Add `chunk` function to split array into smaller

docs/lists.md

@@ -168,6 +168,16 @@ equivalent of `list[n:m]`.
 `slice` panics if there is a problem while `mustSlice` returns an error to the
 template engine if there is a problem.
 
+## chunk
+
+To split a list into chunks of given size, use `chunk size list`. This is useful for pagination.
+
+```
+chunk 3 (list 1 2 3 4 5 6 7 8)
+```
+
+This produces list of lists `[ [ 1 2 3 ] [ 4 5 6 ] [ 7 8 ] ]`.
+
 ## A Note on List Internals
 
 A list is implemented in Go as a `[]interface{}`. For Go developers embedding

functions.go

@@ -313,6 +313,8 @@ var genericMap = map[string]interface{}{
 	"mustSlice":   mustSlice,
 	"concat":      concat,
 	"dig":         dig,
+	"chunk":       chunk,
+	"mustChunk":   mustChunk,
 
 	// Crypto:
 	"bcrypt":            bcrypt,

list.go

@@ -2,6 +2,7 @@ package sprig
 
 import (
 	"fmt"
+	"math"
 	"reflect"
 	"sort"
 )
@@ -72,6 +73,50 @@ func mustPrepend(list interface{}, v interface{}) ([]interface{}, error) {
 	}
 }
 
+func chunk(size int, list interface{}) [][]interface{} {
+	l, err := mustChunk(size, list)
+	if err != nil {
+		panic(err)
+	}
+
+	return l
+}
+
+func mustChunk(size int, list interface{}) ([][]interface{}, error) {
+	tp := reflect.TypeOf(list).Kind()
+	switch tp {
+	case reflect.Slice, reflect.Array:
+		l2 := reflect.ValueOf(list)
+
+		l := l2.Len()
+
+		cs := int(math.Floor(float64(l-1)/float64(size)) + 1)
+		nl := make([][]interface{}, cs)
+
+		for i := 0; i < cs; i++ {
+			clen := size
+			if i == cs-1 {
+				clen = int(math.Floor(math.Mod(float64(l), float64(size))))
+				if clen == 0 {
+					clen = size
+				}
+			}
+
+			nl[i] = make([]interface{}, clen)
+
+			for j := 0; j < clen; j++ {
+				ix := i*size + j
+				nl[i][j] = l2.Index(ix).Interface()
+			}
+		}
+
+		return nl, nil
+
+	default:
+		return nil, fmt.Errorf("Cannot chunk type %s", tp)
+	}
+}
+
 func last(list interface{}) interface{} {
 	l, err := mustLast(list)
 	if err != nil {

list_test.go

@@ -44,6 +44,32 @@ func TestMustPush(t *testing.T) {
 	}
 }
 
+func TestChunk(t *testing.T) {
+	tests := map[string]string{
+		`{{ tuple 1 2 3 4 5 6 7 | chunk 3 | len }}`:                                 "3",
+		`{{ tuple | chunk 3 | len }}`:                                               "0",
+		`{{ range ( tuple 1 2 3 4 5 6 7 8 9 | chunk 3 ) }}{{. | join "-"}}|{{end}}`: "1-2-3|4-5-6|7-8-9|",
+		`{{ range ( tuple 1 2 3 4 5 6 7 8 | chunk 3 ) }}{{. | join "-"}}|{{end}}`:   "1-2-3|4-5-6|7-8|",
+		`{{ range ( tuple 1 2 | chunk 3 ) }}{{. | join "-"}}|{{end}}`:               "1-2|",
+	}
+	for tpl, expect := range tests {
+		assert.NoError(t, runt(tpl, expect))
+	}
+}
+
+func TestMustChunk(t *testing.T) {
+	tests := map[string]string{
+		`{{ tuple 1 2 3 4 5 6 7 | mustChunk 3 | len }}`:                                 "3",
+		`{{ tuple | mustChunk 3 | len }}`:                                               "0",
+		`{{ range ( tuple 1 2 3 4 5 6 7 8 9 | mustChunk 3 ) }}{{. | join "-"}}|{{end}}`: "1-2-3|4-5-6|7-8-9|",
+		`{{ range ( tuple 1 2 3 4 5 6 7 8 | mustChunk 3 ) }}{{. | join "-"}}|{{end}}`:   "1-2-3|4-5-6|7-8|",
+		`{{ range ( tuple 1 2 | mustChunk 3 ) }}{{. | join "-"}}|{{end}}`:               "1-2|",
+	}
+	for tpl, expect := range tests {
+		assert.NoError(t, runt(tpl, expect))
+	}
+}
+
 func TestPrepend(t *testing.T) {
 	tests := map[string]string{
 		`{{ $t := tuple 1 2 3  }}{{ prepend $t 0 | len }}`:                             "4",