From 5eb4de4285e42b7c792d9eec241c2c92fe637f10 Mon Sep 17 00:00:00 2001
From: Fabian Homborg <FHomborg@gmail.com>
Date: Wed, 26 Aug 2020 17:30:33 +0200
Subject: [PATCH] math: Implement tau

---
 doc_src/cmds/math.rst | 2 +-
 src/tinyexpr.cpp      | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/doc_src/cmds/math.rst b/doc_src/cmds/math.rst
index 468d1b50a..8669b2cbe 100644
--- a/doc_src/cmds/math.rst
+++ b/doc_src/cmds/math.rst
@@ -61,7 +61,7 @@ Constants
 ``math`` knows the following constants:
 
 - ``e`` - Euler's number.
-- ``pi`` - You know that one. Half of Tau. (Tau is not implemented)
+- ``pi`` - You know that one. Half of Tau. (``tau`` is also implemented)
 
 Use them without a leading ``$`` - ``pi - 3`` should be about 0.
 
diff --git a/src/tinyexpr.cpp b/src/tinyexpr.cpp
index a44503b2e..8d0716393 100644
--- a/src/tinyexpr.cpp
+++ b/src/tinyexpr.cpp
@@ -137,6 +137,7 @@ void te_free(te_expr *n) {
 }
 
 static constexpr double pi() { return M_PI; }
+static constexpr double tau() { return 2 * M_PI; }
 static constexpr double e() { return M_E; }
 
 static double fac(double a) { /* simplest version of fac */
@@ -209,7 +210,9 @@ static const te_builtin functions[] = {
     {"sinh", reinterpret_cast<const void *>(static_cast<te_fun1>(sinh)), TE_FUNCTION1},
     {"sqrt", reinterpret_cast<const void *>(static_cast<te_fun1>(sqrt)), TE_FUNCTION1},
     {"tan", reinterpret_cast<const void *>(static_cast<te_fun1>(tan)), TE_FUNCTION1},
-    {"tanh", reinterpret_cast<const void *>(static_cast<te_fun1>(tanh)), TE_FUNCTION1}};
+    {"tanh", reinterpret_cast<const void *>(static_cast<te_fun1>(tanh)), TE_FUNCTION1},
+    {"tau", reinterpret_cast<const void *>(static_cast<te_fun0>(tau)), TE_FUNCTION0},
+};
 
 static const te_builtin *find_builtin(const char *name, int len) {
     const auto end = std::end(functions);