fn: add Filter to List

fn/list.go

@@ -300,3 +300,18 @@ func (l *List[A]) PushFrontList(other *List[A]) {
 		n = n.Prev()
 	}
 }
+
+// Filter gives a slice of all of the node values that satisfy the given
+// predicate.
+func (l *List[A]) Filter(f Pred[A]) []A {
+	var acc []A
+
+	for cursor := l.Front(); cursor != nil; cursor = cursor.Next() {
+		a := cursor.Value
+		if f(a) {
+			acc = append(acc, a)
+		}
+	}
+
+	return acc
+}

fn/list_test.go

@@ -5,6 +5,9 @@ import (
 	"reflect"
 	"testing"
 	"testing/quick"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/exp/slices"
 )
 
 func GenList(r *rand.Rand) *List[uint32] {
@@ -727,3 +730,93 @@ func TestMoveUnknownMark(t *testing.T) {
 	checkList(t, &l1, []int{1})
 	checkList(t, &l2, []int{2})
 }
+
+// TestFilterIdempotence ensures that the slice coming out of List.Filter is
+// the same as that slice filtered again by the same predicate.
+func TestFilterIdempotence(t *testing.T) {
+	require.NoError(
+		t, quick.Check(
+			func(l *List[uint32], modSize uint32) bool {
+				pred := func(a uint32) bool {
+					return a%modSize != 0
+				}
+
+				filtered := l.Filter(pred)
+
+				filteredAgain := Filter(pred, filtered)
+
+				return slices.Equal(filtered, filteredAgain)
+			},
+			&quick.Config{
+				Values: func(vs []reflect.Value, r *rand.Rand) {
+					l := GenList(r)
+					vs[0] = reflect.ValueOf(l)
+					vs[1] = reflect.ValueOf(
+						r.Uint32()%5 + 1,
+					)
+				},
+			},
+		),
+	)
+}
+
+// TestFilterShrinks ensures that the length of the slice returned from
+// List.Filter is never larger than the length of the List.
+func TestFilterShrinks(t *testing.T) {
+	require.NoError(
+		t, quick.Check(
+			func(l *List[uint32], modSize uint32) bool {
+				pred := func(a uint32) bool {
+					return a%modSize != 0
+				}
+
+				filteredSize := len(l.Filter(pred))
+
+				return filteredSize <= l.Len()
+			},
+			&quick.Config{
+				Values: func(vs []reflect.Value, r *rand.Rand) {
+					l := GenList(r)
+					vs[0] = reflect.ValueOf(l)
+					vs[1] = reflect.ValueOf(
+						r.Uint32()%5 + 1,
+					)
+				},
+			},
+		),
+	)
+}
+
+// TestFilterLawOfExcludedMiddle ensures that if we intersect a List.Filter
+// with its negation that the intersection is the empty set.
+func TestFilterLawOfExcludedMiddle(t *testing.T) {
+	require.NoError(
+		t, quick.Check(
+			func(l *List[uint32], modSize uint32) bool {
+				pred := func(a uint32) bool {
+					return a%modSize != 0
+				}
+
+				negatedPred := func(a uint32) bool {
+					return !pred(a)
+				}
+
+				positive := NewSet(l.Filter(pred)...)
+				negative := NewSet(l.Filter(negatedPred)...)
+
+				return positive.Intersect(negative).Equal(
+					NewSet[uint32](),
+				)
+			},
+			&quick.Config{
+				Values: func(vs []reflect.Value, r *rand.Rand) {
+					l := GenList(r)
+					vs[0] = reflect.ValueOf(l)
+					vs[1] = reflect.ValueOf(
+						r.Uint32()%5 + 1,
+					)
+				},
+			},
+		),
+	)
+}