feat: add Product and ProductBy functions (#566)

Co-authored-by: John.Devitt@cdp.net <JohnDevitt@Mac.cust.communityfibre.co.uk>

Author: JohnDevitt

README.md

@@ -155,6 +155,8 @@ Supported math helpers:
 - [Clamp](#clamp)
 - [Sum](#sum)
 - [SumBy](#sumby)
+- [Product](#product)
+- [ProductBy](#productby)
 - [Mean](#mean)
 - [MeanBy](#meanby)
 
@@ -866,7 +868,6 @@ l := lo.DropByIndex([]int{0, 1, 2, 3, 4, 5}, 2, 4, -1)
 
 [[play](https://go.dev/play/p/JswS7vXRJP2)]
 
-
 ### Reject
 
 The opposite of Filter, this method returns the elements of collection that predicate does not return truthy for.
@@ -885,6 +886,7 @@ odd := lo.Reject([]int{1, 2, 3, 4}, func(x int, _ int) bool {
 The opposite of FilterMap, this method returns a slice which obtained after both filtering and mapping using the given callback function.
 
 The callback function should return two values:
+
 - the result of the mapping operation and
 - whether the result element should be included or not.
 
@@ -1511,7 +1513,35 @@ sum := lo.SumBy(strings, func(item string) int {
 // 6
 ```
 
-[[play](https://go.dev/play/p/Dz_a_7jN_ca)]
+### Product
+
+Calculates the product of the values in a collection.
+
+If collection is empty 0 is returned.
+
+```go
+list := []int{1, 2, 3, 4, 5}
+product := lo.Product(list)
+// 120
+```
+
+[[play](https://go.dev/play/p/2_kjM_smtAH)]
+
+### ProductBy
+
+Calculates the product of the values in a collection using the given return value from the iteration function.
+
+If collection is empty 0 is returned.
+
+```go
+strings := []string{"foo", "bar"}
+product := lo.ProductBy(strings, func(item string) int {
+    return len(item)
+})
+// 9
+```
+
+[[play](https://go.dev/play/p/wadzrWr9Aer)]
 
 ### Mean
 
@@ -2693,6 +2723,7 @@ first := lo.FirstOrEmpty([]int{1, 2, 3})
 first := lo.FirstOrEmpty([]int{})
 // 0
 ```
+
 ### FirstOr
 
 Returns the first element of a collection or the fallback value if empty.
@@ -2730,6 +2761,7 @@ last := lo.LastOrEmpty([]int{1, 2, 3})
 last := lo.LastOrEmpty([]int{})
 // 0
 ```
+
 ### LastOr
 
 Returns the first element of a collection or the fallback value if empty.
@@ -3644,7 +3676,6 @@ iterations, duration, ok := lo.WaitFor(laterTrue, 10*time.Millisecond, 5*time.Mi
 // false
 ```
 
-
 ### WaitForWithContext
 
 Runs periodically until a condition is validated or context is invalid.

math.go

@@ -85,6 +85,44 @@ func SumBy[T any, R constraints.Float | constraints.Integer | constraints.Comple
 	return sum
 }
 
+// Product gets the product of the values in a collection. If collection is empty 0 is returned.
+// Play: https://go.dev/play/p/2_kjM_smtAH
+func Product[T constraints.Float | constraints.Integer | constraints.Complex](collection []T) T {
+
+	if collection == nil {
+		return 0
+	}
+
+	if len(collection) == 0 {
+		return 0
+	}
+
+	var product T = 1
+	for i := range collection {
+		product *= collection[i]
+	}
+	return product
+}
+
+// ProductBy summarizes the values in a collection using the given return value from the iteration function. If collection is empty 0 is returned.
+// Play: https://go.dev/play/p/wadzrWr9Aer
+func ProductBy[T any, R constraints.Float | constraints.Integer | constraints.Complex](collection []T, iteratee func(item T) R) R {
+
+	if collection == nil {
+		return 0
+	}
+
+	if len(collection) == 0 {
+		return 0
+	}
+
+	var product R = 1
+	for i := range collection {
+		product = product * iteratee(collection[i])
+	}
+	return product
+}
+
 // Mean calculates the mean of a collection of numbers.
 func Mean[T constraints.Float | constraints.Integer](collection []T) T {
 	var length = T(len(collection))

math_test.go

@@ -98,6 +98,46 @@ func TestSumBy(t *testing.T) {
 	is.Equal(result5, complex128(6_6))
 }
 
+func TestProduct(t *testing.T) {
+	is := assert.New(t)
+
+	result1 := Product([]float32{2.3, 3.3, 4, 5.3})
+	result2 := Product([]int32{2, 3, 4, 5})
+	result3 := Product([]int32{7, 8, 9, 0})
+	result4 := Product([]int32{7, -1, 9, 2})
+	result5 := Product([]uint32{2, 3, 4, 5})
+	result6 := Product([]uint32{})
+	result7 := Product([]complex128{4_4, 2_2})
+
+	is.Equal(result1, float32(160.908))
+	is.Equal(result2, int32(120))
+	is.Equal(result3, int32(0))
+	is.Equal(result4, int32(-126))
+	is.Equal(result5, uint32(120))
+	is.Equal(result6, uint32(0))
+	is.Equal(result7, complex128(96_8))
+}
+
+func TestProductBy(t *testing.T) {
+	is := assert.New(t)
+
+	result1 := ProductBy([]float32{2.3, 3.3, 4, 5.3}, func(n float32) float32 { return n })
+	result2 := ProductBy([]int32{2, 3, 4, 5}, func(n int32) int32 { return n })
+	result3 := ProductBy([]int32{7, 8, 9, 0}, func(n int32) int32 { return n })
+	result4 := ProductBy([]int32{7, -1, 9, 2}, func(n int32) int32 { return n })
+	result5 := ProductBy([]uint32{2, 3, 4, 5}, func(n uint32) uint32 { return n })
+	result6 := ProductBy([]uint32{}, func(n uint32) uint32 { return n })
+	result7 := ProductBy([]complex128{4_4, 2_2}, func(n complex128) complex128 { return n })
+
+	is.Equal(result1, float32(160.908))
+	is.Equal(result2, int32(120))
+	is.Equal(result3, int32(0))
+	is.Equal(result4, int32(-126))
+	is.Equal(result5, uint32(120))
+	is.Equal(result6, uint32(0))
+	is.Equal(result7, complex128(96_8))
+}
+
 func TestMean(t *testing.T) {
 	t.Parallel()
 	is := assert.New(t)