using System;
using System.Runtime.InteropServices;
using NetVips.Internal;

namespace NetVips;

/// <summary>
/// Wrap a <see cref="VipsRegion"/> object.
/// </summary>
/// <remarks>
/// A region is a small part of an image. You use regions to read pixels
/// out of images without storing the entire image in memory.
/// At least libvips 8.8 is needed.
/// </remarks>
public class Region : VipsObject
{
    private Region(nint pointer) : base(pointer)
    {
    }

    /// <summary>
    /// Make a region on an image.
    /// </summary>
    /// <param name="image"><see cref="Image"/> to create this region on.</param>
    /// <returns>A new <see cref="Region"/>.</returns>
    /// <exception cref="VipsException">If unable to make a new region on <paramref name="image"/>.</exception>
    public static Region New(Image image)
    {
        var vi = VipsRegion.New(image);
        if (vi == IntPtr.Zero)
        {
            throw new VipsException("unable to make region");
        }

        return new Region(vi);
    }

    /// <summary>
    /// Width of pixels held by region.
    /// </summary>
    public int Width => VipsRegion.Width(this);

    /// <summary>
    /// Height of pixels held by region.
    /// </summary>
    public int Height => VipsRegion.Height(this);

    /// <summary>
    /// Fetch an area of pixels.
    /// </summary>
    /// <param name="left">Left edge of area to fetch.</param>
    /// <param name="top">Top edge of area to fetch.</param>
    /// <param name="width">Width of area to fetch.</param>
    /// <param name="height">Height of area to fetch.</param>
    /// <returns>An array of bytes filled with pixel data.</returns>
    public byte[] Fetch(int left, int top, int width, int height)
    {
        var pointer = VipsRegion.Fetch(this, left, top, width, height, out var size);
        if (pointer == IntPtr.Zero)
        {
            throw new VipsException("unable to fetch from region");
        }

        var managedArray = new byte[size];
        Marshal.Copy(pointer, managedArray, 0, (int)size);

        GLib.GFree(pointer);

        return managedArray;
    }
}