ree_util: Add CompareValuesAt to ReadWriteValue

cpp/src/arrow/compute/kernels/ree_util_internal.h

@@ -81,6 +81,18 @@ class ReadWriteValue<ArrowType, in_has_validity_buffer, out_has_validity_buffer,
     return valid;
   }
 
+  /// Pre-conditions guaranteed by the callers:
+  /// - i and j are valid indices into the values buffer
+  /// - the values in i and j are valid
+  bool CompareValuesAt(int64_t i, int64_t j) const {
+    if constexpr (std::is_same_v<ArrowType, BooleanType>) {
+      return bit_util::GetBit(input_values_, i) == bit_util::GetBit(input_values_, j);
+    } else {
+      return (reinterpret_cast<const ValueRepr*>(input_values_))[i] ==
+             (reinterpret_cast<const ValueRepr*>(input_values_))[j];
+    }
+  }
+
   /// \brief Ensure padding is zeroed in validity bitmap.
   void ZeroValidityPadding(int64_t length) const {
     DCHECK(output_values_);
@@ -164,6 +176,11 @@ class ReadWriteValue<ArrowType, in_has_validity_buffer, out_has_validity_buffer,
     return valid;
   }
 
+  bool CompareValuesAt(int64_t i, int64_t j) const {
+    return 0 == memcmp(input_values_ + (i * byte_width_),
+                       input_values_ + (j * byte_width_), byte_width_);
+  }
+
   /// \brief Ensure padding is zeroed in validity bitmap.
   void ZeroValidityPadding(int64_t length) const {
     DCHECK(output_values_);
@@ -251,6 +268,14 @@ class ReadWriteValue<ArrowType, in_has_validity_buffer, out_has_validity_buffer,
     return valid;
   }
 
+  bool CompareValuesAt(int64_t i, int64_t j) const {
+    const offset_type len_i = input_offsets_[i + 1] - input_offsets_[i];
+    const offset_type len_j = input_offsets_[j + 1] - input_offsets_[j];
+    return len_i == len_j &&
+           memcmp(input_values_ + input_offsets_[i], input_values_ + input_offsets_[j],
+                  static_cast<size_t>(len_i));
+  }
+
   /// \brief Ensure padding is zeroed in validity bitmap.
   void ZeroValidityPadding(int64_t length) const {
     DCHECK(output_values_);