static real32 Tau = 0.9; // tension

static real32
Bezier_SolveYForX(v2 Point_P0, v2 Point_P1, v2 Point_P2, v2 Point_P3, real32 TargetX) {

    real32 Y = 0;

    v2 m1 = (Point_P2 - Point_P0) / (2 * Tau);
    v2 m2 = (Point_P3 - Point_P1) / (2 * Tau);

    real32 Precision = 0.001;
    real32 t = (TargetX - Point_P0.x) / (Point_P3.x - Point_P0.x);

    for (;;) {
        real32 t2 = t * t;
        real32 t3 = t2 * t;
        real32 mt = 1-t;
        real32 mt2 = mt * mt;
        real32 mt3 = mt2 * mt;
        v2 Point = (Point_P0 * mt3) + (3 * Point_P1 * mt2 * t) + (3 * Point_P2 * mt * t2) + (Point_P3 * t3);

        bool32 Cond1 = (Point.x <= (TargetX - Precision));
        bool32 Cond2 = (Point.x >= (TargetX + Precision));

        if (Cond1 || Cond2) {
            t = t * TargetX / Point.x;
        } else {
            Y = Point.y;
            break;
        }
    }

    return Y;
}

#if 0
// A modified version of the bezier code in ImGui with extra features for bitmap and path interaction.

// Function to convert a ratio back into a point for the bezier handles.
v2 Line_RatioToPoint(v2 a, v2 b, real32 ratio)
{
    v2 ab_dir = b - a;
    real32 ab_len_sqr = ab_dir.x * ab_dir.x + ab_dir.y * ab_dir.y;
    real32 dot = ratio*ab_len_sqr;
    return a + ab_dir * dot / V2(ab_len_sqr);
}

// The ratio here is just the dot product divided by the squared length.
v2 Bezier_LineClosestPoint2(v2 a, v2 b, v2 p, real32 *ratio)
{
    v2 ap = p - a;
    v2 ab_dir = b - a;
    real32 dot = ap.x * ab_dir.x + ap.y * ab_dir.y;
    if (dot < 0.0f) {
        *ratio = 0.0f;
        return a;
    }
    real32 ab_len_sqr = ab_dir.x * ab_dir.x + ab_dir.y * ab_dir.y;
    if (dot > ab_len_sqr) {
        *ratio = 1.0f;
        return b;
    }
    *ratio = dot / ab_len_sqr;
    return a + ab_dir * dot / ab_len_sqr;
}

// Following the algorithm, we take the ratio from the _leftmost_ point in each
// subdivision of the cubic spline until we're within tess_tol, and then we
// interpolate it with the subdivision's rightmost point in the ClosestPoint call.
// The pow(0.5, level) represents the ratio of the next subdivision's leftmost
// point (AKA the rightmost point of the current subdivision).

// finds the point closest to p and also fills out its ratio along the curve
static void Bezier_CubicClosestPointCasteljauStep(v2 p, v2 *p_closest, real32 ratio, real32 *r_closest, v2 *p_last, real32 *p_closest_dist2,
                                                    real32 x1, real32 y1, real32 x2, real32 y2, real32 x3, real32 y3, real32 x4, real32 y4, real32 tess_tol, int level)
{
    real32 dx = x4 - x1;
    real32 dy = y4 - y1;
    real32 d2 = ((x2 - x4) * dy - (y2 - y4) * dx);
    real32 d3 = ((x3 - x4) * dy - (y3 - y4) * dx);
    d2 = (d2 >= 0) ? d2 : -d2;
    d3 = (d3 >= 0) ? d3 : -d3;
    if ((d2 + d3) * (d2 + d3) < tess_tol * (dx * dx + dy * dy))
    {
        v2 p_current = V2(x4, y4);
        real32 added_ratio;
        v2 p_line = Bezier_LineClosestPoint2(*p_last, p_current, p, &added_ratio);
        real32 dist2 = LengthSq(p - p_line);
        if (dist2 < *p_closest_dist2)
        {
            *p_closest = p_line;
            *p_closest_dist2 = dist2;
            *r_closest = ratio + pow(0.5, level)*added_ratio;
        }
        *p_last = p_current;
    }
    else if (level < 10)
    {
        real32 x12 = (x1 + x2)*0.5f,       y12 = (y1 + y2)*0.5f;
        real32 x23 = (x2 + x3)*0.5f,       y23 = (y2 + y3)*0.5f;
        real32 x34 = (x3 + x4)*0.5f,       y34 = (y3 + y4)*0.5f;
        real32 x123 = (x12 + x23)*0.5f,    y123 = (y12 + y23)*0.5f;
        real32 x234 = (x23 + x34)*0.5f,    y234 = (y23 + y34)*0.5f;
        real32 x1234 = (x123 + x234)*0.5f, y1234 = (y123 + y234)*0.5f;
        Bezier_CubicClosestPointCasteljauStep(p, p_closest, ratio, r_closest, p_last, p_closest_dist2, x1, y1, x12, y12, x123, y123, x1234, y1234, tess_tol, level + 1);
        Bezier_CubicClosestPointCasteljauStep(p, p_closest, ratio + pow(0.5, level+1), r_closest, p_last, p_closest_dist2, x1234, y1234, x234, y234, x34, y34, x4, y4, tess_tol, level + 1);
    }
}

// finds the min/max bounds of the curve
static void Bezier_CubicMinMaxCasteljauStep(v2 *p_min, v2 *p_max, real32 x1, real32 y1, real32 x2, real32 y2, real32 x3, real32 y3, real32 x4, real32 y4, real32 tess_tol, int level)
{
    real32 dx = x4 - x1;
    real32 dy = y4 - y1;
    real32 d2 = ((x2 - x4) * dy - (y2 - y4) * dx);
    real32 d3 = ((x3 - x4) * dy - (y3 - y4) * dx);
    d2 = (d2 >= 0) ? d2 : -d2;
    d3 = (d3 >= 0) ? d3 : -d3;
    if ((d2 + d3) * (d2 + d3) < tess_tol * (dx * dx + dy * dy))
    {
        v2 p_current = V2(x4, y4);
        if (p_current.x < p_min->x) {
            p_min->x = p_current.x;
        }
        if (p_current.y < p_min->y) {
            p_min->y = p_current.y;
        }
        if (p_current.x > p_max->x) {
            p_max->x = p_current.x;
        }
        if (p_current.y > p_max->y) {
            p_max->y = p_current.y;
        }
    }
    else if (level < 10)
    {
        real32 x12 = (x1 + x2)*0.5f,       y12 = (y1 + y2)*0.5f;
        real32 x23 = (x2 + x3)*0.5f,       y23 = (y2 + y3)*0.5f;
        real32 x34 = (x3 + x4)*0.5f,       y34 = (y3 + y4)*0.5f;
        real32 x123 = (x12 + x23)*0.5f,    y123 = (y12 + y23)*0.5f;
        real32 x234 = (x23 + x34)*0.5f,    y234 = (y23 + y34)*0.5f;
        real32 x1234 = (x123 + x234)*0.5f, y1234 = (y123 + y234)*0.5f;
        Bezier_CubicMinMaxCasteljauStep(p_min, p_max, x1, y1, x12, y12, x123, y123, x1234, y1234, tess_tol, level + 1);
        Bezier_CubicMinMaxCasteljauStep(p_min, p_max, x1234, y1234, x234, y234, x34, y34, x4, y4, tess_tol, level + 1);
    }
}

// return all points
static void Bezier_CubicCalcPointsCasteljauStep(void *Data, uint32 *Increment, real32 x1, real32 y1, real32 x2, real32 y2, real32 x3, real32 y3, real32 x4, real32 y4, real32 tess_tol, int level)
{
    real32 dx = x4 - x1;
    real32 dy = y4 - y1;
    real32 d2 = ((x2 - x4) * dy - (y2 - y4) * dx);
    real32 d3 = ((x3 - x4) * dy - (y3 - y4) * dx);
    d2 = (d2 >= 0) ? d2 : -d2;
    d3 = (d3 >= 0) ? d3 : -d3;
    if ((d2 + d3) * (d2 + d3) < tess_tol * (dx * dx + dy * dy))
    {
        *((real32 *)Data + *Increment*3) = x4;
        *((real32 *)Data + *Increment*3 + 1) = y4;
        *((real32 *)Data + *Increment*3 + 2) = 0;
        *Increment += 1;
    }
    else if (level < 10)
    {
        real32 x12 = (x1 + x2)*0.5f,       y12 = (y1 + y2)*0.5f;
        real32 x23 = (x2 + x3)*0.5f,       y23 = (y2 + y3)*0.5f;
        real32 x34 = (x3 + x4)*0.5f,       y34 = (y3 + y4)*0.5f;
        real32 x123 = (x12 + x23)*0.5f,    y123 = (y12 + y23)*0.5f;
        real32 x234 = (x23 + x34)*0.5f,    y234 = (y23 + y34)*0.5f;
        real32 x1234 = (x123 + x234)*0.5f, y1234 = (y123 + y234)*0.5f;
        Bezier_CubicCalcPointsCasteljauStep(Data, Increment, x1, y1, x12, y12, x123, y123, x1234, y1234, tess_tol, level + 1);
        Bezier_CubicCalcPointsCasteljauStep(Data, Increment, x1234, y1234, x234, y234, x34, y34, x4, y4, tess_tol, level + 1);
    }
}

real32 Bezier_CubicRatioOfPoint(v2 p1, v2 p2, v2 p3, v2 p4, v2 p)
{
    real32 tess_tol = TESS_TOL;
    v2 p_last = p1;
    v2 p_closest;
    real32 p_closest_dist2 = FLT_MAX;
    real32 ratio = 0;
    Bezier_CubicClosestPointCasteljauStep(p, &p_closest, 0, &ratio, &p_last, &p_closest_dist2, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y, p4.x, p4.y, tess_tol, 0);
    return ratio;
}

void Bezier_CubicCalcPoints(v2 p1, v2 p2, v2 p3, v2 p4, void *Data, uint32 *Increment)
{
    real32 tess_tol = TESS_TOL;
    void *Pointer = Data;
    Bezier_CubicCalcPointsCasteljauStep(Pointer, Increment, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y, p4.x, p4.y, tess_tol, 0);
}

// These functions will become more generalized as shapes are added.

static void
Mask_AddPointToLine(mask *Mask, uint16 Index, v2 Pos)
{
    for (int i = Mask->NumberOfPoints - 1; i > Index; i--) {
        Mask->Point[i+1] = Mask->Point[i];
    }
    mask_point *PointToAdd = &Mask->Point[Index+1];
    PointToAdd->Pos = Pos;
    Mask->NumberOfPoints++;
}

static void
Mask_PushPoint(mask *Mask, v2 Pos)
{
    mask_point *PointToAdd = &Mask->Point[Mask->NumberOfPoints];
    PointToAdd->Pos = Pos;
    Mask->NumberOfPoints++;
}

static void
Mask_ShiftPointers(mask *Mask, int16 Increment, int16 StopAt) {
    if (Increment > 0) {
        int16 i = Mask->NumberOfPoints - 1;
        while (i >= StopAt) {
            mask_point *CurrentPoint = &Mask->Point[i];
            mask_point *NextPoint = &Mask->Point[i + Increment];
            *NextPoint = *CurrentPoint;
            i--;
        }
    } else  {
        int16 i = StopAt;
        while (i <= Mask->NumberOfPoints - 1) {
            mask_point *CurrentPoint = &Mask->Point[i];
            mask_point *NextPoint = &Mask->Point[i - Increment];
            *CurrentPoint = *NextPoint;
            i++;
        }
    }
}

static void
Mask_DeletePoint(memory *Memory, mask *Mask, uint16 Index)
{
    History_Entry_Commit(Memory, action_entry_default, "Delete keyframe");
    mask_point *MaskPointIndex = &Mask->Point[Index];
    History_Action_StoreData(Memory, MaskPointIndex, sizeof(mask_point));

    History_Action_Change_Decrement(Memory, &Mask->NumberOfPoints, action_type_change_u16);
    // History_Action_Shift(Memory, action_type_shift_bezier, Mask, -1, Index);
    void *StartingAddress = &Mask->Point[0];
    History_Action_Shift_2(Memory, StartingAddress, sizeof(mask_point), Mask->NumberOfPoints, -1, Index);
    // Mask_ShiftPointers(Mask, -1, Index);
    History_Entry_End(Memory);
}

// It's more useful to input the ratio here instead of the cursor position
// since we have to use it to calculate the four new handle lengths (two on the
// new point and one on each edge).
static void
Mask_AddPointToCurve(memory *Memory, mask *Mask, uint16 Index, real32 ratio)
{
    mask_point *Point0 = &Mask->Point[Index];
    mask_point *Point1 = &Mask->Point[Index+1];
    if (Index + 1 == Mask->NumberOfPoints)
        Point1 = &Mask->Point[0];

    v2 Point0_Pos_Right = Point0->Pos + Point0->TangentRight;
    v2 Point1_Pos_Left = Point1->Pos + Point1->TangentLeft;
    v2 Handle0_Half   = Line_RatioToPoint(Point0->Pos,      Point0_Pos_Right, ratio);
    v2 Handle1_Half   = Line_RatioToPoint(Point1_Pos_Left,  Point1->Pos,      ratio);
    v2 Top_Half       = Line_RatioToPoint(Point0_Pos_Right, Point1_Pos_Left,  ratio);
    v2 NewHandleLeft  = Line_RatioToPoint(Handle0_Half,     Top_Half,         ratio);
    v2 NewHandleRight = Line_RatioToPoint(Top_Half,         Handle1_Half,     ratio);
    v2 NewPos         = Line_RatioToPoint(NewHandleLeft,    NewHandleRight,   ratio);

    History_Entry_Commit(Memory, action_entry_default, "Add point to curve");

    v2 NewPoint0Pos = -(Point0->Pos - Handle0_Half);
    v2 NewPoint1Pos = -(Point1->Pos - Handle1_Half);
    History_Action_Change_V2(Memory, &Point0->TangentRight, &Point0->TangentRight, &NewPoint0Pos);
    History_Action_Change_V2(Memory, &Point1->TangentLeft, &Point1->TangentLeft, &NewPoint1Pos);

    void *StartingAddress = &Mask->Point[0];
    History_Action_Shift_2(Memory, StartingAddress, sizeof(mask_point), Mask->NumberOfPoints, 1, Index);

    mask_point *PointToAdd = &Mask->Point[Index+1];

    // NOTE(fox): The above shift duplicates the keyframe at Index into where
    // we're writing, Index+1. I'm using the Change action (which is normally
    // for changing values that already exist) on this intermediate keyframe
    // slot to save having to write a bunch of special actions for shifting and
    // adding new data.

    History_Action_Change_V2(Memory, &PointToAdd->Pos, &PointToAdd->Pos, &NewPos);
    v2 NewLeftPos = -(NewPos - NewHandleLeft);
    v2 NewRightPos = -(NewPos - NewHandleRight);
    History_Action_Change_V2(Memory, &PointToAdd->TangentLeft, &PointToAdd->TangentLeft, &NewLeftPos);
    History_Action_Change_V2(Memory, &PointToAdd->TangentRight, &PointToAdd->TangentRight, &NewRightPos);

    History_Action_Change_Increment(Memory, &Mask->NumberOfPoints, action_type_change_u16);
    History_Entry_End(Memory);
}

static void
Mask_RasterizePoints(mask *Mask)
{
    Mask->NumberOfVerts = 0;
    for (int i = 0; i < Mask->NumberOfPoints; i++) {
        mask_point Point0 = Mask->Point[i];
        mask_point Point1 = Mask->Point[i+1];
        if (i+1 == Mask->NumberOfPoints)
            Point1 = Mask->Point[0];

        if (Point0.HandleBezier && Point1.HandleBezier) {
            Bezier_CubicCalcPoints(Point0.Pos, Point0.Pos + Point0.TangentRight, Point1.Pos + Point1.TangentLeft, Point1.Pos,
                                   Mask->TriangulatedPointCache, &Mask->NumberOfVerts);
        } else if (Point0.HandleBezier) {
            Bezier_CubicCalcPoints(Point0.Pos, Point0.Pos + Point0.TangentRight, Point1.Pos, Point1.Pos,
                                   Mask->TriangulatedPointCache, &Mask->NumberOfVerts);
        } else if (Point1.HandleBezier) {
            Bezier_CubicCalcPoints(Point0.Pos, Point0.Pos, Point1.Pos + Point1.TangentLeft, Point1.Pos,
                                   Mask->TriangulatedPointCache, &Mask->NumberOfVerts);
        } else {
            real32 *Data = (real32 *)Mask->TriangulatedPointCache + Mask->NumberOfVerts*3;
            *(Data++) = Point0.Pos.x;
            *(Data++) = Point0.Pos.y;
            *(Data++) = 0;
            // NOTE(fox): CubicCalcPoints sometimes misses generating the start
            // point of the next path in the above two cases, so I'm making
            // straight lines always add both points as a hotfix. This leads
            // to cases of duplicate verts, but it doesn't seem like it harms
            // the rendering in any way.
            *(Data++) = Point1.Pos.x;
            *(Data++) = Point1.Pos.y;
            *(Data++) = 0;
            Mask->NumberOfVerts += 2;
        }
    }
}

void Mask_TriangulateAndRasterize(memory *Memory, project_layer *Layer, mask *Mask)
{
    if (!Mask->TriangulatedPointCache) {
        Mask->TriangulatedPointCache = AllocateMemory(Memory, 50*1024, P_VectorPoints);
    }
    Mask_RasterizePoints(Mask);

    GL_RasterizeShape(Layer, Mask);
}
#endif