Implement sort by view (#8)

Author: markisus

README.md

@@ -6,6 +6,7 @@ These are the most useful operations.
 - `.operator[](row_idx)` read data out as tuple of references
 - `.get_column<col_idx>()` direct access to underlying std::vector column
 - `.view<col_idx1, col_idx2, ...>(row_idx)` read subset of the fields out as a tuple of references
+- `.sort_by_view<col_idx1, col_idx2, ...>()` sort all columns in tandem based on a subset of columns
 
 Code Example (scratch.cpp)
 ------------------------
@@ -86,6 +87,24 @@ int main(int argc, char *argv[])
     });
     std::cout << presidents << "\n";
 
+    // We can also sort by a view.
+    std::cout << "Sorting by first name, last name." << "\n";
+    presidents.sort_by_view<FIRST_NAME,LAST_NAME>();
+    std::cout << presidents << "\n";
+
+    // We can also use a comparator when sorting by view.
+    // In that case, the comparator should take a tuple of references.
+    // For example, we can sort by difference in length of last and first name.
+    std::cout << "Sorting by length of first name - length of last name." << "\n";    
+    presidents.sort_by_view<LAST_NAME, FIRST_NAME>([](auto view1, auto view2) {
+        const auto& [lname_a, fname_a] = view1;
+        const auto& [lname_b, fname_b] = view2;
+        int view1_order = -int(lname_a.size()) + int(fname_a.size());
+        int view2_order = -int(lname_b.size()) + int(fname_b.size());
+        return view1_order < view2_order;
+    });
+    std::cout << presidents << "\n";
+
     return 0;
 }
 ```
@@ -164,6 +183,26 @@ soa {
 	0, George, Washington
 }
 
+Sorting by first name, last name.
+soa {
+        3, Barack, Obama
+        1, Bill, Clinton
+        4, Donald, Trump
+        2, George, Bush
+        0, George, Washington
+        5, Joseph, Biden
+}
+
+Sorting by length of first name - length of last name.
+soa {
+        0, George, Washington
+        1, Bill, Clinton
+        3, Barack, Obama
+        4, Donald, Trump
+        5, Joseph, Biden
+        2, George, Bush
+}
+
 ```
 
 Benchmark

scratch.cc

@@ -73,5 +73,23 @@ int main(int argc, char *argv[])
     });
     std::cout << presidents << "\n";
 
+    // We can also sort by a view.
+    std::cout << "Sorting by first name, last name." << "\n";
+    presidents.sort_by_view<FIRST_NAME,LAST_NAME>();
+    std::cout << presidents << "\n";
+
+    // We can also use a comparator when sorting by view.
+    // In that case, the comparator should take a tuple of references.
+    // For example, we can sort by difference in length of last and first name.
+    std::cout << "Sorting by length of first name - length of last name." << "\n";    
+    presidents.sort_by_view<LAST_NAME, FIRST_NAME>([](auto view1, auto view2) {
+        const auto& [lname_a, fname_a] = view1;
+        const auto& [lname_b, fname_b] = view2;
+        int view1_order = -int(lname_a.size()) + int(fname_a.size());
+        int view2_order = -int(lname_b.size()) + int(fname_b.size());
+        return view1_order < view2_order;
+    });
+    std::cout << presidents << "\n";
+
     return 0;
 }

vapid/soa.h

@@ -214,6 +214,31 @@ namespace vapid {
             sort_by_field<col_idx>([](auto&& a, auto&& b) { return a < b; });
         }
 
+        template <size_t... I, typename C>
+        void sort_by_view(C&& comparator) {
+            reset_sort_reference();
+
+            auto comparator_wrapper = [=](size_t a, size_t b) {
+                return comparator(view<I...>(a),
+                                  view<I...>(b));
+            };
+
+            std::stable_sort(sort_order_reference_.begin(),
+                sort_order_reference_.end(),
+                comparator_wrapper);
+
+            if (no_double_buffering_) {
+                sort_order_analysis_.store_analysis(sort_order_reference_);
+            }
+
+            sort_by_reference_impl(std::index_sequence_for<Ts...>{});
+        }
+
+        template <size_t... I>
+        void sort_by_view() {
+            sort_by_view<I...>([](auto&& a, auto&& b) { return a < b; });
+        }
+
         void dump(std::basic_ostream<char>& ss) const {
             constexpr size_t MAX_NUM_ELEMENTS_TO_PRINT = 25;
             size_t num_elements_to_print = size();