using System.Text;

namespace MeterVault.App.Analysis;

/// <summary>
/// <c>GET /export/analysis.csv</c> (D-55): the analysis table of the URL keys the pages use — <c>scope</c>/<c>id</c>/
/// <c>ids</c>, <c>metric</c>, <c>period</c>, <c>from</c>, <c>to</c>, <c>bucket</c>, <c>compare</c> — as a CSV download.
/// Build links to it with <see cref="AnalysisLinks.Export"/>.
/// </summary>
/// <remarks>
/// A UI endpoint, not part of the versioned REST API: it serves the same reader a signed-in browser already sees and
/// needs no API key, like the pages themselves. Invalid input is a 400 with a short plain-text message; nothing a user
/// can type into the URL makes it a 500.
/// </remarks>
public static class AnalysisExportEndpoints
{
    public static IEndpointRouteBuilder MapAnalysisExport(this IEndpointRouteBuilder endpoints)
    {
        ArgumentNullException.ThrowIfNull(endpoints);

        endpoints.MapGet(AnalysisLinks.ExportPath, async (HttpContext http, AnalysisExport export, ILogger<AnalysisExport> logger, CancellationToken ct) =>
        {
            var query = AnalysisQuery.Parse(http.Request.Query, AnalysisDefaults.Export);

            AnalysisExportResult prepared;
            try
            {
                prepared = await export.PrepareAsync(query, ct);
            }
            catch (ArgumentException ex)
            {
                // The readers reject a combination they cannot answer with an ArgumentException; that is the request's
                // fault, not the server's.
                logger.LogWarning(ex, "Analysis export refused {Query}", query);
                return Results.Text("This combination of keys cannot be exported.", "text/plain; charset=utf-8", Encoding.UTF8, StatusCodes.Status400BadRequest);
            }

            if (prepared.Error is { } error)
            {
                return Results.Text(error, "text/plain; charset=utf-8", Encoding.UTF8, StatusCodes.Status400BadRequest);
            }

            return Results.Stream(
                async stream =>
                {
                    // A byte-order mark, so spreadsheet programs read the umlauts of meter names as UTF-8.
                    await using var writer = new StreamWriter(stream, new UTF8Encoding(encoderShouldEmitUTF8Identifier: true), leaveOpen: true);
                    await AnalysisCsvWriter.WriteAsync(writer, prepared.Rows, ct);
                },
                "text/csv; charset=utf-8",
                prepared.FileName);
        })
        .ExcludeFromDescription();

        return endpoints;
    }
}