encoders.go

// Copyright 2020 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 (
	"encoding/base64"
	"math"

	"github.com/google/cel-go/cel"
	"github.com/google/cel-go/checker"
	"github.com/google/cel-go/common/types"
	"github.com/google/cel-go/common/types/ref"
	"github.com/google/cel-go/interpreter"
)

// Encoders returns a cel.EnvOption to configure extended functions for string, byte, and object
// encodings.
//
// # Base64.Decode
//
// Decodes base64-encoded string to bytes.
//
// This function will return an error if the string input is not base64-encoded.
//
//	base64.decode(<string>) -> <bytes>
//
// Examples:
//
//	base64.decode('aGVsbG8=')  // return b'hello'
//	base64.decode('aGVsbG8')   // return b'hello'
//
// # Base64.Encode
//
// Encodes bytes to a base64-encoded string.
//
//	base64.encode(<bytes>)  -> <string>
//
// Examples:
//
//	base64.encode(b'hello') // return b'aGVsbG8='
func Encoders(options ...EncodersOption) cel.EnvOption {
	l := &encoderLib{version: math.MaxUint32}
	for _, o := range options {
		l = o(l)
	}
	return cel.Lib(l)
}

// EncodersOption declares a functional operator for configuring encoder extensions.
type EncodersOption func(*encoderLib) *encoderLib

// EncodersVersion sets the library version for encoder extensions.
func EncodersVersion(version uint32) EncodersOption {
	return func(lib *encoderLib) *encoderLib {
		lib.version = version
		return lib
	}
}

type encoderLib struct {
	version uint32
}

func (*encoderLib) LibraryName() string {
	return "cel.lib.ext.encoders"
}

func (lib *encoderLib) CompileOptions() []cel.EnvOption {
	opts := []cel.EnvOption{
		cel.Function("base64.decode",
			cel.Overload("base64_decode_string", []*cel.Type{cel.StringType}, cel.BytesType,
				cel.UnaryBinding(func(str ref.Val) ref.Val {
					s := str.(types.String)
					return bytesOrError(base64DecodeString(string(s)))
				}))),
		cel.Function("base64.encode",
			cel.Overload("base64_encode_bytes", []*cel.Type{cel.BytesType}, cel.StringType,
				cel.UnaryBinding(func(bytes ref.Val) ref.Val {
					b := bytes.(types.Bytes)
					return stringOrError(base64EncodeBytes([]byte(b)))
				}))),
	}
	if lib.version >= 1 {
		estimators := []checker.CostOption{
			checker.OverloadCostEstimate("base64_decode_string", estimateDecode),
			checker.OverloadCostEstimate("base64_encode_bytes", estimateEncode),
		}
		opts = append(opts, cel.CostEstimatorOptions(estimators...))
	}
	return opts
}

func (lib *encoderLib) ProgramOptions() []cel.ProgramOption {
	var opts []cel.ProgramOption
	if lib.version >= 1 {
		trackers := []interpreter.CostTrackerOption{
			interpreter.OverloadCostTracker("base64_decode_string", trackDecode),
			interpreter.OverloadCostTracker("base64_encode_bytes", trackEncode),
		}
		opts = append(opts, cel.CostTrackerOptions(trackers...))
	}
	return opts
}

func base64DecodeString(str string) ([]byte, error) {
	b, err := base64.StdEncoding.DecodeString(str)
	if err == nil {
		return b, nil
	}
	if _, tryAltEncoding := err.(base64.CorruptInputError); tryAltEncoding {
		return base64.RawStdEncoding.DecodeString(str)
	}
	return nil, err
}

func base64EncodeBytes(bytes []byte) (string, error) {
	return base64.StdEncoding.EncodeToString(bytes), nil
}

func estimateEncode(estimator checker.CostEstimator, target *checker.AstNode, args []checker.AstNode) *checker.CallEstimate {
	if len(args) != 1 {
		return nil
	}
	sz := estimateSize(estimator, args[0])
	cost := sz.MultiplyByCostFactor(stringCostFactor).Add(callCostEstimate)
	resSize := estimateEncodeSize(sz)
	return &checker.CallEstimate{CostEstimate: cost, ResultSize: &resSize}
}

func estimateDecode(estimator checker.CostEstimator, target *checker.AstNode, args []checker.AstNode) *checker.CallEstimate {
	if len(args) != 1 {
		return nil
	}
	sz := estimateSize(estimator, args[0])
	cost := sz.MultiplyByCostFactor(stringCostFactor).Add(callCostEstimate)
	resSize := estimateDecodeSize(sz)
	return &checker.CallEstimate{CostEstimate: cost, ResultSize: &resSize}
}

func trackEncode(args []ref.Val, _ ref.Val) *uint64 {
	sz := actualSize(args[0])
	cost := uint64(math.Ceil(float64(sz)*stringCostFactor)) + callCost
	return &cost
}

func trackDecode(args []ref.Val, _ ref.Val) *uint64 {
	sz := actualSize(args[0])
	cost := uint64(math.Ceil(float64(sz)*stringCostFactor)) + callCost
	return &cost
}

func estimateEncodeSize(sz checker.SizeEstimate) checker.SizeEstimate {
	minVal := (sz.Min*4 + 2) / 3
	maxVal := (sz.Max*4 + 2) / 3
	if sz.Max > math.MaxUint64/4 {
		maxVal = math.MaxUint64
	}
	return checker.SizeEstimate{Min: minVal, Max: maxVal}
}

func estimateDecodeSize(sz checker.SizeEstimate) checker.SizeEstimate {
	minVal := sz.Min * 3 / 4
	maxVal := sz.Max * 3 / 4
	return checker.SizeEstimate{Min: minVal, Max: maxVal}
}