ADT: Add a drop_back() helper to ArrayRef

The slice(N, M) interface is powerful but not concise when wanting to
drop a few elements off of an ArrayRef, fix this by adding a drop_back
method.

llvm-svn: 212370

2 files changed, 13 insertions, 0 deletions

diff --git a/llvm/include/llvm/ADT/ArrayRef.h b/llvm/include/llvm/ADT/ArrayRef.h
index 1b64fee9a59..0fff505d8d0 100644
--- a/llvm/include/llvm/ADT/ArrayRef.h
+++ b/llvm/include/llvm/ADT/ArrayRef.h
@@ -147,6 +147,12 @@ namespace llvm {
       return ArrayRef<T>(data()+N, M);
     }
 
+    // \brief Drop the last \p N elements of the array.
+    ArrayRef<T> drop_back(unsigned N = 1) const {
+      assert(size() >= N && "Dropping more elements than exist");
+      return slice(0, size() - N);
+    }
+
     /// @}
     /// @name Operator Overloads
     /// @{
diff --git a/llvm/unittests/ADT/ArrayRefTest.cpp b/llvm/unittests/ADT/ArrayRefTest.cpp
index 7133ca79c46..293afc6ea37 100644
--- a/llvm/unittests/ADT/ArrayRefTest.cpp
+++ b/llvm/unittests/ADT/ArrayRefTest.cpp
@@ -29,5 +29,12 @@ TEST(ArrayRefTest, AllocatorCopy) {
   EXPECT_NE(Array2.data(), Array2c.data());
 }
 
+TEST(ArrayRefTest, DropBack) {
+  static const int TheNumbers[] = {4, 8, 15, 16, 23, 42};
+  ArrayRef<int> AR1(TheNumbers);
+  ArrayRef<int> AR2(TheNumbers, AR1.size() - 1);
+  EXPECT_TRUE(AR1.drop_back().equals(AR2));
+}
+
 
 } // end anonymous namespace