using System;
using System.Numerics;

namespace AcDream.App.Rendering.Sky;

/// <summary>
/// Builds the temporary sky projection used by retail <c>GameSky::Draw</c>
/// (<c>0x00506FF0</c>): preserve the active viewport's field of view and
/// projection handedness, changing only its near/far depth mapping.
/// </summary>
internal static class SkyProjection
{
    public static Matrix4x4 WithDepthRange(
        in Matrix4x4 activeProjection,
        float near,
        float far)
    {
        if (!float.IsFinite(near) || !float.IsFinite(far)
            || near <= 0f || far <= near)
        {
            throw new ArgumentOutOfRangeException(
                nameof(far),
                "Sky depth range must be finite with 0 < near < far.");
        }

        var result = activeProjection;

        // System.Numerics perspective matrices use M34=-1 (right-handed).
        // Keep the source matrix's X/Y scale and offsets verbatim so a
        // teleport projection remains pixel-aligned with terrain. The
        // positive branch also preserves a left-handed source should one be
        // supplied by a future backend.
        if (activeProjection.M34 < 0f)
        {
            result.M33 = far / (near - far);
            result.M43 = near * far / (near - far);
        }
        else if (activeProjection.M34 > 0f)
        {
            result.M33 = far / (far - near);
            result.M43 = -near * far / (far - near);
        }
        else
        {
            throw new ArgumentException(
                "Sky projection must be perspective (M34 cannot be zero).",
                nameof(activeProjection));
        }

        return result;
    }
}