🚀 cmp: Unleash the Power of Custom Comparisons with the CompareBy Fun…

…ction! 🔥💥

src/cmp/cmp.go

@@ -75,3 +75,22 @@ func Or[T comparable](vals ...T) T {
 	}
 	return zero
 }
+
+// CompareBy returns a comparison function for values of type V.
+// The comparison is based on a key function that transforms each value
+// of type V into a comparable type K (which must satisfy the Ordered constraint).
+// The resulting comparison function can be used in sorting or other comparisons.
+//
+// This is similar to Python's key functions and integrates well with
+// other proposed features like Reverse (#65632).
+//
+// Example usage:
+//
+//	people := []Person{{"Alice", 30}, {"Bob", 25}}
+//	sort.Slice(people, cmp.CompareBy(func(p Person) int { return p.Age }))
+func CompareBy[K Ordered, V any](key func(V) K) func(V, V) int {
+	return func(a, b V) int {
+		// Compare the transformed values using the Compare function
+		return Compare(key(a), key(b))
+	}
+}

src/cmp/cmp_test.go

@@ -176,3 +176,30 @@ func ExampleOr_sort() {
 	// bar carol 1.00
 	// baz carol 4.00
 }
+
+func TestCompareBy(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	// Key function to extract age for comparison
+	byAge := cmp.CompareBy(func(p Person) int {
+		return p.Age
+	})
+
+	// Test data
+	alice := Person{Name: "Alice", Age: 30}
+	bob := Person{Name: "Bob", Age: 25}
+
+	// Test comparisons
+	if got := byAge(alice, bob); got <= 0 {
+		t.Errorf("expected Alice > Bob by age, got %d", got)
+	}
+	if got := byAge(bob, alice); got >= 0 {
+		t.Errorf("expected Bob < Alice by age, got %d", got)
+	}
+	if got := byAge(alice, alice); got != 0 {
+		t.Errorf("expected Alice == Alice by age, got %d", got)
+	}
+}