From d21d7520046d605fa34eb8003393ba0450e4dd3a Mon Sep 17 00:00:00 2001
From: Simon Gardling <titaniumtown@gmail.com>
Date: Fri, 4 Mar 2022 09:23:48 -0500
Subject: [PATCH] derivative regenerative caching

---
 src/function.rs | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/function.rs b/src/function.rs
index 1fd34b1..c76ca34 100644
--- a/src/function.rs
+++ b/src/function.rs
@@ -132,7 +132,26 @@ impl Function {
                     })
                     .collect(),
             );
-            self.derivative_cache = None; // TODO: setup this caching system for derivatives
+
+            if self.derivative_cache.is_some() {
+                let derivative_cache = self.derivative_cache.as_ref().unwrap();
+
+                self.derivative_cache = Some(
+                    (0..=self.pixel_width)
+                        .map(|x| (x as f64 / resolution as f64) + min_x)
+                        .map(|x| {
+                            if let Some(i) = x_data.get_index(x) {
+                                derivative_cache[i]
+                            } else {
+                                let (x1, x2) = (x - EPSILON, x + EPSILON);
+                                let (y1, y2) = (self.run_func(x1), self.run_func(x2));
+                                let slope = (y2 - y1) / (EPSILON * 2.0);
+                                Value::new(x, slope)
+                            }
+                        })
+                        .collect(),
+                );
+            }
         } else {
             self.back_cache = None;
             self.derivative_cache = None;