# HG changeset patch
# User Lin Jiang <lnjng07@protonmail.com>
# Date 1724262840 25200
#      Wed Aug 21 10:54:00 2024 -0700
# Node ID 3d1dcda1995e28400dbba7525f58cb80b9fe84e6
# Parent  8a73e1d140bf3c40daec3a3a4e0e39c8034e10a2
add intersect functionality

diff --git a/examples/centroid.et b/examples/centroid.et
--- a/examples/centroid.et
+++ b/examples/centroid.et
@@ -3,9 +3,15 @@
 (setq C (point 7 3))
 (setq triangle_a (triangle A B C))
 (setq O (centroid triangle_a))
+(setq line_A (lineseg A O))
+(setq line_B (lineseg B O))
+(setq line_C (lineseg C O))
 
 triangle_a
 O
-(lineseg A O)
-(lineseg B O)
-(lineseg C O)
+line_A
+line_B
+line_C
+(intersect line_A (lineseg B C))
+(intersect line_B (lineseg A C))
+(intersect line_C (lineseg A B))
diff --git a/examples/intersect.et b/examples/intersect.et
new file mode 100644
--- /dev/null
+++ b/examples/intersect.et
@@ -0,0 +1,7 @@
+(setq C (circle (point 0 0) 5))
+(setq L (lineseg (point 0 0) (point 3 0)))
+
+C
+L
+(intersect L C 0)
+(intersect L C 1)
diff --git a/src/lang/functions.rs b/src/lang/functions.rs
--- a/src/lang/functions.rs
+++ b/src/lang/functions.rs
@@ -397,6 +397,119 @@
 */
 
 #[derive(Clone)]
+pub struct FnIntersect;
+
+impl FnIntersect {
+    /// Case 1: Two line segments
+    fn from_linesegs(&self, args: &[Value]) -> Result<Value, String> {
+        // check for 2 arguments
+        if args.len() != 2 {
+            return Err("Intersect requires exactly 2 arguments".to_string());
+        }
+
+        // check for 2 line segments
+        let lineseg1 = match &args[0] {
+            Value::Lineseg(l) => l.clone(),
+            _ => return Err("Invalid types for line segment".to_string()),
+        };
+        let lineseg2 = match &args[1] {
+            Value::Lineseg(l) => l.clone(),
+            _ => return Err("Invalid types for line segment".to_string()),
+        };
+
+        // check if line segments are parallel
+        if lineseg1.slope() == lineseg2.slope() {
+            return Err("Line segments are parallel".to_string());
+        }
+
+        // handle vertical line segments
+        if lineseg1.slope().abs() == f64::INFINITY {
+            let x = lineseg1.start.x;
+            let y = lineseg2.slope() * x + lineseg2.y_intercept();
+            return Ok(Value::Point(Point { x, y }));
+        } else if lineseg2.slope().abs() == f64::INFINITY {
+            let x = lineseg2.start.x;
+            let y = lineseg1.slope() * x + lineseg1.y_intercept();
+            return Ok(Value::Point(Point { x, y }));
+        }
+
+        // otherwise, find the intersection point
+        let x = (lineseg2.y_intercept() - lineseg1.y_intercept())
+            / (lineseg1.slope() - lineseg2.slope());
+        let y = lineseg1.slope() * x + lineseg1.y_intercept();
+
+        Ok(Value::Point(Point { x, y }))
+    }
+
+    /// Case 2: One line segment and one circle
+    fn from_lineseg_circle(&self, args: &[Value]) -> Result<Value, String> {
+        // check for 3 arguments
+        if args.len() != 3 {
+            return Err("Intersect requires exactly 3 arguments".to_string());
+        }
+
+        // check for 1 line segment, 1 circle, and 1 index either 0 or 1
+        let lineseg = match &args[0] {
+            Value::Lineseg(l) => l.clone(),
+            _ => return Err("Invalid types for line segment".to_string()),
+        };
+        let circle = match &args[1] {
+            Value::Circle(c) => c.clone(),
+            _ => return Err("Invalid types for circle".to_string()),
+        };
+        let index = match &args[2] {
+            Value::Int(i) => *i,
+            _ => return Err("Invalid types for index".to_string()),
+        };
+        if index != 0 && index != 1 {
+            return Err("Index must be either 0 or 1".to_string());
+        }
+
+        // calculate the intersection points without methods
+        let a = lineseg.start.y;
+        let b = lineseg.end.y;
+        let c = circle.center.x;
+        let d = circle.center.y;
+        let r = circle.radius;
+        let m = (b - a) / (lineseg.start.x - lineseg.end.x);
+        let n = (a * lineseg.end.y - b * lineseg.start.y) / (lineseg.end.x - lineseg.start.x);
+        let A = 1.0 + m * m;
+        let B = 2.0 * (m * n - m * d - c);
+        let C = c * c + d * d + n * n - 2.0 * n * d - r * r;
+        let D = B * B - 4.0 * A * C;
+        if D < 0.0 {
+            return Err("No intersection points".to_string());
+        }
+        let x1 = (-B + D.sqrt()) / (2.0 * A);
+        let x2 = (-B - D.sqrt()) / (2.0 * A);
+        let y1 = m * x1 + n;
+        let y2 = m * x2 + n;
+
+        // return the intersection point
+        if index == 0 {
+            Ok(Value::Point(Point { x: x1, y: y1 }))
+        } else {
+            Ok(Value::Point(Point { x: x2, y: y2 }))
+        }
+    }
+}
+
+impl Operation for FnIntersect {
+    clone_impl!(FnIntersect);
+    fn call(&self, args: &[Value]) -> Result<Value, String> {
+        match self.from_linesegs(args) {
+            Ok(point) => return Ok(point),
+            _ => {}
+        }
+
+        match self.from_lineseg_circle(args) {
+            Ok(point) => Ok(point),
+            Err(e) => Err(e),
+        }
+    }
+}
+
+#[derive(Clone)]
 pub struct FnInradius;
 impl Operation for FnInradius {
     clone_impl!(FnInradius);
diff --git a/src/lang/types.rs b/src/lang/types.rs
--- a/src/lang/types.rs
+++ b/src/lang/types.rs
@@ -73,6 +73,18 @@
     pub end: Point,
 }
 
+impl Lineseg {
+    /// Return the slope of the lineseg
+    pub fn slope(&self) -> f64 {
+        (self.end.y - self.start.y) / (self.end.x - self.start.x)
+    }
+
+    /// Return the y intercept of the lineseg
+    pub fn y_intercept(&self) -> f64 {
+        self.start.y - self.slope() * self.start.x
+    }
+}
+
 impl Element for Lineseg {
     /// Turn lineseg into a SVG element
     fn to_svg(&self) -> Vec<Box<dyn Render>> {
@@ -95,7 +107,8 @@
     fn to_svg(&self) -> Vec<Box<dyn Render>> {
         vec![Box::new(SvgCircle {
             center: *self,
-            radius: 0.01,
+            radius: 0.05,
+            fill: true,
         })]
     }
 }
@@ -134,6 +147,7 @@
         vec![Box::new(SvgCircle {
             center: self.center,
             radius: self.radius,
+            fill: false,
         })]
     }
 }
diff --git a/src/lexer.rs b/src/lexer.rs
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -1,7 +1,6 @@
 use crate::lang::functions;
 use crate::lang::types::{Operation, Value};
 use std::fmt::{Debug, Error, Formatter};
-use std::rc::Weak;
 
 #[derive(Clone, Debug, PartialEq)]
 pub enum Token {
@@ -131,6 +130,11 @@
         },
 
         // functions that return properties
+        "intersect" => Function {
+            name,
+            args: Vec::new(),
+            function: Box::new(functions::FnIntersect),
+        },
         "inradius" => Function {
             name,
             args: Vec::new(),
diff --git a/src/renderer.rs b/src/renderer.rs
--- a/src/renderer.rs
+++ b/src/renderer.rs
@@ -306,14 +306,20 @@
 pub struct SvgCircle {
     pub center: Point,
     pub radius: f64,
+    pub fill: bool,
 }
 
 impl Render for SvgCircle {
     impl_as_any!(SvgCircle);
     fn render(&self) -> String {
+        let mut fill_value = "none";
+        if self.fill {
+            fill_value = "black";
+        }
+
         format!(
-            "\t<circle cx=\"{}\" cy=\"{}\" r=\"{}\" fill=\"none\" stroke=\"black\" stroke-width=\"0.02\"/>\n",
-            self.center.x, self.center.y, self.radius
+            "\t<circle cx=\"{}\" cy=\"{}\" r=\"{}\" fill=\"{}\" stroke=\"black\" stroke-width=\"0.02\"/>\n",
+            self.center.x, self.center.y, self.radius, fill_value
         )
     }