feat(CollectMap): allow flexible stream collection into builtin Go map

hmap.go

@@ -0,0 +1,26 @@
+package fungi
+
+import "io"
+
+// CollectMap exhaustively collects items from a given stream into a builtin map
+// type. As its first argument, it requires a function that knows how to take an
+// item and split it into map Key and Value of appropriate type.
+func CollectMap[
+	T, V any,
+	K comparable,
+](keyval func(T) (K, V)) StreamReducer[T, map[K]V] {
+	return func(items Stream[T]) (hmap map[K]V, err error) {
+		hmap = make(map[K]V)
+		var item T
+		for err == nil {
+			if item, err = items.Next(); err == nil {
+				key, value := keyval(item)
+				hmap[key] = value
+			}
+		}
+		if err == io.EOF {
+			err = nil
+		}
+		return
+	}
+}

hmap_test.go

@@ -0,0 +1,22 @@
+package fungi
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCollectMap(t *testing.T) {
+	source := SliceStream([]int{1, 2, 3})
+	result, err := CollectMap(duplicate)(source)
+	assert.NoError(t, err)
+	assert.Equal(t, map[int]int{
+		1: 1,
+		2: 2,
+		3: 3,
+	}, result)
+}
+
+func duplicate(i int) (int, int) {
+	return i, i
+}