[LLVM][ADT] Add consume_front and consume_back to ArrayRef (#146741)

Add `consume_front` that returns the first element and drops it from the
current ArrayRef, and `consume_back` that returns the last element and
drops it from the current ArrayRef.

Author: jurahul

llvm/include/llvm/ADT/ArrayRef.h

@@ -158,6 +158,20 @@ namespace llvm {
       return Data[Length-1];
     }
 
+    /// consume_front() - Returns the first element and drops it from ArrayRef.
+    const T &consume_front() {
+      const T &Ret = front();
+      *this = drop_front();
+      return Ret;
+    }
+
+    /// consume_back() - Returns the last element and drops it from ArrayRef.
+    const T &consume_back() {
+      const T &Ret = back();
+      *this = drop_back();
+      return Ret;
+    }
+
     // copy - Allocate copy in Allocator and return ArrayRef<T> to it.
     template <typename Allocator> MutableArrayRef<T> copy(Allocator &A) {
       T *Buff = A.template Allocate<T>(Length);
@@ -352,6 +366,20 @@ namespace llvm {
       return data()[this->size()-1];
     }
 
+    /// consume_front() - Returns the first element and drops it from ArrayRef.
+    T &consume_front() {
+      T &Ret = front();
+      *this = drop_front();
+      return Ret;
+    }
+
+    /// consume_back() - Returns the last element and drops it from ArrayRef.
+    T &consume_back() {
+      T &Ret = back();
+      *this = drop_back();
+      return Ret;
+    }
+
     /// slice(n, m) - Chop off the first N elements of the array, and keep M
     /// elements in the array.
     MutableArrayRef<T> slice(size_t N, size_t M) const {

llvm/unittests/ADT/ArrayRefTest.cpp

@@ -104,6 +104,48 @@ TEST(ArrayRefTest, DropFront) {
   EXPECT_TRUE(AR1.drop_front(2).equals(AR2));
 }
 
+TEST(ArrayRefTest, ConsumeFront) {
+  static const int TheNumbers[] = {4, 8, 15, 16, 23, 42};
+  ArrayRef<int> AR1(TheNumbers);
+  ArrayRef<int> AR2(&TheNumbers[2], AR1.size() - 2);
+  EXPECT_EQ(&AR1.consume_front(), &TheNumbers[0]);
+  EXPECT_EQ(&AR1.consume_front(), &TheNumbers[1]);
+  EXPECT_TRUE(AR1.equals(AR2));
+}
+
+TEST(ArrayRefTest, ConsumeBack) {
+  static const int TheNumbers[] = {4, 8, 15, 16, 23, 42};
+  ArrayRef<int> AR1(TheNumbers);
+  ArrayRef<int> AR2(TheNumbers, AR1.size() - 2);
+  EXPECT_EQ(&AR1.consume_back(), &TheNumbers[5]);
+  EXPECT_EQ(&AR1.consume_back(), &TheNumbers[4]);
+  EXPECT_TRUE(AR1.equals(AR2));
+}
+
+TEST(ArrayRefTest, MutableArryaRefConsumeFront) {
+  int TheNumbers[] = {4, 8, 15, 16, 23, 42};
+  MutableArrayRef<int> AR1(TheNumbers);
+  MutableArrayRef<int> AR2(&TheNumbers[2], AR1.size() - 2);
+  EXPECT_EQ(&AR1.consume_front(), &TheNumbers[0]);
+  EXPECT_EQ(&AR1.consume_front(), &TheNumbers[1]);
+  EXPECT_TRUE(AR1.equals(AR2));
+
+  AR1.consume_front() = 33;
+  EXPECT_EQ(TheNumbers[2], 33);
+}
+
+TEST(ArrayRefTest, MutableArryaRefConsumeBack) {
+  int TheNumbers[] = {4, 8, 15, 16, 23, 42};
+  MutableArrayRef<int> AR1(TheNumbers);
+  MutableArrayRef<int> AR2(TheNumbers, AR1.size() - 2);
+  EXPECT_EQ(&AR1.consume_back(), &TheNumbers[5]);
+  EXPECT_EQ(&AR1.consume_back(), &TheNumbers[4]);
+  EXPECT_TRUE(AR1.equals(AR2));
+
+  AR1.consume_back() = 33;
+  EXPECT_EQ(TheNumbers[3], 33);
+}
+
 TEST(ArrayRefTest, DropWhile) {
   static const int TheNumbers[] = {1, 3, 5, 8, 10, 11};
   ArrayRef<int> AR1(TheNumbers);