package funcs

import (
	"encoding/base64"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"unicode/utf8"

	homedir "github.com/mitchellh/go-homedir"
	"github.com/zclconf/go-cty/cty"
	"github.com/zclconf/go-cty/cty/function"
)

// MakeFileFunc constructs a function that takes a file path and returns the
// contents of that file, either directly as a string (where valid UTF-8 is
// required) or as a string containing base64 bytes.
func MakeFileFunc(baseDir string, encBase64 bool) function.Function {
	return function.New(&function.Spec{
		Params: []function.Parameter{
			{
				Name: "path",
				Type: cty.String,
			},
		},
		Type: function.StaticReturnType(cty.String),
		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
			path := args[0].AsString()
			path, err := homedir.Expand(path)
			if err != nil {
				return cty.UnknownVal(cty.String), fmt.Errorf("failed to expand ~: %s", err)
			}

			if !filepath.IsAbs(path) {
				path = filepath.Join(baseDir, path)
			}

			// Ensure that the path is canonical for the host OS
			path = filepath.Clean(path)

			src, err := ioutil.ReadFile(path)
			if err != nil {
				// ReadFile does not return Terraform-user-friendly error
				// messages, so we'll provide our own.
				if os.IsNotExist(err) {
					return cty.UnknownVal(cty.String), fmt.Errorf("no file exists at %s", path)
				}
				return cty.UnknownVal(cty.String), fmt.Errorf("failed to read %s", path)
			}

			switch {
			case encBase64:
				enc := base64.StdEncoding.EncodeToString(src)
				return cty.StringVal(enc), nil
			default:
				if !utf8.Valid(src) {
					return cty.UnknownVal(cty.String), fmt.Errorf("contents of %s are not valid UTF-8; to read arbitrary bytes, use the filebase64 function instead", path)
				}
				return cty.StringVal(string(src)), nil
			}
		},
	})
}

// File reads the contents of the file at the given path.
//
// The file must contain valid UTF-8 bytes, or this function will return an error.
//
// The underlying function implementation works relative to a particular base
// directory, so this wrapper takes a base directory string and uses it to
// construct the underlying function before calling it.
func File(baseDir string, path cty.Value) (cty.Value, error) {
	fn := MakeFileFunc(baseDir, false)
	return fn.Call([]cty.Value{path})
}

// FileBase64 reads the contents of the file at the given path.
//
// The bytes from the file are encoded as base64 before returning.
//
// The underlying function implementation works relative to a particular base
// directory, so this wrapper takes a base directory string and uses it to
// construct the underlying function before calling it.
func FileBase64(baseDir string, path cty.Value) (cty.Value, error) {
	fn := MakeFileFunc(baseDir, true)
	return fn.Call([]cty.Value{path})
}