From d2372464a28d9ce8e6b78dc538c38759b54dddbb Mon Sep 17 00:00:00 2001
From: Lucas CHOLLET <lucas.chollet@free.fr>
Date: Sun, 1 Jan 2023 15:55:57 +0100
Subject: [PATCH] LibGfx: Put the Midpoint Ellipse Algorithm in its own
 function

---
 Userland/Libraries/LibGfx/Painter.cpp | 101 ++++++++++++++------------
 1 file changed, 53 insertions(+), 48 deletions(-)

diff --git a/Userland/Libraries/LibGfx/Painter.cpp b/Userland/Libraries/LibGfx/Painter.cpp
index a8133e433b..f604804e4b 100644
--- a/Userland/Libraries/LibGfx/Painter.cpp
+++ b/Userland/Libraries/LibGfx/Painter.cpp
@@ -472,6 +472,55 @@ void Painter::draw_circle_arc_intersecting(IntRect const& a_rect, IntPoint cente
     return draw_circle_arc_intersecting(a_rect, center, radius - 1, color, thickness - 1);
 }
 
+// The callback will only be called for a quarter of the ellipse, the user is intended to deduce other points.
+// As the coordinate space is relative to the center of the rectangle, it's simply (x, y), (x, -y), (-x, y) and (-x, -y).
+static void on_each_ellipse_point(IntRect const& rect, Function<void(IntPoint)>&& callback)
+{
+    // Note: This is an implementation of the Midpoint Ellipse Algorithm.
+    double const a = rect.width() / 2;
+    double const a_square = a * a;
+    double const b = rect.height() / 2;
+    double const b_square = b * b;
+
+    int x = 0;
+    auto y = static_cast<int>(b);
+
+    double dx = 2 * b_square * x;
+    double dy = 2 * a_square * y;
+
+    // For region 1:
+    auto decision_parameter = b_square - a_square * b + .25 * a_square;
+
+    while (dx < dy) {
+        callback({ x, y });
+
+        if (decision_parameter >= 0) {
+            y--;
+            dy -= 2 * a_square;
+            decision_parameter -= dy;
+        }
+        x++;
+        dx += 2 * b_square;
+        decision_parameter += dx + b_square;
+    }
+
+    // For region 2:
+    decision_parameter = b_square * ((x + 0.5) * (x + 0.5)) + a_square * ((y - 1) * (y - 1)) - a_square * b_square;
+
+    while (y >= 0) {
+        callback({ x, y });
+
+        if (decision_parameter <= 0) {
+            x++;
+            dx += 2 * b_square;
+            decision_parameter += dx;
+        }
+        y--;
+        dy -= 2 * a_square;
+        decision_parameter += a_square - dy;
+    }
+}
+
 void Painter::fill_ellipse(IntRect const& a_rect, Color color)
 {
     VERIFY(scale() == 1); // FIXME: Add scaling support.
@@ -498,57 +547,13 @@ void Painter::draw_ellipse_intersecting(IntRect const& rect, Color color, int th
 
     auto const center = rect.center();
 
-    auto const draw_real_world_x4 = [this, &color, thickness, center](int x, int y) {
-        IntPoint const directions[4] = { { x, y }, { x, -y }, { -x, y }, { -x, -y } };
-        for (auto const& delta : directions) {
+    on_each_ellipse_point(rect, [this, &color, thickness, center](IntPoint position) {
+        IntPoint const directions[4] = { { position.x(), position.y() }, { position.x(), -position.y() }, { -position.x(), position.y() }, { -position.x(), -position.y() } };
+        for (auto const delta : directions) {
             auto const point = center + delta;
             draw_line(point, point, color, thickness);
         }
-    };
-
-    // Note: This is an implementation of the Midpoint Ellipse Algorithm:
-    double const a = rect.width() / 2;
-    double const a_square = a * a;
-    double const b = rect.height() / 2;
-    double const b_square = b * b;
-
-    int x = 0;
-    auto y = static_cast<int>(b);
-
-    double dx = 2 * b_square * x;
-    double dy = 2 * a_square * y;
-
-    // For region 1:
-    auto decision_parameter = b_square - a_square * b + .25 * a_square;
-
-    while (dx < dy) {
-        draw_real_world_x4(x, y);
-
-        if (decision_parameter >= 0) {
-            y--;
-            dy -= 2 * a_square;
-            decision_parameter -= dy;
-        }
-        x++;
-        dx += 2 * b_square;
-        decision_parameter += dx + b_square;
-    }
-
-    // For region 2:
-    decision_parameter = b_square * ((x + 0.5) * (x + 0.5)) + a_square * ((y - 1) * (y - 1)) - a_square * b_square;
-
-    while (y >= 0) {
-        draw_real_world_x4(x, y);
-
-        if (decision_parameter <= 0) {
-            x++;
-            dx += 2 * b_square;
-            decision_parameter += dx;
-        }
-        y--;
-        dy -= 2 * a_square;
-        decision_parameter += a_square - dy;
-    }
+    });
 }
 
 template<typename RectType, typename Callback>