package main

import (
    "database/sql"
    "flag"
    "html/template"
    "log/slog"
    "net/http"
    "os"

    "snippetbox.alexedwards.net/internal/models"

    "github.com/go-playground/form/v4" // New import
    _ "github.com/go-sql-driver/mysql"
)

// Add a formDecoder field to hold a pointer to a form.Decoder instance.
type application struct {
    logger        *slog.Logger
    snippets      *models.SnippetModel
    templateCache map[string]*template.Template
    formDecoder   *form.Decoder
}

func main() {
    addr := flag.String("addr", ":4000", "HTTP network address")
    dsn := flag.String("dsn", "web:pass@/snippetbox?parseTime=true", "MySQL data source name")
    flag.Parse()

    logger := slog.New(slog.NewTextHandler(os.Stdout, nil))

    db, err := openDB(*dsn)
    if err != nil {
        logger.Error(err.Error())
        os.Exit(1)
    }
    defer db.Close()

    templateCache, err := newTemplateCache()
    if err != nil {
        logger.Error(err.Error())
        os.Exit(1)
    }

    // Initialize a decoder instance...
    formDecoder := form.NewDecoder()

    // And add it to the application dependencies.
    app := &application{
        logger:        logger,
        snippets:      &models.SnippetModel{DB: db},
        templateCache: templateCache,
        formDecoder:   formDecoder,
    }

    logger.Info("starting server", "addr", *addr)

    err = http.ListenAndServe(*addr, app.routes())
    logger.Error(err.Error())
    os.Exit(1)
}

...

package main

...

// Update our snippetCreateForm struct to include struct tags which tell the
// decoder how to map HTML form values into the different struct fields. So, for
// example, here we're telling the decoder to store the value from the HTML form
// input with the name "title" in the Title field. The struct tag `form:"-"` 
// tells the decoder to completely ignore a field during decoding.
type snippetCreateForm struct {
    Title               string `form:"title"`
    Content             string `form:"content"`
    Expires             int    `form:"expires"`
    validator.Validator `form:"-"`
}

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

    // Declare a new empty instance of the snippetCreateForm struct.
    var form snippetCreateForm

    // Call the Decode() method of the form decoder, passing in the current
    // request and *a pointer* to our snippetCreateForm struct. This will
    // essentially fill our struct with the relevant values from the HTML form.
    // If there is a problem, we return a 400 Bad Request response to the client.
    err = app.formDecoder.Decode(&form, r.PostForm)
    if err != nil {
        app.clientError(w, http.StatusBadRequest)
        return
    }

    // Then validate and use the data as normal...
    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.PermittedValue(form.Expires, 1, 7, 365), "expires", "This field must equal 1, 7 or 365")

    if !form.Valid() {
        data := app.newTemplateData(r)
        data.Form = form
        app.render(w, r, http.StatusUnprocessableEntity, "create.tmpl", data)
        return
    }

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

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

package main

import (
    "bytes"
    "errors" // New import
    "fmt"
    "net/http"
    "time"

    "github.com/go-playground/form/v4" // New import
)

...

// Create a new decodePostForm() helper method. The second parameter here, dst,
// is the target destination that we want to decode the form data into.
func (app *application) decodePostForm(r *http.Request, dst any) error {
    // Call ParseForm() on the request, in the same way that we did in our
    // snippetCreatePost handler.
    err := r.ParseForm()
    if err != nil {
        return err
    }

    // Call Decode() on our decoder instance, passing the target destination as
    // the first parameter.
    err = app.formDecoder.Decode(dst, r.PostForm)
    if err != nil {
        // If we try to use an invalid target destination, the Decode() method
        // will return an error with the type *form.InvalidDecoderError.We use 
        // errors.As() to check for this and raise a panic rather than returning
        // the error.
        var invalidDecoderError *form.InvalidDecoderError
        
        if errors.As(err, &invalidDecoderError) {
            panic(err)
        }

        // For all other errors, we return them as normal.
        return err
    }

    return nil
}

package main

...

func (app *application) snippetCreatePost(w http.ResponseWriter, r *http.Request) {
    var form snippetCreateForm

    err := app.decodePostForm(r, &form)
    if err != nil {
        app.clientError(w, http.StatusBadRequest)
        return
    }

    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.PermittedValue(form.Expires, 1, 7, 365), "expires", "This field must equal 1, 7 or 365")

    if !form.Valid() {
        data := app.newTemplateData(r)
        data.Form = form
        app.render(w, r, http.StatusUnprocessableEntity, "create.tmpl", data)
        return
    }

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

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