ext: Add list library with slice method

ext/README.md

@@ -149,6 +149,23 @@ Example:
 
     proto.hasExt(msg, google.expr.proto2.test.int32_ext) // returns true || false
 
+## Lists
+
+Extended functions for list manipulation. As a general note, all indices are
+zero-based.
+
+### Slice
+
+
+Returns a new sub-list using the indexes provided.
+
+    <list>.slice(<int>, <int>) -> <list>
+
+Examples:
+
+    [1,2,3,4].slice(1, 3) // return [2, 3]
+    [1,2,3,4].slice(2, 4) // return [3 ,4]
+
 ## Sets
 
 Sets provides set relationship tests.

ext/lists.go

@@ -0,0 +1,94 @@
+// Copyright 2023 Google LLC
+//
+// 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 ext
+
+import (
+	"fmt"
+
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
+)
+
+// Lists returns a cel.EnvOption to configure extended functions for list manipulation.
+// As a general note, all indices are zero-based.
+// # Slice
+//
+// Returns a new sub-list using the indexes provided.
+//
+//	<list>.slice(<int>, <int>) -> <list>
+//
+// Examples:
+//
+//	[1,2,3,4].slice(1, 3) // return [2, 3]
+//	[1,2,3,4].slice(2, 4) // return [3 ,4]
+func Lists() cel.EnvOption {
+	return cel.Lib(listsLib{})
+}
+
+type listsLib struct{}
+
+// LibraryName implements the SingletonLibrary interface method.
+func (listsLib) LibraryName() string {
+	return "cel.lib.ext.lists"
+}
+
+// CompileOptions implements the Library interface method.
+func (listsLib) CompileOptions() []cel.EnvOption {
+	listType := cel.ListType(cel.TypeParamType("T"))
+	return []cel.EnvOption{
+		cel.Function("slice",
+			cel.MemberOverload("list_slice",
+				[]*cel.Type{listType, cel.IntType, cel.IntType}, listType,
+				cel.FunctionBinding(func(args ...ref.Val) ref.Val {
+					list := args[0].(traits.Lister)
+					start := args[1].(types.Int)
+					end := args[2].(types.Int)
+					result, err := slice(list, start, end)
+					if err != nil {
+						return types.WrapErr(err)
+					}
+					return result
+				}),
+			),
+		),
+	}
+}
+
+// ProgramOptions implements the Library interface method.
+func (listsLib) ProgramOptions() []cel.ProgramOption {
+	return []cel.ProgramOption{}
+}
+
+func slice(list traits.Lister, start, end types.Int) (ref.Val, error) {
+	listLength := list.Size().(types.Int)
+	if start < 0 || end < 0 {
+		return nil, fmt.Errorf("cannot slice(%d, %d), negative indexes not supported", start, end)
+	}
+	if start > end {
+		return nil, fmt.Errorf("cannot slice(%d, %d), start index must be less than or equal to end index", start, end)
+	}
+	if listLength < end {
+		return nil, fmt.Errorf("cannot slice(%d, %d), list is length %d", start, end, listLength)
+	}
+
+	var newList []ref.Val
+	for i := types.Int(start); i < end; i++ {
+		val := list.Get(i)
+		newList = append(newList, val)
+	}
+	return types.DefaultTypeAdapter.NativeToValue(newList), nil
+}

ext/lists_test.go

@@ -0,0 +1,89 @@
+// Copyright 2023 Google LLC
+//
+// 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 ext
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/google/cel-go/cel"
+)
+
+func TestLists(t *testing.T) {
+	listsTests := []struct {
+		expr string
+		err  string
+	}{
+		{expr: `[1,2,3,4].slice(0, 4) == [1,2,3,4]`},
+		{expr: `[1,2,3,4].slice(0, 0) == []`},
+		{expr: `[1,2,3,4].slice(1, 1) == []`},
+		{expr: `[1,2,3,4].slice(4, 4) == []`},
+		{expr: `[1,2,3,4].slice(1, 3) == [2, 3]`},
+		{expr: `[1,2,3,4].slice(3, 0)`, err: "cannot slice(3, 0), start index must be less than or equal to end index"},
+		{expr: `[1,2,3,4].slice(0, 10)`, err: "cannot slice(0, 10), list is length 4"},
+		{expr: `[1,2,3,4].slice(-5, 10)`, err: "cannot slice(-5, 10), negative indexes not supported"},
+		{expr: `[1,2,3,4].slice(-5, -3)`, err: "cannot slice(-5, -3), negative indexes not supported"},
+	}
+
+	env := testListsEnv(t)
+	for i, tst := range listsTests {
+		tc := tst
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			var asts []*cel.Ast
+			pAst, iss := env.Parse(tc.expr)
+			if iss.Err() != nil {
+				t.Fatalf("env.Parse(%v) failed: %v", tc.expr, iss.Err())
+			}
+			asts = append(asts, pAst)
+			cAst, iss := env.Check(pAst)
+			if iss.Err() != nil {
+				t.Fatalf("env.Check(%v) failed: %v", tc.expr, iss.Err())
+			}
+			asts = append(asts, cAst)
+
+			for _, ast := range asts {
+				prg, err := env.Program(ast)
+				if err != nil {
+					t.Fatalf("env.Program() failed: %v", err)
+				}
+				out, _, err := prg.Eval(cel.NoVars())
+				if tc.err != "" {
+					if err == nil {
+						t.Fatalf("got value %v, wanted error %s for expr: %s",
+							out.Value(), tc.err, tc.expr)
+					}
+					if !strings.Contains(err.Error(), tc.err) {
+						t.Errorf("got error %v, wanted error %s for expr: %s", err, tc.err, tc.expr)
+					}
+				} else if err != nil {
+					t.Fatal(err)
+				} else if out.Value() != true {
+					t.Errorf("got %v, wanted true for expr: %s", out.Value(), tc.expr)
+				}
+			}
+		})
+	}
+}
+
+func testListsEnv(t *testing.T, opts ...cel.EnvOption) *cel.Env {
+	t.Helper()
+	baseOpts := []cel.EnvOption{Lists()}
+	env, err := cel.NewEnv(append(baseOpts, opts...)...)
+	if err != nil {
+		t.Fatalf("cel.NewEnv(Lists()) failed: %v", err)
+	}
+	return env
+}