diff: make Diff() better

Turned it into a DisjunctiveUnion which is also better because it uses
terminology from set theory to make things more clear. And it's just a
superset of the functionality!

Author: adam000

diff/diff.go

@@ -1,19 +1,38 @@
 package diff
 
-// Assumes that a is a superset of b and that they are sorted.
-// TODO need a better name.
-func Diff[C comparable](a []C, b []C) []C {
-	result := make([]C, 0, len(a)-len(b))
+import "golang.org/x/exp/constraints"
+
+// DisjunctiveUnion returns 2 slices containing the values not shared by both.
+// DisjunctiveUnion assumes that there are not duplicate values in the input
+// slices and that the input slices are sorted.
+func DisjunctiveUnion[C constraints.Ordered](a []C, b []C) ([]C, []C) {
+	notInA := make([]C, 0)
+	notInB := make([]C, 0)
+	aIdx := 0
 	bIdx := 0
-	for _, i := range a {
-		if bIdx < len(b) {
-			if i == b[bIdx] {
-				bIdx++
-				continue
-			}
+
+	for aIdx < len(a) && bIdx < len(b) {
+		if a[aIdx] < b[bIdx] {
+			notInB = append(notInB, a[aIdx])
+			aIdx++
+		} else if a[aIdx] == b[bIdx] {
+			aIdx++
+			bIdx++
+		} else {
+			notInA = append(notInA, b[bIdx])
+			bIdx++
 		}
-		result = append(result, i)
 	}
 
-	return result
+	for aIdx < len(a) {
+		notInB = append(notInB, a[aIdx])
+		aIdx++
+	}
+
+	for bIdx < len(b) {
+		notInA = append(notInA, b[bIdx])
+		bIdx++
+	}
+
+	return notInA, notInB
 }

diff/diff_test.go

@@ -1,28 +1,45 @@
 package diff
 
-import "testing"
+import (
+	"reflect"
+	"testing"
+)
 
-func Test_Diff(t *testing.T) {
-	a := []int{1, 2, 3, 4, 5}
-	b := []int{2, 4, 5}
-
-	allOfA := Diff[int](a, []int{})
-	if len(allOfA) != len(a) {
-		t.Errorf("Expected empty b to return full list, got %v", allOfA)
-	}
-
-	empty := Diff([]int{}, []int{})
-	if len(empty) != 0 {
-		t.Errorf("Expected empty from empty / empty")
+func Test_DisjunctiveUnion(t *testing.T) {
+	tests := []struct {
+		a      []int
+		b      []int
+		notInA []int
+		notInB []int
+	}{
+		{
+			[]int{1, 2, 3, 8, 9, 10},
+			[]int{3, 4, 5},
+			[]int{4, 5},
+			[]int{1, 2, 8, 9, 10},
+		},
+		{
+			[]int{1, 2, 3},
+			[]int{1, 2, 3},
+			[]int{},
+			[]int{},
+		},
+		{
+			[]int{1, 2, 3},
+			[]int{1, 2, 3, 4},
+			[]int{4},
+			[]int{},
+		},
 	}
 
-	result := Diff(a, b)
-	if len(result) != len(a)-len(b) {
-		t.Errorf("Expected diff of %v and %v to be %d long, but got %v", a, b, len(a)-len(b), result)
-	}
+	for _, test := range tests {
+		notInA, notInB := DisjunctiveUnion(test.a, test.b)
 
-	same := Diff(a, a)
-	if len(same) != 0 {
-		t.Errorf("Expected diffing the same thing against itself to come up empty, got %v", same)
+		if !reflect.DeepEqual(notInA, test.notInA) {
+			t.Errorf("Expected notInA to be %v, got %v", test.notInA, notInA)
+		}
+		if !reflect.DeepEqual(notInB, test.notInB) {
+			t.Errorf("Expected notInB to be %v, got %v", test.notInB, notInB)
+		}
 	}
 }

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/gorilla/mux v1.8.0
 	go4.org v0.0.0-20230225012048-214862532bf5
 	golang.org/x/crypto v0.10.0
+	golang.org/x/exp v0.0.0-20230626212559-97b1e661b5df
 )
 
 require (

go.sum

@@ -99,6 +99,8 @@ golang.org/x/exp v0.0.0-20191030013958-a1ab85dbe136/go.mod h1:JXzH8nQsPlswgeRAPE
 golang.org/x/exp v0.0.0-20191129062945-2f5052295587/go.mod h1:2RIsYlXP63K8oxa1u096TMicItID8zy7Y6sNkU49FU4=
 golang.org/x/exp v0.0.0-20191227195350-da58074b4299/go.mod h1:2RIsYlXP63K8oxa1u096TMicItID8zy7Y6sNkU49FU4=
 golang.org/x/exp v0.0.0-20200207192155-f17229e696bd/go.mod h1:J/WKrq2StrnmMY6+EHIKF9dgMWnmCNThgcyBT1FY9mM=
+golang.org/x/exp v0.0.0-20230626212559-97b1e661b5df h1:UA2aFVmmsIlefxMk29Dp2juaUSth8Pyn3Tq5Y5mJGME=
+golang.org/x/exp v0.0.0-20230626212559-97b1e661b5df/go.mod h1:FXUEEKJgO7OQYeo8N01OfiKP8RXMtf6e8aTskBGqWdc=
 golang.org/x/image v0.0.0-20190227222117-0694c2d4d067/go.mod h1:kZ7UVZpmo3dzQBMxlp+ypCbDeSB+sBbTgSJuh5dn5js=
 golang.org/x/image v0.0.0-20190802002840-cff245a6509b/go.mod h1:FeLwcggjj3mMvU+oOTbSwawSJRM1uh48EjtB4UJZlP0=
 golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=