# define IMGUI_DEFINE_MATH_OPERATORS
# include "imgui_canvas.h"
# include <type_traits>

// https://stackoverflow.com/a/36079786
# define DECLARE_HAS_MEMBER(__trait_name__, __member_name__)                         \
                                                                                     \
    template <typename __boost_has_member_T__>                                       \
    class __trait_name__                                                             \
    {                                                                                \
        using check_type = ::std::remove_const_t<__boost_has_member_T__>;            \
        struct no_type {char x[2];};                                                 \
        using  yes_type = char;                                                      \
                                                                                     \
        struct  base { void __member_name__() {}};                                   \
        struct mixin : public base, public check_type {};                            \
                                                                                     \
        template <void (base::*)()> struct aux {};                                   \
                                                                                     \
        template <typename U> static no_type  test(aux<&U::__member_name__>*);       \
        template <typename U> static yes_type test(...);                             \
                                                                                     \
        public:                                                                      \
                                                                                     \
        static constexpr bool value = (sizeof(yes_type) == sizeof(test<mixin>(0)));  \
    }

namespace ImCanvasDetails {

DECLARE_HAS_MEMBER(HasFringeScale, _FringeScale);

struct FringeScaleRef
{
    // Overload is present when ImDrawList does have _FringeScale member variable.
    template <typename T>
    static float& Get(typename std::enable_if<HasFringeScale<T>::value, T>::type* drawList)
    {
        return drawList->_FringeScale;
    }

    // Overload is present when ImDrawList does not have _FringeScale member variable.
    template <typename T>
    static float& Get(typename std::enable_if<!HasFringeScale<T>::value, T>::type*)
    {
        static float placeholder = 1.0f;
        return placeholder;
    }
};

DECLARE_HAS_MEMBER(HasVtxCurrentOffset, _VtxCurrentOffset);

struct VtxCurrentOffsetRef
{
    // Overload is present when ImDrawList does have _FringeScale member variable.
    template <typename T>
    static unsigned int& Get(typename std::enable_if<HasVtxCurrentOffset<T>::value, T>::type* drawList)
    {
        return drawList->_VtxCurrentOffset;
    }

    // Overload is present when ImDrawList does not have _FringeScale member variable.
    template <typename T>
    static unsigned int& Get(typename std::enable_if<!HasVtxCurrentOffset<T>::value, T>::type* drawList)
    {
        return drawList->_CmdHeader.VtxOffset;
    }
};

} // namespace ImCanvasDetails

// Returns a reference to _FringeScale extension to ImDrawList
//
// If ImDrawList does not have _FringeScale a placeholder is returned.
static inline float& ImFringeScaleRef(ImDrawList* drawList)
{
    using namespace ImCanvasDetails;
    return FringeScaleRef::Get<ImDrawList>(drawList);
}

static inline unsigned int& ImVtxOffsetRef(ImDrawList* drawList)
{
    using namespace ImCanvasDetails;
    return VtxCurrentOffsetRef::Get<ImDrawList>(drawList);
}

static inline ImVec2 ImSelectPositive(const ImVec2& lhs, const ImVec2& rhs) { return ImVec2(lhs.x > 0.0f ? lhs.x : rhs.x, lhs.y > 0.0f ? lhs.y : rhs.y); }

bool ImGuiEx::Canvas::Begin(const char* id, const ImVec2& size)
{
    return Begin(ImGui::GetID(id), size);
}

bool ImGuiEx::Canvas::Begin(ImGuiID id, const ImVec2& size)
{
    IM_ASSERT(m_InBeginEnd == false);

    m_WidgetPosition = ImGui::GetCursorScreenPos();
    m_WidgetSize = ImSelectPositive(size, ImGui::GetContentRegionAvail());
    m_WidgetRect = ImRect(m_WidgetPosition, m_WidgetPosition + m_WidgetSize);
    m_DrawList = ImGui::GetWindowDrawList();

    UpdateViewTransformPosition();

    if (ImGui::IsClippedEx(m_WidgetRect, id, false))
        return false;

    // Save current channel, so we can assert when user
    // call canvas API with different one.
    m_ExpectedChannel = m_DrawList->_Splitter._Current;

    // #debug: Canvas content.
    //m_DrawList->AddRectFilled(m_StartPos, m_StartPos + m_CurrentSize, IM_COL32(0, 0, 0, 64));
    m_DrawList->AddRect(m_WidgetRect.Min, m_WidgetRect.Max, IM_COL32(255, 0, 255, 64));

    ImGui::SetCursorScreenPos(ImVec2(0.0f, 0.0f));

# if IMGUI_EX_CANVAS_DEFERED()
    m_Ranges.resize(0);
# endif

    SaveInputState();
    SaveViewportState();

    EnterLocalSpace();

    // Emit dummy widget matching bounds of the canvas.
    ImGui::SetCursorScreenPos(m_ViewRect.Min);
    ImGui::Dummy(m_ViewRect.GetSize());

    ImGui::SetCursorScreenPos(ImVec2(0.0f, 0.0f));

    m_InBeginEnd = true;

    return true;
}

void ImGuiEx::Canvas::End()
{
    // If you're here your call to Begin() returned false,
    // or Begin() wasn't called at all.
    IM_ASSERT(m_InBeginEnd == true);

    // If you're here, please make sure you do not interleave
    // channel splitter with canvas.
    // Always call canvas function with using same channel.
    IM_ASSERT(m_DrawList->_Splitter._Current == m_ExpectedChannel);

    //auto& io = ImGui::GetIO();

    // Check: Unmatched calls to Suspend() / Resume(). Please check your code.
    IM_ASSERT(m_SuspendCounter == 0);

    LeaveLocalSpace();

    // Emit dummy widget matching bounds of the canvas.
    ImGui::SetCursorScreenPos(m_WidgetPosition);
    ImGui::Dummy(m_WidgetSize);

    // #debug: Rect around canvas. Content should be inside these bounds.
    //m_DrawList->AddRect(m_WidgetPosition - ImVec2(1.0f, 1.0f), m_WidgetPosition + m_WidgetSize + ImVec2(1.0f, 1.0f), IM_COL32(196, 0, 0, 255));

    m_InBeginEnd = false;
}

void ImGuiEx::Canvas::SetView(const ImVec2& origin, float scale)
{
    SetView(CanvasView(origin, scale));
}

void ImGuiEx::Canvas::SetView(const CanvasView& view)
{
    if (m_InBeginEnd)
        LeaveLocalSpace();

    if (m_View.Origin.x != view.Origin.x || m_View.Origin.y != view.Origin.y)
    {
        m_View.Origin = view.Origin;

        UpdateViewTransformPosition();
    }

    if (m_View.Scale != view.Scale)
    {
        m_View.Scale    = view.Scale;
        m_View.InvScale = view.InvScale;
    }

    if (m_InBeginEnd)
        EnterLocalSpace();
}

void ImGuiEx::Canvas::CenterView(const ImVec2& canvasPoint)
{
    auto view = CalcCenterView(canvasPoint);
    SetView(view);
}

ImGuiEx::CanvasView ImGuiEx::Canvas::CalcCenterView(const ImVec2& canvasPoint) const
{
    auto localCenter = ToLocal(m_WidgetPosition + m_WidgetSize * 0.5f);
    auto localOffset = canvasPoint - localCenter;
    auto offset      = FromLocalV(localOffset);

    return CanvasView{ m_View.Origin - offset, m_View.Scale };
}

void ImGuiEx::Canvas::CenterView(const ImRect& canvasRect)
{
    auto view = CalcCenterView(canvasRect);

    SetView(view);
}

ImGuiEx::CanvasView ImGuiEx::Canvas::CalcCenterView(const ImRect& canvasRect) const
{
    auto canvasRectSize = canvasRect.GetSize();

    if (canvasRectSize.x <= 0.0f || canvasRectSize.y <= 0.0f)
        return View();

    auto widgetAspectRatio     = m_WidgetSize.y   > 0.0f ? m_WidgetSize.x   / m_WidgetSize.y   : 0.0f;
    auto canvasRectAspectRatio = canvasRectSize.y > 0.0f ? canvasRectSize.x / canvasRectSize.y : 0.0f;

    if (widgetAspectRatio <= 0.0f || canvasRectAspectRatio <= 0.0f)
        return View();

    auto newOrigin = m_View.Origin;
    auto newScale  = m_View.Scale;
    if (canvasRectAspectRatio > widgetAspectRatio)
    {
        // width span across view
        newScale = m_WidgetSize.x / canvasRectSize.x;
        newOrigin = canvasRect.Min * -newScale;
        newOrigin.y += (m_WidgetSize.y - canvasRectSize.y * newScale) * 0.5f;
    }
    else
    {
        // height span across view
        newScale = m_WidgetSize.y / canvasRectSize.y;
        newOrigin = canvasRect.Min * -newScale;
        newOrigin.x += (m_WidgetSize.x - canvasRectSize.x * newScale) * 0.5f;
    }

    return CanvasView{ newOrigin, newScale };
}

void ImGuiEx::Canvas::Suspend()
{
    // If you're here, please make sure you do not interleave
    // channel splitter with canvas.
    // Always call canvas function with using same channel.
    IM_ASSERT(m_DrawList->_Splitter._Current == m_ExpectedChannel);

    if (m_SuspendCounter == 0)
        LeaveLocalSpace();

    ++m_SuspendCounter;
}

void ImGuiEx::Canvas::Resume()
{
    // If you're here, please make sure you do not interleave
    // channel splitter with canvas.
    // Always call canvas function with using same channel.
    IM_ASSERT(m_DrawList->_Splitter._Current == m_ExpectedChannel);

    // Check: Number of calls to Resume() do not match calls to Suspend(). Please check your code.
    IM_ASSERT(m_SuspendCounter > 0);
    if (--m_SuspendCounter == 0)
        EnterLocalSpace();
}

ImVec2 ImGuiEx::Canvas::FromLocal(const ImVec2& point) const
{
    return point * m_View.Scale + m_ViewTransformPosition;
}

ImVec2 ImGuiEx::Canvas::FromLocal(const ImVec2& point, const CanvasView& view) const
{
    return point * view.Scale + view.Origin + m_WidgetPosition;
}

ImVec2 ImGuiEx::Canvas::FromLocalV(const ImVec2& vector) const
{
    return vector * m_View.Scale;
}

ImVec2 ImGuiEx::Canvas::FromLocalV(const ImVec2& vector, const CanvasView& view) const
{
    return vector * view.Scale;
}

ImVec2 ImGuiEx::Canvas::ToLocal(const ImVec2& point) const
{
    return (point - m_ViewTransformPosition) * m_View.InvScale;
}

ImVec2 ImGuiEx::Canvas::ToLocal(const ImVec2& point, const CanvasView& view) const
{
    return (point - view.Origin - m_WidgetPosition) * view.InvScale;
}

ImVec2 ImGuiEx::Canvas::ToLocalV(const ImVec2& vector) const
{
    return vector * m_View.InvScale;
}

ImVec2 ImGuiEx::Canvas::ToLocalV(const ImVec2& vector, const CanvasView& view) const
{
    return vector * view.InvScale;
}

ImRect ImGuiEx::Canvas::CalcViewRect(const CanvasView& view) const
{
    ImRect result;
    result.Min = ImVec2(-view.Origin.x, -view.Origin.y) * view.InvScale;
    result.Max = (m_WidgetSize - view.Origin) * view.InvScale;
    return result;
}

void ImGuiEx::Canvas::UpdateViewTransformPosition()
{
    m_ViewTransformPosition = m_View.Origin + m_WidgetPosition;
}

void ImGuiEx::Canvas::SaveInputState()
{
    auto& io = ImGui::GetIO();
    m_MousePosBackup = io.MousePos;
    m_MousePosPrevBackup = io.MousePosPrev;
    for (auto i = 0; i < IM_ARRAYSIZE(m_MouseClickedPosBackup); ++i)
        m_MouseClickedPosBackup[i] = io.MouseClickedPos[i];

    // Record cursor max to prevent scrollbars from appearing.
    m_WindowCursorMaxBackup = ImGui::GetCurrentWindow()->DC.CursorMaxPos;
}

void ImGuiEx::Canvas::RestoreInputState()
{
    auto& io = ImGui::GetIO();
    io.MousePos = m_MousePosBackup;
    io.MousePosPrev = m_MousePosPrevBackup;
    for (auto i = 0; i < IM_ARRAYSIZE(m_MouseClickedPosBackup); ++i)
        io.MouseClickedPos[i] = m_MouseClickedPosBackup[i];
    ImGui::GetCurrentWindow()->DC.CursorMaxPos = m_WindowCursorMaxBackup;
}

void ImGuiEx::Canvas::SaveViewportState()
{
# if defined(IMGUI_HAS_VIEWPORT)
    auto viewport = ImGui::GetWindowViewport();

    m_ViewportPosBackup = viewport->Pos;
    m_ViewportSizeBackup = viewport->Size;
# endif
}

void ImGuiEx::Canvas::RestoreViewportState()
{
# if defined(IMGUI_HAS_VIEWPORT)
    auto viewport = ImGui::GetWindowViewport();

    viewport->Pos = m_ViewportPosBackup;
    viewport->Size = m_ViewportSizeBackup;
# endif
}

void ImGuiEx::Canvas::EnterLocalSpace()
{
    // Prepare ImDrawList for drawing in local coordinate system:
    //   - determine visible part of the canvas
    //   - start unique draw command
    //   - add clip rect matching canvas size
    //   - record current command index
    //   - record current vertex write index

    // Determine visible part of the canvas. Make it before
    // adding new command, to avoid round rip where command
    // is removed in PopClipRect() and added again next PushClipRect().
    ImGui::PushClipRect(m_WidgetPosition, m_WidgetPosition + m_WidgetSize, true);
    auto clipped_clip_rect = m_DrawList->_ClipRectStack.back();
    ImGui::PopClipRect();

    // Make sure we do not share draw command with anyone. We don't want to mess
    // with someones clip rectangle.

    // #FIXME:
    //     This condition is not enough to avoid when user choose
    //     to use channel splitter.
    //
    //     To deal with Suspend()/Resume() calls empty draw command
    //     is always added then splitter is active. Otherwise
    //     channel merger will collapse our draw command one with
    //     different clip rectangle.
    //
    //     More investigation is needed. To get to the bottom of this.
    if ((!m_DrawList->CmdBuffer.empty() && m_DrawList->CmdBuffer.back().ElemCount > 0) || m_DrawList->_Splitter._Count > 1)
        m_DrawList->AddDrawCmd();

# if IMGUI_EX_CANVAS_DEFERED()
    m_Ranges.resize(m_Ranges.Size + 1);
    m_CurrentRange = &m_Ranges.back();
    m_CurrentRange->BeginComandIndex = ImMax(m_DrawList->CmdBuffer.Size - 1, 0);
    m_CurrentRange->BeginVertexIndex = m_DrawList->_VtxCurrentIdx + ImVtxOffsetRef(m_DrawList);
# endif
    m_DrawListCommadBufferSize       = ImMax(m_DrawList->CmdBuffer.Size - 1, 0);
    m_DrawListStartVertexIndex       = m_DrawList->_VtxCurrentIdx + ImVtxOffsetRef(m_DrawList);

# if defined(IMGUI_HAS_VIEWPORT)
    auto viewport_min = m_ViewportPosBackup;
    auto viewport_max = m_ViewportPosBackup + m_ViewportSizeBackup;

    viewport_min.x = (viewport_min.x - m_ViewTransformPosition.x) * m_View.InvScale;
    viewport_min.y = (viewport_min.y - m_ViewTransformPosition.y) * m_View.InvScale;
    viewport_max.x = (viewport_max.x - m_ViewTransformPosition.x) * m_View.InvScale;
    viewport_max.y = (viewport_max.y - m_ViewTransformPosition.y) * m_View.InvScale;

    auto viewport = ImGui::GetWindowViewport();
    viewport->Pos  = viewport_min;
    viewport->Size = viewport_max - viewport_min;
# endif

    // Clip rectangle in parent canvas space and move it to local space.
    clipped_clip_rect.x = (clipped_clip_rect.x - m_ViewTransformPosition.x) * m_View.InvScale;
    clipped_clip_rect.y = (clipped_clip_rect.y - m_ViewTransformPosition.y) * m_View.InvScale;
    clipped_clip_rect.z = (clipped_clip_rect.z - m_ViewTransformPosition.x) * m_View.InvScale;
    clipped_clip_rect.w = (clipped_clip_rect.w - m_ViewTransformPosition.y) * m_View.InvScale;
    ImGui::PushClipRect(ImVec2(clipped_clip_rect.x, clipped_clip_rect.y), ImVec2(clipped_clip_rect.z, clipped_clip_rect.w), false);

    // Transform mouse position to local space.
    auto& io = ImGui::GetIO();
    io.MousePos     = (m_MousePosBackup - m_ViewTransformPosition) * m_View.InvScale;
    io.MousePosPrev = (m_MousePosPrevBackup - m_ViewTransformPosition) * m_View.InvScale;
    for (auto i = 0; i < IM_ARRAYSIZE(m_MouseClickedPosBackup); ++i)
        io.MouseClickedPos[i] = (m_MouseClickedPosBackup[i] - m_ViewTransformPosition) * m_View.InvScale;

    m_ViewRect = CalcViewRect(m_View);;

    auto& fringeScale = ImFringeScaleRef(m_DrawList);
    m_LastFringeScale = fringeScale;
    fringeScale *= m_View.InvScale;
}

void ImGuiEx::Canvas::LeaveLocalSpace()
{
    IM_ASSERT(m_DrawList->_Splitter._Current == m_ExpectedChannel);

# if IMGUI_EX_CANVAS_DEFERED()
    IM_ASSERT(m_CurrentRange != nullptr);

    m_CurrentRange->EndVertexIndex  = m_DrawList->_VtxCurrentIdx + ImVtxOffsetRef(m_DrawList);
    m_CurrentRange->EndCommandIndex = m_DrawList->CmdBuffer.size();
    if (m_CurrentRange->BeginVertexIndex == m_CurrentRange->EndVertexIndex)
    {
        // Drop empty range
        m_Ranges.resize(m_Ranges.Size - 1);
    }
    m_CurrentRange = nullptr;
# endif

    // Move vertices to screen space.
    auto vertex    = m_DrawList->VtxBuffer.Data + m_DrawListStartVertexIndex;
    auto vertexEnd = m_DrawList->VtxBuffer.Data + m_DrawList->_VtxCurrentIdx + ImVtxOffsetRef(m_DrawList);

    // If canvas view is not scaled take a faster path.
    if (m_View.Scale != 1.0f)
    {
        while (vertex < vertexEnd)
        {
            vertex->pos.x = vertex->pos.x * m_View.Scale + m_ViewTransformPosition.x;
            vertex->pos.y = vertex->pos.y * m_View.Scale + m_ViewTransformPosition.y;
            ++vertex;
        }

        // Move clip rectangles to screen space.
        for (int i = m_DrawListCommadBufferSize; i < m_DrawList->CmdBuffer.size(); ++i)
        {
            auto& command = m_DrawList->CmdBuffer[i];
            command.ClipRect.x = command.ClipRect.x * m_View.Scale + m_ViewTransformPosition.x;
            command.ClipRect.y = command.ClipRect.y * m_View.Scale + m_ViewTransformPosition.y;
            command.ClipRect.z = command.ClipRect.z * m_View.Scale + m_ViewTransformPosition.x;
            command.ClipRect.w = command.ClipRect.w * m_View.Scale + m_ViewTransformPosition.y;
        }
    }
    else
    {
        while (vertex < vertexEnd)
        {
            vertex->pos.x = vertex->pos.x + m_ViewTransformPosition.x;
            vertex->pos.y = vertex->pos.y + m_ViewTransformPosition.y;
            ++vertex;
        }

        // Move clip rectangles to screen space.
        for (int i = m_DrawListCommadBufferSize; i < m_DrawList->CmdBuffer.size(); ++i)
        {
            auto& command = m_DrawList->CmdBuffer[i];
            command.ClipRect.x = command.ClipRect.x + m_ViewTransformPosition.x;
            command.ClipRect.y = command.ClipRect.y + m_ViewTransformPosition.y;
            command.ClipRect.z = command.ClipRect.z + m_ViewTransformPosition.x;
            command.ClipRect.w = command.ClipRect.w + m_ViewTransformPosition.y;
        }
    }

    auto& fringeScale = ImFringeScaleRef(m_DrawList);
    fringeScale = m_LastFringeScale;

    // And pop \o/
    ImGui::PopClipRect();

    RestoreInputState();
    RestoreViewportState();
}
