diff --git a/hashset/hashset.go b/hashset/hashset.go
new file mode 100644
index 0000000..76fde67
--- /dev/null
+++ b/hashset/hashset.go
@@ -0,0 +1,47 @@
+// Package hashset provides a map-based Set.
+package hashset
+
+import "iter"
+
+// HashSet is a generic set based on a hash table (map).
+type HashSet[T comparable] struct {
+	m map[T]struct{}
+}
+
+// New creates a new HashSet.
+func New[T comparable]() *HashSet[T] {
+	return &HashSet[T]{m: make(map[T]struct{})}
+}
+
+// Add adds a value to the set.
+func (hs *HashSet[T]) Add(val T) {
+	hs.m[val] = struct{}{}
+}
+
+// Contains reports whether the set contains the given value.
+func (hs *HashSet[T]) Contains(val T) bool {
+	_, ok := hs.m[val]
+	return ok
+}
+
+// Len returns the size/length of the set - the number of values it contains.
+func (hs *HashSet[T]) Len() int {
+	return len(hs.m)
+}
+
+// Delete removes a value from the set; if the value doesn't exist in the
+// set, this is a no-op.
+func (hs *HashSet[T]) Delete(val T) {
+	delete(hs.m, val)
+}
+
+// All returns an iterator over all the values in the set.
+func (hs *HashSet[T]) All() iter.Seq[T] {
+	return func(yield func(T) bool) {
+		for val := range hs.m {
+			if !yield(val) {
+				return
+			}
+		}
+	}
+}
diff --git a/hashset/hashset_test.go b/hashset/hashset_test.go
new file mode 100644
index 0000000..9061f3e
--- /dev/null
+++ b/hashset/hashset_test.go
@@ -0,0 +1,96 @@
+package hashset
+
+import (
+	"slices"
+	"testing"
+)
+
+func TestAll(t *testing.T) {
+	hs := New[int]()
+
+	checkAll := func(wantSorted []int) {
+		t.Helper()
+		if hs.Len() != len(wantSorted) {
+			t.Errorf("got len=%v, want %v", hs.Len(), len(wantSorted))
+		}
+
+		got := slices.Sorted(hs.All())
+		if !slices.Equal(got, wantSorted) {
+			t.Errorf("got %v, want %v", got, wantSorted)
+		}
+	}
+	checkAll([]int{})
+	hs.Add(10)
+	checkAll([]int{10})
+
+	hs.Add(20)
+	hs.Add(13)
+	checkAll([]int{10, 13, 20})
+
+	hs.Add(18)
+	checkAll([]int{10, 13, 18, 20})
+
+	hs.Delete(18)
+	checkAll([]int{10, 13, 20})
+	hs.Delete(10)
+	checkAll([]int{13, 20})
+
+	hs.Add(50)
+	hs.Add(5)
+	checkAll([]int{5, 13, 20, 50})
+
+	hs.Add(60)
+	hs.Add(60)
+	hs.Add(60)
+	checkAll([]int{5, 13, 20, 50, 60})
+
+	hs.Delete(60)
+	hs.Delete(60)
+	hs.Delete(60)
+	checkAll([]int{5, 13, 20, 50})
+
+	hs.Delete(50)
+	hs.Delete(20)
+	hs.Delete(5)
+	checkAll([]int{13})
+
+	hs.Delete(13)
+	checkAll([]int{})
+}
+
+func TestContains(t *testing.T) {
+	hs := New[string]()
+
+	checkContains := func(v string, want bool) {
+		t.Helper()
+		got := hs.Contains(v)
+		if got != want {
+			t.Errorf("contains(%v)=%v, want %v", v, got, want)
+		}
+	}
+
+	checkContains("joe", false)
+	hs.Add("joe")
+	checkContains("joe", true)
+	hs.Delete("joe")
+	checkContains("joe", false)
+
+	hs.Add("bee")
+	hs.Add("geranium")
+	checkContains("joe", false)
+	checkContains("bee", true)
+	checkContains("geranium", true)
+
+	hs.Add("cheese")
+	hs.Add("io")
+	hs.Add("joe")
+
+	for _, v := range []string{"joe", "bee", "geranium", "io", "cheese"} {
+		checkContains(v, true)
+		hs.Delete(v)
+	}
+
+	for _, v := range []string{"joe", "bee", "geranium", "io", "cheese"} {
+		checkContains(v, false)
+	}
+}