pkg/list: adding aggregation for decimal lists
this commit adds the aggregation functions avg, max, min, product, sum
on lists containing only numbers. it also adds a new type to builtin.go
and gen.go to have a direct transformation of cue.Value to
[]*internal.Decimal
Issue #78
Change-Id: I94640726b93f8bb23f43cf85330e2e9056cbcf70
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3103
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/pkg/list/math.go b/pkg/list/math.go
new file mode 100644
index 0000000..4ce4ba3
--- /dev/null
+++ b/pkg/list/math.go
@@ -0,0 +1,99 @@
+// Copyright 2018 The CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package list
+
+import (
+ "fmt"
+
+ "cuelang.org/go/internal"
+ "github.com/cockroachdb/apd/v2"
+)
+
+// Avg returns the average value of a non empty list xs.
+func Avg(xs []*internal.Decimal) (*internal.Decimal, error) {
+ if 0 == len(xs) {
+ return nil, fmt.Errorf("empty list")
+ }
+
+ s := apd.New(0, 0)
+ for _, x := range xs {
+ _, err := internal.BaseContext.Add(s, x, s)
+ if err != nil {
+ return nil, err
+ }
+ }
+
+ var d apd.Decimal
+ l := apd.New(int64(len(xs)), 0)
+ _, err := internal.BaseContext.Quo(&d, s, l)
+ if err != nil {
+ return nil, err
+ }
+ return &d, nil
+}
+
+// Max returns the maximum value of a non empty list xs.
+func Max(xs []*internal.Decimal) (*internal.Decimal, error) {
+ if 0 == len(xs) {
+ return nil, fmt.Errorf("empty list")
+ }
+
+ max := xs[0]
+ for _, x := range xs[1:] {
+ if -1 == max.Cmp(x) {
+ max = x
+ }
+ }
+ return max, nil
+}
+
+// Min returns the minimum value of a non empty list xs.
+func Min(xs []*internal.Decimal) (*internal.Decimal, error) {
+ if 0 == len(xs) {
+ return nil, fmt.Errorf("empty list")
+ }
+
+ min := xs[0]
+ for _, x := range xs[1:] {
+ if +1 == min.Cmp(x) {
+ min = x
+ }
+ }
+ return min, nil
+}
+
+// Product returns the product of a non empty list xs.
+func Product(xs []*internal.Decimal) (*internal.Decimal, error) {
+ d := apd.New(1, 0)
+ for _, x := range xs {
+ _, err := internal.BaseContext.Mul(d, x, d)
+ if err != nil {
+ return nil, err
+ }
+ }
+ return d, nil
+}
+
+// Sum returns the sum of a list non empty xs.
+func Sum(xs []*internal.Decimal) (*internal.Decimal, error) {
+ d := apd.New(0, 0)
+ for _, x := range xs {
+ _, err := internal.BaseContext.Add(d, x, d)
+ if err != nil {
+ return nil, err
+ }
+ }
+ return d, nil
+}
