using System.Globalization;
using Elternbeirat.Web.Shared;

namespace Elternbeirat.Web.Features.Posts;

/// <summary>
///     Formats a post's calendar day for display and for a machine-readable
///     <c>datetime</c> attribute.
/// </summary>
/// <remarks>
///     Posts carry a <see cref="DateOnly"/> (a calendar day, no time of day). Their
///     <c>&lt;time&gt;</c> element needs the day twice: once in German for the
///     visitor (<see cref="Display"/>) and once as ISO 8601 for the <c>datetime</c>
///     attribute (<see cref="Iso"/>). Keeping both here means the two never drift
///     apart and the markup no longer repeats the same <c>ToString</c> pair.
///     <para>
///         This is the post counterpart to
///         <see cref="Events.EventDateFormat"/>, which formats a
///         <see cref="System.DateTime"/> range instead.
///     </para>
/// </remarks>
public static class PostDateFormat
{
    /// <summary>
    ///     Display format for a calendar day, e.g. <c>8. Oktober 2026</c>. The
    ///     weekday is omitted, unlike the event format.
    /// </summary>
    private const string LongDate = "d. MMMM yyyy";

    /// <summary>
    ///     Formats a calendar day in German for a visitor to read.
    /// </summary>
    /// <param name="value">
    ///     The calendar day to format.
    /// </param>
    /// <returns>
    ///     The day as e.g. <c>8. Oktober 2026</c>, using German month names.
    /// </returns>
    public static string Display(DateOnly value)
        => value.ToString(LongDate, Cultures.German);

    /// <summary>
    ///     Formats a calendar day as ISO 8601 (<c>yyyy-MM-dd</c>) for a
    ///     machine-readable <c>datetime</c> attribute.
    /// </summary>
    /// <param name="value">
    ///     The calendar day to format.
    /// </param>
    /// <returns>
    ///     The day as e.g. <c>2026-10-08</c>, culture-invariant so the value is
    ///     stable regardless of the request's culture.
    /// </returns>
    public static string Iso(DateOnly value)
        => value.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);
}