package main

import (
	"bytes"
	"flag"
	"fmt"
	"go/ast"
	"go/token"
	"go/types"
	"io"
	"os"
	"reflect"
	"strings"

	"golang.org/x/tools/go/packages"
	"golang.org/x/tools/imports"
)

const (
	defaultMerge = `
		if s.%[1]s != %[2]s {
			s.%[1]s = other.%[1]s
		}
	`
	defaultValidator = `
		if s.%[1]s == %[2]s {
			return errors.New("missing %[3]s")
		}
	`
	ipValidator = `
		if net.ParseIP(s.%[1]s) == nil {
			return fmt.Errorf("invalid %[3]s: %%q", s.%[1]s)
		}
	`
	positiveValidator = `
		if s.%[1]s < 1 {
			return fmt.Errorf("invalid %[3]s: %%q", s.%[1]s)
		}
	`
)

var (
	structName  string
	packageName string
	fileName    string
)

type Validator struct {
	code string
}

type Field struct {
	name         string
	prettyName   string
	defaultValue string
	validators   []string
}

func NewField(name, prettyName, defaultValue, tag string) *Field {
	validators := make([]string, 0, 2)

	switch tag {
	case "":
		validators = append(validators, defaultValidator)
	case "ip":
		validators = append(validators, defaultValidator, ipValidator)
	case "positive":
		validators = append(validators, positiveValidator)
	case "no":
		break
	default:
		panic(fmt.Sprintf("invalid tag %v", tag))
	}

	return &Field{
		name:         name,
		prettyName:   prettyName,
		defaultValue: defaultValue,
		validators:   validators,
	}
}

func init() {
	namePtr := flag.String("name", "", "name of the struct")
	flag.Parse()
	if namePtr == nil || *namePtr == "" {
		fmt.Fprintln(os.Stderr, "Invalid name")
		os.Exit(1)
	}
	structName = *namePtr

	packageName = os.Getenv("GOPACKAGE")

	fileName = fmt.Sprintf("%s_gen.go", strings.TrimSuffix(os.Getenv("GOFILE"), ".go"))
}

func defaultValue(t types.Type) string {
	b, ok := t.Underlying().(*types.Basic)
	if !ok {
		return "nil"
	}

	switch b.Kind() {
	case types.Int, types.Int8, types.Int16, types.Int32, types.Int64,
		types.Uint, types.Uint8, types.Uint16, types.Uint32, types.Uint64:
		return "0"
	case types.String:
		return `""`
	case types.Bool:
		return "false"
	default:
		return "nil"
	}
}

type Package struct {
	fields  []*Field
	imports map[string]struct{}
}

func enumerateFields(root *types.Struct) []*Field {
	fields := make([]*Field, 0, 16)

	for i := range root.NumFields() {
		field := root.Field(i)
		tag := reflect.StructTag(root.Tag(i))

		extraValidators := tag.Get("gen")
		prettyName := tag.Get("toml")
		if embed, ok := field.Type().Underlying().(*types.Struct); ok {
			fields = append(fields, enumerateFields(embed)...)
		} else {
			fields = append(fields, NewField(field.Name(), prettyName, defaultValue(field.Type()), extraValidators))
		}
	}

	return fields
}

func writeMerge(w io.Writer, name string, fields []*Field) {
	fmt.Fprintf(w, "func (s *%[1]s) Merge(other %[1]s) {", name)
	for _, f := range fields {
		fmt.Fprintf(w, defaultMerge, f.name, f.defaultValue)
	}
	fmt.Fprintf(w, "}\n\n")
}

func writeValidators(w io.Writer, name string, fields []*Field) {
	fmt.Fprintf(w, "func (s %s) Validate() error {", name)
	for _, f := range fields {
		for _, v := range f.validators {
			fmt.Fprintf(w, v, f.name, f.defaultValue, f.prettyName)
		}
	}
	fmt.Fprintf(w, "\nreturn nil\n}\n\n")
}

func main() {
	cfg := &packages.Config{
		Mode: packages.NeedName |
			packages.NeedTypes |
			packages.NeedTypesInfo |
			packages.NeedSyntax |
			packages.NeedFiles,
	}
	pkgs, _ := packages.Load(cfg, ".")
	config := *pkgs[0]
	for _, v := range config.Syntax {
		ast.Inspect(v, func(node ast.Node) bool {
			decl, ok := node.(*ast.GenDecl)
			if !ok || decl.Tok != token.TYPE {
				return true
			}

			for _, spec := range decl.Specs {
				ts := spec.(*ast.TypeSpec)
				if ts.Name.String() != structName {
					continue
				}

				t := config.TypesInfo.TypeOf(ts.Type.(*ast.StructType)).(*types.Struct)
				fields := enumerateFields(t)

				buf := bytes.NewBuffer(make([]byte, 0))
				fmt.Fprintf(buf, "// Code generated by roleconfig; DO NOT EDIT.\n\npackage %s\n", packageName)
				writeValidators(buf, structName, fields)
				writeMerge(buf, structName, fields)
				x, _ := imports.Process(fileName, buf.Bytes(), nil)
				os.WriteFile(fileName, x, 0644)

				break
			}
			return false
		})
	}
}