From 78a0e2a289a076814cb404190a3ae6556b9f5a1a Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Thu, 6 Apr 2017 10:29:32 -0400
Subject: math: Add pi constant.

* chickadee/math.scm (pi): New variable.
* doc/api.texi (Math): Add "Basics" subsection.
---
 chickadee/math.scm |  7 ++++++-
 doc/api.texi       | 15 +++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/chickadee/math.scm b/chickadee/math.scm
index 3bcda52..adb7d8b 100644
--- a/chickadee/math.scm
+++ b/chickadee/math.scm
@@ -16,12 +16,17 @@
 ;;; <http://www.gnu.org/licenses/>.
 
 (define-module (chickadee math)
-  #:export (square
+  #:export (pi
+            pi/2
+            square
             clamp
             min
             max)
   #:replace (min max))
 
+(define pi 3.14159265358979323846)
+(define pi/2 (/ pi 2.0))
+
 (define-inlinable (square x)
   (* x x))
 
diff --git a/doc/api.texi b/doc/api.texi
index 9c568d5..e1f6acc 100644
--- a/doc/api.texi
+++ b/doc/api.texi
@@ -406,11 +406,26 @@ with vectors and matrices and axis-aligned bounding box collision
 detection.
 
 @menu
+* Basics::                      Commonly used, miscellaneous things.
 * Vectors::                     Euclidean vectors.
 * Matrices::                    Transformation matrices.
 * Rectangles::                  Axis-aligned bounding boxes.
 @end menu
 
+@node Basics
+@subsection Basics
+
+@defvar {Scheme Variable} pi
+An essential constant for all trigonometry.  @code{@U{03C0}} is the ratio
+of a circle's circumferences to its diameter.  Since @code{@U{03C0}} is an
+irrational number, the @var{pi} in Chickadee is a mere floating point
+approximation that is ``good enough.''
+@end defvar
+
+@defvar {Scheme Variable} pi/2
+Half of @var{pi}.
+@end defvar
+
 @node Vectors
 @subsection Vectors
 
-- 
cgit v1.2.3