std.MultiArrayList: add a swap method

lib/std/multi_array_list.zig

@@ -133,6 +133,13 @@ pub fn MultiArrayList(comptime T: type) type {
                 };
             }
 
+            pub fn swap(self: Slice, a: usize, b: usize) void {
+                inline for (@typeInfo(Field).@"enum".fields) |field| {
+                    const its = self.items(@field(Field, field.name));
+                    std.mem.swap(@FieldType(T, field.name), &its[a], &its[b]);
+                }
+            }
+
             pub fn toMultiArrayList(self: Slice) Self {
                 if (self.ptrs.len == 0 or self.capacity == 0) {
                     return .{};
@@ -267,6 +274,10 @@ pub fn MultiArrayList(comptime T: type) type {
             return self.slice().get(index);
         }
 
+        pub fn swap(self: Self, a: usize, b: usize) void {
+            return self.slice().swap(a, b);
+        }
+
         /// Extend the list by 1 element.
         ///
         /// Allocates more memory as necessary.
@@ -771,6 +782,19 @@ test "basic usage" {
     try testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 1, 2, 3 });
     try testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'a', 'b', 'c' });
 
+    list.swap(0, 2);
+    try testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 3, 2, 1 });
+    try testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'c', 'b', 'a' });
+    list.swap(2, 1);
+    try testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 3, 1, 2 });
+    try testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'c', 'a', 'b' });
+    list.swap(2, 0);
+    try testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 2, 1, 3 });
+    try testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'b', 'a', 'c' });
+    list.swap(0, 1);
+    try testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 1, 2, 3 });
+    try testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'a', 'b', 'c' });
+
     try testing.expectEqual(@as(usize, 3), list.items(.b).len);
     try testing.expectEqualStrings("foobar", list.items(.b)[0]);
     try testing.expectEqualStrings("zigzag", list.items(.b)[1]);