// Copyright 2019 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 yaml converts YAML encodings to and from CUE. When converting to CUE,
// comments and position information are retained.
package yaml

import (
	"bytes"
	"io"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/ast"
	"cuelang.org/go/internal/third_party/yaml"
	goyaml "github.com/ghodss/yaml"
)

// TODO: replace the ghodss YAML encoder. It has a few major issues:
//   - it does not expose the underlying error, which means we lose valuable
//     information.
//   - comments and other meta data are lost.

// Extract parses the YAML to a CUE expression. Streams are returned as a list
// of the streamed values.
func Extract(filename string, src interface{}) (*ast.File, error) {
	a := []ast.Expr{}
	d, err := yaml.NewDecoder(filename, src)
	if err != nil {
		return nil, err
	}
	for {
		expr, err := d.Decode()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, err
		}
		a = append(a, expr)
	}
	f := &ast.File{Filename: filename}
	switch len(a) {
	case 0:
	case 1:
		switch x := a[0].(type) {
		case *ast.StructLit:
			f.Decls = x.Elts
		default:
			f.Decls = []ast.Decl{&ast.EmitDecl{Expr: x}}
		}
	default:
		f.Decls = []ast.Decl{&ast.EmitDecl{Expr: &ast.ListLit{Elts: a}}}
	}
	return f, nil
}

// Decode converts a YAML file to a CUE value. Streams are returned as a list
// of the streamed values.
func Decode(r *cue.Runtime, filename string, src interface{}) (*cue.Instance, error) {
	file, err := Extract(filename, src)
	if err != nil {
		return nil, err
	}
	return r.CompileFile(file)
}

// Encode returns the YAML encoding of v.
func Encode(v cue.Value) ([]byte, error) {
	b, err := goyaml.Marshal(v)
	return b, err
}

// EncodeStream returns the YAML encoding of iter, where consecutive values
// of iter are separated with a `---`.
func EncodeStream(iter cue.Iterator) ([]byte, error) {
	// TODO: return an io.Reader and allow asynchronous processing.
	buf := &bytes.Buffer{}
	for i := 0; iter.Next(); i++ {
		if i > 0 {
			buf.WriteString("---\n")
		}
		b, err := goyaml.Marshal(iter.Value())
		if err != nil {
			return nil, err
		}
		buf.Write(b)
	}
	return buf.Bytes(), nil
}