From a7f49c07192375823c129591b3a9a3349682a599 Mon Sep 17 00:00:00 2001
From: Carlos Eduardo Santin <carlos_santin@sicredi.com.br>
Date: Wed, 10 May 2023 18:51:06 -0300
Subject: [PATCH] Add conversion of slices to map grouping by field

Add the method ToMapGroupedBy that receives a slice and a function to extract a key from each slice item. As result it returns a map grouping all items having the same key.
---
 slices/slice.go      | 11 +++++++++++
 slices/slice_test.go | 29 +++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/slices/slice.go b/slices/slice.go
index 9139241..7a58391 100644
--- a/slices/slice.go
+++ b/slices/slice.go
@@ -577,6 +577,17 @@ func ToMap[S ~[]V, V any](items S) map[int]V {
 	return result
 }
 
+// ToMapGroupedBy converts the given slice into a map where keys are values returned
+// from keyExtractor function and values are items from the given slice
+func ToMapGroupedBy[V any, T comparable](items []V, keyExtractor func(V) T) map[T][]V {
+	result := make(map[T][]V)
+	for _, item := range items {
+		key := keyExtractor(item)
+		result[key] = append(result[key], item)
+	}
+	return result
+}
+
 // ToKeys converts the given slice into a map where items from the slice are the keys
 // of the resulting map and all values are equal to the given `val` value.
 func ToKeys[S ~[]K, K comparable, V any](items S, val V) map[K]V {
diff --git a/slices/slice_test.go b/slices/slice_test.go
index 7ac1f95..3328aab 100644
--- a/slices/slice_test.go
+++ b/slices/slice_test.go
@@ -532,3 +532,32 @@ func TestWithout(t *testing.T) {
 
 	f([]int{1, 1, 2, 3, 1, 4, 1}, []int{1}, []int{2, 3, 4})
 }
+
+func TestToMapGroupedBy(t *testing.T) {
+	is := is.New(t)
+
+	type employee struct {
+		Name       string
+		Department string
+	}
+
+	f := func(given []employee, expected map[string][]employee) {
+		actual := slices.ToMapGroupedBy(given, func(emp employee) string {
+			return emp.Department
+		})
+		is.Equal(expected, actual)
+	}
+
+	employeeJohn := employee{Name: "John", Department: "Engineering"}
+	employeeEva := employee{Name: "Eva", Department: "HR"}
+	employeeCarlos := employee{Name: "Carlos", Department: "Engineering"}
+
+	f([]employee{employeeJohn, employeeEva, employeeCarlos}, map[string][]employee{
+		"Engineering": {employeeJohn, employeeCarlos},
+		"HR":          {employeeEva},
+	})
+
+	f([]employee{}, map[string][]employee{})
+
+	f(nil, map[string][]employee{})
+}