From e942c62ad6ceb20c4072f06b89257357b6f504bd Mon Sep 17 00:00:00 2001
From: Simon Bruder <simon.bruder@mailbox.tu-dresden.de>
Date: Sun, 7 May 2023 13:25:57 +0200
Subject: [PATCH] u02: Implement bresenham circle tool

---
 u02/src/tests.cpp | 69 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/u02/src/tests.cpp b/u02/src/tests.cpp
index a2611dd..90dda1e 100644
--- a/u02/src/tests.cpp
+++ b/u02/src/tests.cpp
@@ -5,6 +5,7 @@
 #include <catch2/generators/catch_generators_adapters.hpp>
 #include <catch2/generators/catch_generators_random.hpp>
 
+#include "bresenham_circle_tool.h"
 #include "bresenham_line_tool.h"
 #include "canvas_buffer.h"
 #include "dda_line_tool.h"
@@ -298,3 +299,71 @@ TEST_CASE("Rectangle (prop)") {
   }
   REQUIRE(pass);
 }
+
+TEST_CASE("Bresenham circle (prop: √(x²+y²)-r<ε)") {
+  // Let s be the size of the canvas (s,s).
+  // Let m be the smallest coordinate (x and y) for random points
+  // and M be the largest coordinate (x and y) for random points.
+  //
+  // The largest radius that can fit on canvas (for arbitrary centres) is m.
+  // The largest radius that is possible to create is √(2)(M-m).
+  //
+  // ⇒ √(2)(M-m)   ≥ m
+  // ⇔ √(2)M-√(2)m ≥ m
+  // ⇔ √(2)M       ≥ (1+√(2))m
+  // ⇔ m           ≤ (√(2)/(1+√(2)))m
+  // ⇔ m           ≤ (2-√(2))M    (1)
+  //
+  // Additionally, to have a centered point field,
+  // s-M=m must hold
+  //
+  //   s-M = m
+  // ⇒ s-M ≤ (2-√(2))M
+  // ⇔ s   ≤ (3-√(2))M
+  // ⇔ M   ≥ s/(3-√(2))    (2)
+  //
+  // With this, it now is possible to express M and m in terms of s:
+  //
+  //   m ≤ (2-√(2))M    (1)
+  // ⇔ m ≤ ((2-√(2))/(3-√(2)))s
+  // ⇔ m ≤ ((4-√(2))/7)s (3)
+  //
+  // When the points are rounded to the nearest integer,
+  // M must be rounded down and m rounded down.
+  const int size = 100;                                         // s
+  const int max_c = std::floor(size / (3 - std::sqrt(2)));      // M (2)
+  const int min_c = std::ceil(((4 - std::sqrt(2)) / 7) * size); // m (3)
+
+  const int x0 = GENERATE_COPY(take(10, random(min_c, max_c)));
+  const int y0 = GENERATE_COPY(take(10, random(min_c, max_c)));
+  const int x1 = GENERATE_COPY(take(10, random(min_c, max_c)));
+  const int y1 = GENERATE_COPY(take(10, random(min_c, max_c)));
+
+  if ((x0 == min_c || x0 == max_c) && (x1 == min_c || x1 == max_c) &&
+      (y0 == min_c || y0 == max_c) && (y1 == min_c || y1 == max_c)) {
+    SKIP("All coordinates have extreme value, skipping (avoid rounding error)");
+  }
+
+  const int r =
+      round(std::sqrt(std::pow((x1 - x0), 2) + std::pow((y1 - y0), 2)));
+
+  canvas_buffer *canvas = new canvas_buffer(size, size);
+  bresenham_circle_tool *tool = new bresenham_circle_tool(*canvas);
+
+  tool->draw(x0, y0, x1, y1);
+
+  bool pass = true;
+  for (int x = 0; x < size; x++) {
+    for (int y = 0; y < size; y++) {
+      double distance =
+          std::abs(std::sqrt(std::pow(x0 - x, 2) + std::pow(y0 - y, 2)) - r);
+      // Because of rounding errors, an exact test (for all pixels) is not
+      // feasible.
+      // Therefore, it is only tested if set pixels have a distance <= 1.
+      if (canvas->get_pixel(x, y) && distance > 1) {
+        pass = false;
+      }
+    }
+  }
+  REQUIRE(pass);
+}