package validator

import (
    "strings"
    "unicode/utf8"
)

// Define a new Validator type which contains a map of validation errors for our
// form fields.
type Validator struct {
    FieldErrors map[string]string
}

// Valid() returns true if the FieldErrors map doesn't contain any entries.
func (v *Validator) Valid() bool {
    return len(v.FieldErrors) == 0
}

// AddFieldError() adds an error message to the FieldErrors map (so long as no
// entry already exists for the given key).
func (v *Validator) AddFieldError(key, message string) {
    // Note: We need to initialize the map first, if it isn't already
    // initialized.
    if v.FieldErrors == nil {
        v.FieldErrors = make(map[string]string)
    }

    if _, exists := v.FieldErrors[key]; !exists {
        v.FieldErrors[key] = message
    }
}

// CheckField() adds an error message to the FieldErrors map only if a
// validation check is not 'ok'.
func (v *Validator) CheckField(ok bool, key, message string) {
    if !ok {
        v.AddFieldError(key, message)
    }
}

// NotBlank() returns true if a value is not an empty string.
func NotBlank(value string) bool {
    return strings.TrimSpace(value) != ""
}

// MaxChars() returns true if a value contains no more than n characters.
func MaxChars(value string, n int) bool {
    return utf8.RuneCountInString(value) <= n
}

// PermittedInt() returns true if a value is in a list of permitted integers.
func PermittedInt(value int, permittedValues ...int) bool {
    for i := range permittedValues {
        if value == permittedValues[i] {
            return true
        }
    }
    return false
}

package main

import (
    "errors"
    "fmt"
    "net/http"
    "strconv"

    "snippetbox.alexedwards.net/internal/models"
    "snippetbox.alexedwards.net/internal/validator" // New import

    "github.com/julienschmidt/httprouter"
)

...

// Remove the explicit FieldErrors struct field and instead embed the Validator
// type. Embedding this means that our snippetCreateForm "inherits" all the
// fields and methods of our Validator type (including the FieldErrors field).
type snippetCreateForm struct {
    Title               string 
    Content             string 
    Expires             int    
    validator.Validator
}

func (app *application) snippetCreatePost(w http.ResponseWriter, r *http.Request) {
    err := r.ParseForm()
    if err != nil {
        app.clientError(w, http.StatusBadRequest)
        return
    }

    expires, err := strconv.Atoi(r.PostForm.Get("expires"))
    if err != nil {
        app.clientError(w, http.StatusBadRequest)
        return
    }

    form := snippetCreateForm{
        Title:   r.PostForm.Get("title"),
        Content: r.PostForm.Get("content"),
        Expires: expires,
        // Remove the FieldErrors assignment from here.
    }

    // Because the Validator type is embedded by the snippetCreateForm struct,
    // we can call CheckField() directly on it to execute our validation checks.
    // CheckField() will add the provided key and error message to the
    // FieldErrors map if the check does not evaluate to true. For example, in
    // the first line here we "check that the form.Title field is not blank". In
    // the second, we "check that the form.Title field has a maximum character
    // length of 100" and so on.
    form.CheckField(validator.NotBlank(form.Title), "title", "This field cannot be blank")
    form.CheckField(validator.MaxChars(form.Title, 100), "title", "This field cannot be more than 100 characters long")
    form.CheckField(validator.NotBlank(form.Content), "content", "This field cannot be blank")
    form.CheckField(validator.PermittedInt(form.Expires, 1, 7, 365), "expires", "This field must equal 1, 7 or 365")

    // Use the Valid() method to see if any of the checks failed. If they did,
    // then re-render the template passing in the form in the same way as
    // before.
    if !form.Valid() {
        data := app.newTemplateData(r)
        data.Form = form
        app.render(w, http.StatusUnprocessableEntity, "create.tmpl", data)
        return
    }

    id, err := app.snippets.Insert(form.Title, form.Content, form.Expires)
    if err != nil {
        app.serverError(w, err)
        return
    }

    http.Redirect(w, r, fmt.Sprintf("/snippet/view/%d", id), http.StatusSeeOther)
}