From 692eb45def187e2f2dbe7e40eacf27b2058c88bd Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Thu, 12 Aug 2021 07:31:36 -0400
Subject: math: Rename 2pi to tau.

---
 chickadee/graphics/path.scm | 10 +++++-----
 chickadee/math.scm          |  4 ++--
 doc/api.texi                |  2 +-
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/chickadee/graphics/path.scm b/chickadee/graphics/path.scm
index 0d218fb..fb75efd 100644
--- a/chickadee/graphics/path.scm
+++ b/chickadee/graphics/path.scm
@@ -118,8 +118,8 @@
   ;; Curves" by Joe Cridge:
   ;; https://web.archive.org/web/20170829122313/https://www.joecridge.me/content/pdf/bezier-arcs.pdf
   (define (adjust-angle angle)
-    ;; Clamp within [0, 2pi] range.
-    (let* ((clamped (mod angle 2pi))
+    ;; Clamp within [0, tau] range.
+    (let* ((clamped (mod angle tau))
            (adjusted (atan (* (/ rx ry) (tan clamped)))))
       ;; Adjust angles to counter linear scaling.
       (cond
@@ -129,11 +129,11 @@
              (<= clamped (* pi 1.5)))
         (+ adjusted pi))
        (else
-        (+ adjusted 2pi)))))
+        (+ adjusted tau)))))
   (let* ((angle-start (adjust-angle angle-start))
          (angle-end* (adjust-angle angle-end))
          (angle-end (if (> angle-start angle-end*)
-                        (+ angle-end* 2pi)
+                        (+ angle-end* tau)
                         angle-end*))
          ;; Don't bother making a curve for an angle smaller than
          ;; this.
@@ -416,7 +416,7 @@
           (close-path))))
 
 (define (regular-polygon center num-sides radius)
-  (let ((theta-step (/ 2pi num-sides)))
+  (let ((theta-step (/ tau num-sides)))
     (apply path
            (let loop ((i 0))
              (cond
diff --git a/chickadee/math.scm b/chickadee/math.scm
index c61b862..9286fe4 100644
--- a/chickadee/math.scm
+++ b/chickadee/math.scm
@@ -18,7 +18,7 @@
 (define-module (chickadee math)
   #:export (pi
             pi/2
-            2pi
+            tau
             cotan
             clamp
             min
@@ -30,7 +30,7 @@
 
 (define pi 3.14159265358979323846)
 (define pi/2 (/ pi 2.0))
-(define 2pi (* pi 2.0))
+(define tau 6.283185307179586) ;; AKA 2pi
 
 (define-inlinable (cotan z)
   "Return the cotangent of Z."
diff --git a/doc/api.texi b/doc/api.texi
index 96ac394..68cffef 100644
--- a/doc/api.texi
+++ b/doc/api.texi
@@ -581,7 +581,7 @@ approximation that is ``good enough.''
 Half of @var{pi}.
 @end defvar
 
-@defvar 2pi
+@defvar tau
 Twice @var{pi}.
 @end defvar
 
-- 
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