using System;

namespace Swan {
  /// <summary>
  /// A utility class to compute paging or batching offsets.
  /// </summary>
  public class Paginator {
    /// <summary>
    /// Initializes a new instance of the <see cref="Paginator" /> class.
    /// </summary>
    /// <param name="totalCount">The total count of items to page over.</param>
    /// <param name="pageSize">The desired size of individual pages.</param>
    public Paginator(Int32 totalCount, Int32 pageSize) {
      this.TotalCount = totalCount;
      this.PageSize = pageSize;
      this.PageCount = this.ComputePageCount();
    }

    /// <summary>
    /// Gets the desired number of items per page.
    /// </summary>
    public Int32 PageSize {
      get;
    }

    /// <summary>
    /// Gets the total number of items to page over.
    /// </summary>
    public Int32 TotalCount {
      get;
    }

    /// <summary>
    /// Gets the computed number of pages.
    /// </summary>
    public Int32 PageCount {
      get;
    }

    /// <summary>
    /// Gets the start item index of the given page.
    /// </summary>
    /// <param name="pageIndex">Zero-based index of the page.</param>
    /// <returns>The start item index.</returns>
    public Int32 GetFirstItemIndex(Int32 pageIndex) {
      pageIndex = this.FixPageIndex(pageIndex);
      return pageIndex * this.PageSize;
    }

    /// <summary>
    /// Gets the end item index of the given page.
    /// </summary>
    /// <param name="pageIndex">Zero-based index of the page.</param>
    /// <returns>The end item index.</returns>
    public Int32 GetLastItemIndex(Int32 pageIndex) {
      Int32 startIndex = this.GetFirstItemIndex(pageIndex);
      return Math.Min(startIndex + this.PageSize - 1, this.TotalCount - 1);
    }

    /// <summary>
    /// Gets the item count of the given page index.
    /// </summary>
    /// <param name="pageIndex">Zero-based index of the page.</param>
    /// <returns>The number of items that the page contains.</returns>
    public Int32 GetItemCount(Int32 pageIndex) {
      pageIndex = this.FixPageIndex(pageIndex);
      return (pageIndex >= this.PageCount - 1) ? this.GetLastItemIndex(pageIndex) - this.GetFirstItemIndex(pageIndex) + 1 : this.PageSize;
    }

    /// <summary>
    /// Fixes the index of the page by applying bound logic.
    /// </summary>
    /// <param name="pageIndex">Index of the page.</param>
    /// <returns>A limit-bound index.</returns>
    private Int32 FixPageIndex(Int32 pageIndex) => pageIndex < 0 ? 0 : pageIndex >= this.PageCount ? this.PageCount - 1 : pageIndex;

    /// <summary>
    /// Computes the number of pages for the paginator.
    /// </summary>
    /// <returns>The page count.</returns>
    private Int32 ComputePageCount() =>
      // include this if when you always want at least 1 page 
      this.TotalCount == 0 ? 0 : this.TotalCount % this.PageSize != 0 ? (this.TotalCount / this.PageSize) + 1 : this.TotalCount / this.PageSize;
  }
}