package main

import (
    "fmt"
    "reflect"
    "regexp"
    "strings"
)

const tagName = "validate"

//邮箱验证
var mailRe = regexp.MustCompile(`\A[\w+\-.]+@[a-z\d\-]+(\.[a-z]+)*\.[a-z]+\z`)

//验证接口
type Validator interface {
    Validate(interface{}) (bool, error)
}

type DefaultValidator struct{}

func (v DefaultValidator) Validate(val interface{}) (bool, error) {
    return true, nil
}

type StringValidator struct {
    Min int
    Max int
}

func (v StringValidator) Validate(val interface{}) (bool, error) {
    l := len(val.(string))
    if l == 0 {
        return false, fmt.Errorf("cannot be blank")
    }
    if l < v.Min {
        return false, fmt.Errorf("should be at least %v chars long", v.Min)
    }
    if v.Max >= v.Min && l > v.Max {
        return false, fmt.Errorf("should be less than %v chars long", v.Max)
    }
    return true, nil
}

type NumberValidator struct {
    Min int
    Max int
}

func (v NumberValidator) Validate(val interface{}) (bool, error) {
    num := val.(int)
    if num < v.Min {
        return false, fmt.Errorf("should be greater than %v", v.Min)
    }
    if v.Max >= v.Min && num > v.Max {
        return false, fmt.Errorf("should be less than %v", v.Max)
    }
    return true, nil
}

type EmailValidator struct{}

func (v EmailValidator) Validate(val interface{}) (bool, error) {
    if !mailRe.MatchString(val.(string)) {
        return false, fmt.Errorf("is not a valid email address")
    }
    return true, nil
}

func getValidatorFromTag(tag string) Validator {
    args := strings.Split(tag, ",")
    switch args[0] {
    case "number":
        validator := NumberValidator{}
        fmt.Sscanf(strings.Join(args[1:], ","), "min=%d,max=%d", &validator.Min, &validator.Max)
        return validator
    case "string":
        validator := StringValidator{}
        fmt.Sscanf(strings.Join(args[1:], ","), "min=%d,max=%d", &validator.Min, &validator.Max)
        return validator
    case "email":
        return EmailValidator{}
    }
    return DefaultValidator{}
}

func validateStruct(s interface{}) []error {
    errs := []error{}
    v := reflect.ValueOf(s)
    for i := 0; i < v.NumField(); i++ {
        tag := v.Type().Field(i).Tag.Get(tagName)
        if tag == "" || tag == "-" {
            continue
        }
        validator := getValidatorFromTag(tag)
        valid, err := validator.Validate(v.Field(i).Interface())
        if !valid && err != nil {
            errs = append(errs, fmt.Errorf("%s %s", v.Type().Field(i).Name, err.Error()))
        }
    }
    return errs
}

type User struct {
    Id    int    `validate:"number,min=1,max=1000"`
    Name  string `validate:"string,min=2,max=10"`
    Bio   string `validate:"string,min=2,max=10"`
    Email string `validate:"email"`
}

func main() {
    user := User{
        Id:    0,
        Name:  "superlongstring",
        Bio:   "superlongstring",
        Email: "foobar",
    }
    fmt.Println("Errors:")
    for i, err := range validateStruct(user) {
        fmt.Printf("\t%d. %s\n", i+1, err.Error())
    }
}