package data

import (
    "greenlight.alexedwards.net/internal/validator" // New import
)

// Add a SortSafelist field to hold the supported sort values.
type Filters struct {
    Page         int
    PageSize     int
    Sort         string
    SortSafelist []string
}

func ValidateFilters(v *validator.Validator, f Filters) {
    // Check that the page and page_size parameters contain sensible values.
    v.Check(f.Page > 0, "page", "must be greater than zero")
    v.Check(f.Page <= 10_000_000, "page", "must be a maximum of 10 million")
    v.Check(f.PageSize > 0, "page_size", "must be greater than zero")
    v.Check(f.PageSize <= 100, "page_size", "must be a maximum of 100")

    // Check that the sort parameter matches a value in the safelist.
    v.Check(validator.PermittedValue(f.Sort, f.SortSafelist...), "sort", "invalid sort value")
}

package main

...

func (app *application) listMoviesHandler(w http.ResponseWriter, r *http.Request) {
    var input struct {
        Title  string
        Genres []string
        data.Filters
    }

    v := validator.New()

    qs := r.URL.Query()

    input.Title = app.readString(qs, "title", "")
    input.Genres = app.readCSV(qs, "genres", []string{})

    input.Filters.Page = app.readInt(qs, "page", 1, v)
    input.Filters.PageSize = app.readInt(qs, "page_size", 20, v)

    input.Filters.Sort = app.readString(qs, "sort", "id")
    // Add the supported sort values for this endpoint to the sort safelist.
    input.Filters.SortSafelist = []string{"id", "title", "year", "runtime", "-id", "-title", "-year", "-runtime"}

    // Execute the validation checks on the Filters struct and send a response 
    // containing the errors if necessary.
    if data.ValidateFilters(v, input.Filters); !v.Valid() {
        app.failedValidationResponse(w, r, v.Errors)
        return
    }

    fmt.Fprintf(w, "%+v\n", input)
}