/*
   Copyright 2020 The Compose Specification 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 interpolation

import (
	"os"
	"strings"

	"github.com/compose-spec/compose-go/template"
	"github.com/pkg/errors"
)

// Options supported by Interpolate
type Options struct {
	// LookupValue from a key
	LookupValue LookupValue
	// TypeCastMapping maps key paths to functions to cast to a type
	TypeCastMapping map[Path]Cast
	// Substitution function to use
	Substitute func(string, template.Mapping) (string, error)
}

// LookupValue is a function which maps from variable names to values.
// Returns the value as a string and a bool indicating whether
// the value is present, to distinguish between an empty string
// and the absence of a value.
type LookupValue func(key string) (string, bool)

// Cast a value to a new type, or return an error if the value can't be cast
type Cast func(value string) (interface{}, error)

// Interpolate replaces variables in a string with the values from a mapping
func Interpolate(config map[string]interface{}, opts Options) (map[string]interface{}, error) {
	if opts.LookupValue == nil {
		opts.LookupValue = os.LookupEnv
	}
	if opts.TypeCastMapping == nil {
		opts.TypeCastMapping = make(map[Path]Cast)
	}
	if opts.Substitute == nil {
		opts.Substitute = template.Substitute
	}

	out := map[string]interface{}{}

	for key, value := range config {
		interpolatedValue, err := recursiveInterpolate(value, NewPath(key), opts)
		if err != nil {
			return out, err
		}
		out[key] = interpolatedValue
	}

	return out, nil
}

func recursiveInterpolate(value interface{}, path Path, opts Options) (interface{}, error) {
	switch value := value.(type) {
	case string:
		newValue, err := opts.Substitute(value, template.Mapping(opts.LookupValue))
		if err != nil {
			return value, newPathError(path, err)
		}
		caster, ok := opts.getCasterForPath(path)
		if !ok {
			return newValue, nil
		}
		casted, err := caster(newValue)
		return casted, newPathError(path, errors.Wrap(err, "failed to cast to expected type"))

	case map[string]interface{}:
		out := map[string]interface{}{}
		for key, elem := range value {
			interpolatedElem, err := recursiveInterpolate(elem, path.Next(key), opts)
			if err != nil {
				return nil, err
			}
			out[key] = interpolatedElem
		}
		return out, nil

	case []interface{}:
		out := make([]interface{}, len(value))
		for i, elem := range value {
			interpolatedElem, err := recursiveInterpolate(elem, path.Next(PathMatchList), opts)
			if err != nil {
				return nil, err
			}
			out[i] = interpolatedElem
		}
		return out, nil

	default:
		return value, nil
	}
}

func newPathError(path Path, err error) error {
	switch err := err.(type) {
	case nil:
		return nil
	case *template.InvalidTemplateError:
		return errors.Errorf(
			"invalid interpolation format for %s.\nYou may need to escape any $ with another $.\n%s",
			path, err.Template)
	default:
		return errors.Wrapf(err, "error while interpolating %s", path)
	}
}

const pathSeparator = "."

// PathMatchAll is a token used as part of a Path to match any key at that level
// in the nested structure
const PathMatchAll = "*"

// PathMatchList is a token used as part of a Path to match items in a list
const PathMatchList = "[]"

// Path is a dotted path of keys to a value in a nested mapping structure. A *
// section in a path will match any key in the mapping structure.
type Path string

// NewPath returns a new Path
func NewPath(items ...string) Path {
	return Path(strings.Join(items, pathSeparator))
}

// Next returns a new path by append part to the current path
func (p Path) Next(part string) Path {
	return Path(string(p) + pathSeparator + part)
}

func (p Path) parts() []string {
	return strings.Split(string(p), pathSeparator)
}

func (p Path) matches(pattern Path) bool {
	patternParts := pattern.parts()
	parts := p.parts()

	if len(patternParts) != len(parts) {
		return false
	}
	for index, part := range parts {
		switch patternParts[index] {
		case PathMatchAll, part:
			continue
		default:
			return false
		}
	}
	return true
}

func (o Options) getCasterForPath(path Path) (Cast, bool) {
	for pattern, caster := range o.TypeCastMapping {
		if path.matches(pattern) {
			return caster, true
		}
	}
	return nil, false
}