
+template <typename T>
+span<const uint8_t> as_bytes(span<T> s) noexcept {
+  return {reinterpret_cast<const uint8_t*>(s.data()), s.size_bytes()};
+}
+
+template <typename T,
+          typename U = typename std::enable_if<!std::is_const<T>::value>::type>
+span<uint8_t> as_writable_bytes(span<T> s) noexcept {
+  return {reinterpret_cast<uint8_t*>(s.data()), s.size_bytes()};
+}
+
 // Type-deducing helpers for constructing a span.
 template <typename T>
 constexpr span<T> make_span(T* data, size_t size) noexcept {