From db8a0462a25753bb9b9bf05b9d83cfdddb18b7ff Mon Sep 17 00:00:00 2001
From: Francesco Magliocca <franciman12@gmail.com>
Date: Mon, 9 Nov 2020 23:33:30 +0100
Subject: [PATCH] Start correct implementation of line-rect intersection test

---
 src/canvas.rs | 46 +++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 39 insertions(+), 7 deletions(-)

diff --git a/src/canvas.rs b/src/canvas.rs
index 01b843e..1ee8c13 100644
--- a/src/canvas.rs
+++ b/src/canvas.rs
@@ -1,17 +1,49 @@
 use druid::kurbo::BezPath;
 
+struct Interval {
+    start_pos: f64,
+    end_pos: f64,
+}
+
+impl Interval {
+    fn new(x1: f64, x2: f64) -> Interval {
+        Interval {
+            start_pos: x1.min(x2),
+            end_pos: x1.max(x2),
+        }
+    }
+
+    fn new_ordered(start: f64, end: f64) -> Interval {
+        Interval {
+            start_pos: start,
+            end_pos: end,
+        }
+    }
+
+    fn overlaps(&self, other: &Self) -> bool {
+        self.start_pos < other.end_pos && other.start_pos < self.end_pos
+    }
+}
+
 // O(1) complexity
-// TODO: This is only a toy implementation, this is not at all correct!
+// TODO: Implement this in a way that is not so flamboyant
+// TODO: Implement intersection test for bezier curves
 fn segment_intersects_rect(segment: &druid::kurbo::PathSeg, rect: druid::Rect) -> bool {
     match segment {
-        druid::kurbo::PathSeg::Line(line) => rect.contains(line.p0) || rect.contains(line.p1),
+        druid::kurbo::PathSeg::Line(line) => {
+            // A Segment intersects the rect
+            // if their projections on the x and y line both overlap
+            let line_x_proj = Interval::new(line.p0.x, line.p1.x);
+            let line_y_proj = Interval::new(line.p0.y, line.p1.y);
 
-        druid::kurbo::PathSeg::Quad(quad) => {
-            rect.contains(quad.p0) || rect.contains(quad.p1) || rect.contains(quad.p2)
-        }
+            let rect_x_proj = Interval::new_ordered(rect.min_x(), rect.max_x());
+            let rect_y_proj = Interval::new_ordered(rect.min_y(), rect.max_y());
 
-        druid::kurbo::PathSeg::Cubic(cubic) => {
-            rect.contains(cubic.p0) || rect.contains(cubic.p1) || rect.contains(cubic.p2) || rect.contains(cubic.p3)
+            line_x_proj.overlaps(&rect_x_proj) && line_y_proj.overlaps(&rect_y_proj)
+        },
+
+        _ => {
+            unimplemented!();
         }
     }
 }