Implement Span::first and Span::last from C++20 (#274)

This implements `first` and `last` methods on `Span` that mimics
ones in `std::span`.

2 files changed, 68 insertions, 0 deletions

diff --git a/absl/types/span.h b/absl/types/span.h
index bce18ebcb8ce..ea1808d3bae8 100644
--- a/absl/types/span.h
+++ b/absl/types/span.h
@@ -485,6 +485,40 @@ class Span {
                : (base_internal::ThrowStdOutOfRange("pos > size()"), Span());
   }
 
+  // Span::first()
+  //
+  // Returns a `Span` containing first `len` elements. Parameter `len` is of
+  // type `size_type` and thus non-negative. `len` value must be <= size().
+  //
+  // Examples:
+  //
+  //   std::vector<int> vec = {10, 11, 12, 13};
+  //   absl::MakeSpan(vec).first(1);  // {10}
+  //   absl::MakeSpan(vec).first(3);  // {10, 11, 12}
+  //   absl::MakeSpan(vec).first(5);  // throws std::out_of_range
+  constexpr Span first(size_type len) const {
+    return (len <= size())
+               ? Span(data(), len)
+               : (base_internal::ThrowStdOutOfRange("len > size()"), Span());
+  }
+
+  // Span::last()
+  //
+  // Returns a `Span` containing last `len` elements. Parameter `len` is of
+  // type `size_type` and thus non-negative. `len` value must be <= size().
+  //
+  // Examples:
+  //
+  //   std::vector<int> vec = {10, 11, 12, 13};
+  //   absl::MakeSpan(vec).last(1);  // {13}
+  //   absl::MakeSpan(vec).last(3);  // {11, 12, 13}
+  //   absl::MakeSpan(vec).last(5);  // throws std::out_of_range
+  constexpr Span last(size_type len) const {
+    return (len <= size())
+               ? Span(data() + size() - len, len)
+               : (base_internal::ThrowStdOutOfRange("len > size()"), Span());
+  }
+
   // Support for absl::Hash.
   template <typename H>
   friend H AbslHashValue(H h, Span v) {
diff --git a/absl/types/span_test.cc b/absl/types/span_test.cc
index 294229ea07fe..9269f911f042 100644
--- a/absl/types/span_test.cc
+++ b/absl/types/span_test.cc
@@ -295,6 +295,38 @@ TEST(IntSpan, Subspan) {
 #endif
 }
 
+TEST(IntSpan, First) {
+  std::vector<int> empty;
+  EXPECT_THAT(absl::MakeSpan(empty).first(0), SpanIs(empty));
+
+  auto ramp = MakeRamp(10);
+  EXPECT_THAT(absl::MakeSpan(ramp).first(0), SpanIs(ramp.data(), 0));
+  EXPECT_THAT(absl::MakeSpan(ramp).first(10), SpanIs(ramp));
+  EXPECT_THAT(absl::MakeSpan(ramp).first(3), SpanIs(ramp.data(), 3));
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(absl::MakeSpan(ramp).first(11), std::out_of_range);
+#else
+  EXPECT_DEATH_IF_SUPPORTED(absl::MakeSpan(ramp).first(11), "");
+#endif
+}
+
+TEST(IntSpan, Last) {
+  std::vector<int> empty;
+  EXPECT_THAT(absl::MakeSpan(empty).last(0), SpanIs(empty));
+
+  auto ramp = MakeRamp(10);
+  EXPECT_THAT(absl::MakeSpan(ramp).last(0), SpanIs(ramp.data() + 10, 0));
+  EXPECT_THAT(absl::MakeSpan(ramp).last(10), SpanIs(ramp));
+  EXPECT_THAT(absl::MakeSpan(ramp).last(3), SpanIs(ramp.data() + 7, 3));
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(absl::MakeSpan(ramp).last(11), std::out_of_range);
+#else
+  EXPECT_DEATH_IF_SUPPORTED(absl::MakeSpan(ramp).last(11), "");
+#endif
+}
+
 TEST(IntSpan, MakeSpanPtrLength) {
   std::vector<int> empty;
   auto s_empty = absl::MakeSpan(empty.data(), empty.size());
@@ -769,6 +801,8 @@ TEST(ConstIntSpan, ConstexprTest) {
   ABSL_TEST_CONSTEXPR(span.begin());
   ABSL_TEST_CONSTEXPR(span.cbegin());
   ABSL_TEST_CONSTEXPR(span.subspan(0, 0));
+  ABSL_TEST_CONSTEXPR(span.first(1));
+  ABSL_TEST_CONSTEXPR(span.last(1));
   ABSL_TEST_CONSTEXPR(span[0]);
 }