using System.Runtime.CompilerServices;
using Microsoft.SharePoint.Client;
using SharepointToolbox.Core.Models;

namespace SharepointToolbox.Core.Helpers;

public static class SharePointPaginationHelper
{
    /// <summary>
    /// Enumerates all items in a SharePoint list, bypassing the 5,000-item threshold.
    /// Uses CamlQuery with RowLimit=2000 and ListItemCollectionPosition for pagination.
    /// Never call ExecuteQuery directly on a list — always use this helper.
    /// </summary>
    public static async IAsyncEnumerable<ListItem> GetAllItemsAsync(
        ClientContext ctx,
        List list,
        CamlQuery? baseQuery = null,
        [EnumeratorCancellation] CancellationToken ct = default)
    {
        var query = baseQuery ?? CamlQuery.CreateAllItemsQuery();
        query.ViewXml = BuildPagedViewXml(query.ViewXml, rowLimit: 2000);
        query.ListItemCollectionPosition = null;

        do
        {
            ct.ThrowIfCancellationRequested();
            var items = list.GetItems(query);
            ctx.Load(items);
            await ctx.ExecuteQueryAsync();

            foreach (var item in items)
                yield return item;

            query.ListItemCollectionPosition = items.ListItemCollectionPosition;
        }
        while (query.ListItemCollectionPosition != null);
    }

    internal static string BuildPagedViewXml(string? existingXml, int rowLimit)
    {
        // Inject or replace RowLimit in existing CAML, or create minimal view
        if (string.IsNullOrWhiteSpace(existingXml))
            return $"<View><RowLimit>{rowLimit}</RowLimit></View>";

        // Simple replacement approach — adequate for Phase 1
        if (existingXml.Contains("<RowLimit>", StringComparison.OrdinalIgnoreCase))
        {
            return System.Text.RegularExpressions.Regex.Replace(
                existingXml, @"<RowLimit[^>]*>\d+</RowLimit>",
                $"<RowLimit>{rowLimit}</RowLimit>",
                System.Text.RegularExpressions.RegexOptions.IgnoreCase);
        }
        return existingXml.Replace("</View>", $"<RowLimit>{rowLimit}</RowLimit></View>",
            StringComparison.OrdinalIgnoreCase);
    }
}