From 84b81c888893ea6f321c26382bc18330b6687ee6 Mon Sep 17 00:00:00 2001
From: Ryan Brott <rcbrott@gmail.com>
Date: Fri, 5 Jun 2020 06:25:27 -0700
Subject: [PATCH] Fix bad numerical derivative (fixes #79)

---
 .../ftc/teamcode/util/RegressionUtil.java             | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/TeamCode/src/main/java/org/firstinspires/ftc/teamcode/util/RegressionUtil.java b/TeamCode/src/main/java/org/firstinspires/ftc/teamcode/util/RegressionUtil.java
index 47eff956..fbe7823c 100644
--- a/TeamCode/src/main/java/org/firstinspires/ftc/teamcode/util/RegressionUtil.java
+++ b/TeamCode/src/main/java/org/firstinspires/ftc/teamcode/util/RegressionUtil.java
@@ -52,11 +52,14 @@ public AccelResult(double kA, double rSquare) {
      */
     private static List<Double> numericalDerivative(List<Double> x, List<Double> y) {
         List<Double> deriv = new ArrayList<>(x.size());
-        deriv.add(0.0);
-        for (int i = 0; i < x.size(); i++) {
-            deriv.add((y.get(i + 1) - y.get(i - 1)) / (x.get(i + 1) - x.get(i - 1)));
+        for (int i = 1; i < x.size() - 1; i++) {
+            deriv.add(
+                    (y.get(i + 1) - y.get(i - 1)) /
+                    (x.get(i + 1) - x.get(i - 1))
+            );
         }
-        deriv.set(0, deriv.get(1));
+        // copy endpoints to pad output
+        deriv.add(0, deriv.get(0));
         deriv.add(deriv.get(deriv.size() - 1));
         return deriv;
     }