using MeterVault.Core.Normalization;

namespace MeterVault.Infrastructure.Options;

/// <summary>
/// The instance timezone (<see cref="MeterVaultOptions.TimeZone"/>, SDD §10) as every part of the
/// application uses it: the normalizer divides months in it, readers bucket in it and turn requested
/// dates into instants with it. One resolution, so none of them disagrees about where a month starts.
/// </summary>
public static class InstanceTimeZone
{
    /// <summary>The configured zone, or UTC when <paramref name="id"/> is empty or not a known zone.</summary>
    public static TimeZoneInfo Resolve(string? id)
    {
        if (string.IsNullOrWhiteSpace(id))
        {
            return TimeZoneInfo.Utc;
        }

        return TimeZoneInfo.TryFindSystemTimeZoneById(id, out var zone) ? zone : TimeZoneInfo.Utc;
    }

    /// <summary>
    /// The IANA id for a configured zone. .NET also accepts Windows ids ("W. Europe Standard Time"), but
    /// PostgreSQL buckets by IANA names only, so a Windows id is translated rather than passed to SQL.
    /// Anything unrecognised is returned unchanged, to be reported at startup.
    /// </summary>
    public static string Canonical(string id)
    {
        if (!TimeZoneInfo.TryFindSystemTimeZoneById(id, out var zone) || zone.HasIanaId)
        {
            return id;
        }

        return TimeZoneInfo.TryConvertWindowsIdToIanaId(zone.Id, out var iana) ? iana : id;
    }

    /// <summary>The instant a local date starts — the lower bound a range of that date begins at.</summary>
    public static DateTimeOffset StartOf(DateOnly date, TimeZoneInfo zone) => GapAttribution.LocalMidnight(date, zone);
}