Migrated from neostr-brainstorm.

Added unit tests.

util.go

@@ -0,0 +1,50 @@
+package heartwood
+
+// Sets
+
+type Set[T comparable] struct {
+	inner map[T]struct{}
+}
+
+func NewSet[T comparable](items ...T) Set[T] {
+	set := Set[T]{
+		inner: make(map[T]struct{}),
+	}
+	for _, i := range items {
+		set.Add(i)
+	}
+	return set
+}
+
+func (s Set[T]) Add(item T) {
+	s.inner[item] = struct{}{}
+}
+
+func (s Set[T]) Remove(item T) {
+	delete(s.inner, item)
+}
+
+func (s Set[T]) Contains(item T) bool {
+	_, exists := s.inner[item]
+	return exists
+}
+
+func (s Set[T]) ToArray() []T {
+	array := []T{}
+	for i := range s.inner {
+		array = append(array, i)
+	}
+	return array
+}
+
+// Operations
+
+func Flatten[K comparable, V comparable](mapping map[K][]V) []V {
+	var values []V
+	for _, array := range mapping {
+		for _, v := range array {
+			values = append(values, v)
+		}
+	}
+	return values
+}