From adda6869cb2a07984b48c39fcd70ee76449c353d Mon Sep 17 00:00:00 2001
From: Elizabeth Hunt <elizabeth.hunt@simponic.xyz>
Date: Mon, 9 Oct 2023 21:08:25 -0600
Subject: updates 10/9

---
 src/approx/derivative.lisp | 10 +++++++++-
 src/approx/maceps.lisp     |  6 +++---
 src/approx/package.lisp    |  3 ++-
 3 files changed, 14 insertions(+), 5 deletions(-)

(limited to 'src')

diff --git a/src/approx/derivative.lisp b/src/approx/derivative.lisp
index 02fcb4c..8aa171a 100644
--- a/src/approx/derivative.lisp
+++ b/src/approx/derivative.lisp
@@ -1,9 +1,17 @@
 (in-package :lizfcm.approx)
 
-(defun derivative-at (f x &optional (delta 0.01))
+(defun central-derivative-at (f x &optional (delta 0.01))
   (let* ((x2 (+ x delta))
          (x1 (- x delta))
          (y2 (apply f (list x2)))
          (y1 (apply f (list x1))))
     (/ (- y2 y1)
        (- x2 x1))))
+
+(defun fwd-derivative-at (f x &optional (delta 0.01))
+  (let* ((x2 (+ x delta))
+         (x1 x)
+         (y2 (apply f (list x2)))
+         (y1 (apply f (list x1))))
+    (/ (- y2 y1)
+       (- x2 x1))))
diff --git a/src/approx/maceps.lisp b/src/approx/maceps.lisp
index debaa67..e2738e4 100644
--- a/src/approx/maceps.lisp
+++ b/src/approx/maceps.lisp
@@ -3,10 +3,10 @@
 (defun compute-maceps (f a init)
   (let ((h init)
         (err init))
-    (loop while (> err 0)
+    (loop collect (list a h err)
           do
           (setf h (/ h 2)
                 err (abs (- (funcall f (+ a h))
                             (funcall f a))))
-          when (> err 0)
-          collect (list a h err))))
+          while (> err 0))))
+
diff --git a/src/approx/package.lisp b/src/approx/package.lisp
index c11eb60..67c6a90 100644
--- a/src/approx/package.lisp
+++ b/src/approx/package.lisp
@@ -1,5 +1,6 @@
 (in-package :cl-user)
 (defpackage lizfcm.approx
   (:use :cl)
-  (:export :derivative-at
+  (:export :central-derivative-at
+           :fwd-derivative-at
            :compute-maceps))
-- 
cgit v1.2.3-70-g09d2