From 344d892ae09571d384a1a63ec4ae49ab870b208e Mon Sep 17 00:00:00 2001
From: Alain Emilia Anna Zscheile <fogti+devel@ytrizja.de>
Date: Sun, 5 Jan 2025 01:01:20 +0100
Subject: [PATCH] chore,docs(math): rename 'cross_product' to
 'perp_dot_product', add some documentation

---
 src/math/mod.rs      | 16 +++++++++-------
 src/math/tangents.rs |  6 +++---
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/src/math/mod.rs b/src/math/mod.rs
index fdc51d7..f0bb08a 100644
--- a/src/math/mod.rs
+++ b/src/math/mod.rs
@@ -87,12 +87,12 @@ pub fn intersect_circle_segment(circle: &Circle, segment: &Line) -> Vec<Point> {
 }
 
 pub fn between_vectors(p: Point, from: Point, to: Point) -> bool {
-    let cross = cross_product(from, to);
+    let cross = perp_dot_product(from, to);
 
     if cross > 0.0 {
-        cross_product(from, p) >= 0.0 && cross_product(p, to) >= 0.0
+        perp_dot_product(from, p) >= 0.0 && perp_dot_product(p, to) >= 0.0
     } else if cross < 0.0 {
-        cross_product(from, p) >= 0.0 || cross_product(p, to) >= 0.0
+        perp_dot_product(from, p) >= 0.0 || perp_dot_product(p, to) >= 0.0
     } else {
         false
     }
@@ -229,22 +229,24 @@ impl ops::MulAssign<f64> for NormalizedAngle {
 pub fn angle_between(v1: Point, v2: Point) -> NormalizedAngle {
     NormalizedAngle::atan2(geo::point! {
         x: dot_product(v1, v2),
-        y: cross_product(v1, v2)
+        y: perp_dot_product(v1, v2)
     })
 }
 
-pub fn seq_cross_product(start: Point, stop: Point, reference: Point) -> f64 {
+pub fn seq_perp_dot_product(start: Point, stop: Point, reference: Point) -> f64 {
     let dx1 = stop.x() - start.x();
     let dy1 = stop.y() - start.y();
     let dx2 = reference.x() - stop.x();
     let dy2 = reference.y() - stop.y();
-    cross_product((dx1, dy1).into(), (dx2, dy2).into())
+    perp_dot_product((dx1, dy1).into(), (dx2, dy2).into())
 }
 
 pub fn dot_product(v1: Point, v2: Point) -> f64 {
     v1.x() * v2.x() + v1.y() * v2.y()
 }
 
-pub fn cross_product(v1: Point, v2: Point) -> f64 {
+/// This is often called [perp dot product](https://mathworld.wolfram.com/PerpDotProduct.html),
+/// or "2D cross product".
+pub fn perp_dot_product(v1: Point, v2: Point) -> f64 {
     v1.x() * v2.y() - v1.y() * v2.x()
 }
diff --git a/src/math/tangents.rs b/src/math/tangents.rs
index 484d8de..b36ed2b 100644
--- a/src/math/tangents.rs
+++ b/src/math/tangents.rs
@@ -7,7 +7,7 @@ use geo::{geometry::Point, point, Line};
 use specctra_core::math::Circle;
 use thiserror::Error;
 
-use super::seq_cross_product;
+use super::seq_perp_dot_product;
 
 #[derive(Error, Debug, Clone, Copy, PartialEq)]
 #[error("no tangents for {0:?} and {1:?}")] // TODO add real error message
@@ -101,7 +101,7 @@ pub fn tangent_segments(
         .filter_map(move |tangent_point_pair| {
             if let Some(cw1) = cw1 {
                 let cross1 =
-                    seq_cross_product(tangent_point_pair.0, tangent_point_pair.1, circle1.pos);
+                    seq_perp_dot_product(tangent_point_pair.0, tangent_point_pair.1, circle1.pos);
 
                 if (cw1 && cross1 <= 0.0) || (!cw1 && cross1 >= 0.0) {
                     return None;
@@ -110,7 +110,7 @@ pub fn tangent_segments(
 
             if let Some(cw2) = cw2 {
                 let cross2 =
-                    seq_cross_product(tangent_point_pair.0, tangent_point_pair.1, circle2.pos);
+                    seq_perp_dot_product(tangent_point_pair.0, tangent_point_pair.1, circle2.pos);
 
                 if (cw2 && cross2 >= 0.0) || (!cw2 && cross2 <= 0.0) {
                     return None;