adding the maps.Diff function

Changes.md

@@ -2,6 +2,7 @@ WIP  TBD
 
  * Added slices.FirstIndex and slices.GrepIndex.
  * Added set.NewSized and set.Diff.
+ * Added maps.Diff.
  * Fixed missing test coverage on generic.FirstNonNil and generic.FirstNonZero.
 
 v0.2.0  2023-07-08

README.md

@@ -87,6 +87,7 @@ There are a lot of common map operations that are missing from the built-in
 standard library:
 
  * `Merge(...maps)`
+ * `Diff(a, b)`
 
 ## Generic Sets
 

maps/merge.go

@@ -1,5 +1,7 @@
 package maps
 
+import "github.com/zostay/go-std/generic"
+
 // Merge maps into a single map. Keys in maps later in the list will overwrite
 // keys found in earlier maps.
 func Merge[K comparable, V any](maps ...map[K]V) map[K]V {
@@ -11,3 +13,33 @@ func Merge[K comparable, V any](maps ...map[K]V) map[K]V {
 	}
 	return out
 }
+
+// Diff returns three maps from two. The first map returned is a map of
+// key/value pairs where the keys are found in both the first and second input
+// maps. The second map returned contains a map of key/value pairs for keys only
+// found in the first map. The third map returned contains a map of key/value
+// pairs for keys only found in the third map.
+//
+// If values differ for keys held in common, the value in the first input map
+// are kept.
+func Diff[K comparable, V any](a, b map[K]V) (common, inFirst, inSecond map[K]V) {
+	common = make(map[K]V, generic.Max(len(a), len(b)))
+	inFirst = make(map[K]V, len(a))
+	inSecond = make(map[K]V, len(b))
+
+	for akey, avalue := range a {
+		if _, found := b[akey]; found {
+			common[akey] = avalue
+		} else {
+			inFirst[akey] = avalue
+		}
+	}
+
+	for bkey, bvalue := range b {
+		if _, found := a[bkey]; !found {
+			inSecond[bkey] = bvalue
+		}
+	}
+
+	return
+}

maps/merge_test.go

@@ -27,3 +27,13 @@ func TestMerge(t *testing.T) {
 		"a": 5, "b": 6, "c": 7, "d": 4,
 	}, maps.Merge(a, b, c))
 }
+
+func TestDiff(t *testing.T) {
+	a := map[string]int{"one": 1, "two": 2, "three": 3, "four": 4}
+	b := map[string]int{"one": 5, "three": 6, "five": 7, "seven": 8}
+
+	c, one, two := maps.Diff(a, b)
+	assert.Equal(t, map[string]int{"one": 1, "three": 3}, c)
+	assert.Equal(t, map[string]int{"two": 2, "four": 4}, one)
+	assert.Equal(t, map[string]int{"five": 7, "seven": 8}, two)
+}