From 646b276a9ba766a7e2acfe2d35e83228afc0d1f7 Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Mon, 9 Jun 2025 23:26:52 +0100
Subject: Handle edge cases

...where extending a network results in something that can be a straight
or collapsed into a single arc from a bi-arc.
---
 lib/maths.cpp |  9 +++++++++
 lib/maths.h   | 32 +++++++++++++++-----------------
 2 files changed, 24 insertions(+), 17 deletions(-)

(limited to 'lib')

diff --git a/lib/maths.cpp b/lib/maths.cpp
index 000cea7..7dd313e 100644
--- a/lib/maths.cpp
+++ b/lib/maths.cpp
@@ -1,5 +1,6 @@
 #include "maths.h"
 #include <cmath>
+#include <functional>
 #include <glm/glm.hpp>
 #include <glm/gtx/rotate_vector.hpp>
 #include <glm/gtx/transform.hpp>
@@ -32,6 +33,7 @@ static_assert(pow(3, 1) == 3);
 static_assert(pow(3, 2) == 9);
 static_assert(pow(pi, 3) == 31.006278991699219F);
 
+#ifndef __clang__
 static_assert(!linesIntersectAt<int>({0, 10}, {10, 10}, {10, 0}, {0, 0}).has_value());
 static_assert(*linesIntersectAt<int>({0, 0}, {10, 10}, {10, 0}, {0, 10}) == GlobalPosition2D {5, 5});
 static_assert(*linesIntersectAt<int>({300'000'000, 400'000'00}, {300'010'000, 400'010'00}, {310'010'000, 410'000'00},
@@ -43,6 +45,13 @@ constexpr auto EAST2D = RelativePosition2D(east);
 static_assert(!linesIntersectAtDirs<int>({0, 0}, NORTH2D, {10, 10}, NORTH2D).has_value());
 static_assert(linesIntersectAtDirs<int>({0, 0}, NORTH2D, {10, 10}, EAST2D) == GlobalPosition2D {0, 10});
 static_assert(linesIntersectAtDirs<int>({0, 0}, EAST2D, {10, 10}, NORTH2D) == GlobalPosition2D {10, 0});
+
+static_assert(find_arc_centre<int>({0, 0}, -NORTH2D, {10, 10}) == std::make_pair<GlobalPosition2D>({10, 0}, 10.F));
+static_assert(find_arc_centre<int>({20, 0}, -NORTH2D, {10, 10}) == std::make_pair<GlobalPosition2D>({10, 0}, -10.F));
+static_assert(find_arc_centre<int>({10, 0}, -NORTH2D, {10, 10}).second == 0.F);
+static_assert(isWithinLimit(find_arc_centre<int>({0, 0}, pi + quarter_pi, {1000, 1000}).second, 0.F, 0.1F));
+#endif
+
 // NOLINTEND(readability-magic-numbers)
 
 float
diff --git a/lib/maths.h b/lib/maths.h
index 93a4322..09af048 100644
--- a/lib/maths.h
+++ b/lib/maths.h
@@ -427,38 +427,36 @@ linesIntersectAt(const glm::vec<2, T, Q> Aabs, const glm::vec<2, T, Q> Babs, con
 template<std::floating_point T> constexpr auto EPSILON = 0.0001F;
 
 template<std::floating_point T>
-auto
+[[nodiscard]] constexpr auto
 isWithinLimit(T lhs, T rhs, T limit = EPSILON<T>)
 {
 	return std::abs(lhs - rhs) <= limit;
 }
 
-template<Arithmetic T, glm::qualifier Q = glm::defaultp>
-std::pair<glm::vec<2, T, Q>, bool>
-find_arc_centre(glm::vec<2, T, Q> start, Angle entrys, glm::vec<2, T, Q> end, Angle entrye)
+template<Arithmetic T, std::floating_point D, glm::qualifier Q = glm::defaultp>
+constexpr std::pair<glm::vec<2, T, Q>, D>
+find_arc_centre(glm::vec<2, T, Q> start, glm::vec<2, D, Q> entrys, glm::vec<2, T, Q> end)
 {
 	if (start == end) {
-		return {start, false};
+		return {start, 0};
 	}
-	return find_arc_centre(start, sincos(entrys + half_pi), end, sincos(entrye - half_pi));
+	const auto diffEnds = difference(end, start);
+	const auto offset = entrys.x * diffEnds.y - entrys.y * diffEnds.x;
+	if (offset == 0.F) {
+		return {start, offset};
+	}
+	const auto midEnds = start + ((end - start) / 2);
+	const auto centre = linesIntersectAtDirs(start, vector_normal(entrys), midEnds, vector_normal(diffEnds));
+	return {*centre, offset};
 }
 
 template<Arithmetic T, glm::qualifier Q = glm::defaultp>
-std::pair<glm::vec<2, T, Q>, bool>
+constexpr std::pair<glm::vec<2, T, Q>, float>
 find_arc_centre(glm::vec<2, T, Q> start, Angle entrys, glm::vec<2, T, Q> end)
 {
-	if (start == end) {
-		return {start, false};
-	}
-	const auto startNormal = vector_normal(sincos(entrys) * 10'000.F);
-	const auto diffEnds = difference(end, start);
-	const auto midEnds = start + ((end - start) / 2);
-	const auto diffNormal = vector_normal(diffEnds);
-	const auto centre = linesIntersectAt(start, start + startNormal, midEnds, midEnds + diffNormal);
-	return {*centre, normalize(vector_yaw(diffEnds) - entrys) < 0};
+	return find_arc_centre(start, sincos(entrys), end);
 }
 
-
 template<Arithmetic T>
 auto
 midpoint(const std::pair<T, T> & v)
-- 
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