Feature: Line Equation

CMakeLists.txt

@@ -21,6 +21,7 @@ install(DIRECTORY "include" DESTINATION ".")
 
 add_library(${PROJECT_NAME} SHARED
   "src/angle/angle.cpp"
+  "src/geometry/line.cpp"
   "src/geometry/point_2.cpp"
   "src/geometry/point_3.cpp"
   "src/matrix/matrix.cpp"

include/keisan/geometry/line.hpp

@@ -0,0 +1,51 @@
+// Copyright (c) 2026 ICHIRO ITS
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+#ifndef KEISAN__GEOMETRY__LINE_HPP_
+#define KEISAN__GEOMETRY__LINE_HPP_
+
+#include "keisan/geometry/point_2.hpp"
+
+namespace keisan
+{
+
+struct Line
+{
+  Line();
+  Line(double a, double b, double c);
+
+  static Line from_points(const Point2 & p1, const Point2 & p2);
+  static Line from_point(const Point2 & point, const Angle<double> & angle);
+
+  double distance(const Point2 & point) const;
+
+  std::optional<double> slope() const;
+  Angle<double> angle() const;
+
+  std::optional<Point2> intersection(const Line & other) const;
+
+  double a;
+  double b;
+  double c;
+};
+
+}  // namespace keisan
+
+#endif  // KEISAN__GEOMETRY__LINE_HPP_

src/geometry/line.cpp

@@ -0,0 +1,78 @@
+// Copyright (c) 2026 ICHIRO ITS
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+#include "keisan/geometry/line.hpp"
+
+namespace keisan
+{
+
+Line::Line() {}
+
+Line::Line(double a, double b, double c) : a(a), b(b), c(a) {}
+
+Line Line::from_points(const Point2 & p1, const Point2 & p2)
+{
+  double a = p2.y - p1.y;
+  double b = p1.x - p2.x;
+  double c = -(a * p1.x + b * p1.y);
+  return Line(a, b, c);
+}
+
+Line Line::from_point(const Point2 & point, const Angle<double> & angle)
+{
+  double a = -angle.sin();
+  double b = angle.cos();
+  double c = -(a * point.x + b * point.y);
+
+  return Line(a, b, c);
+}
+
+double Line::distance(const Point2 & point) const
+{
+  double denominator = std::hypot(a, b);
+
+  if (denominator < 1e-9) {
+    throw std::runtime_error(
+      "Invalid line equations: coefficients 'a' and 'b' cannot both be zero.");
+  }
+
+  return std::abs(a * point.x + b * point.y + c) / denominator;
+}
+
+std::optional<Point2> Line::intersection(const Line & other) const
+{
+  double determinant = a * other.b - other.a * b;
+  if (std::abs(determinant) < 1e-9) {
+    return std::nullopt;  // Lines are parallel or coincident
+  }
+  double x = (b * other.c - other.b * c) / determinant;
+  double y = (other.a * c - a * other.c) / determinant;
+  return Point2{x, y};
+}
+
+std::optional<double> Line::slope() const
+{
+  if (std::abs(b) < 1e-9) return std::nullopt;  // Line is vertical
+  return -a / b;
+}
+
+Angle<double> Line::angle() const { return signed_arctan(a, -b); }
+
+}  // namespace keisan