From 5da9327e128ff9eb46983b7769e98528ce90fbc0 Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Sat, 18 Aug 2018 17:35:33 -0700
Subject: [PATCH] Implement factorial powered by the Gamma function.

---
 external/libabacus         |  2 +-
 include/functions.hpp      |  1 +
 src/main.cpp               |  2 ++
 src/operator_functions.cpp | 16 ++++++++++++++++
 4 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/external/libabacus b/external/libabacus
index 8c9acaf..0172091 160000
--- a/external/libabacus
+++ b/external/libabacus
@@ -1 +1 @@
-Subproject commit 8c9acafc932541d9a1c09f1fa0a93ac992254309
+Subproject commit 01720914e0899023747c94ef7cf4c5d17782d7dd
diff --git a/include/functions.hpp b/include/functions.hpp
index d12a276..20f2357 100644
--- a/include/functions.hpp
+++ b/include/functions.hpp
@@ -55,6 +55,7 @@ FUNCTION(gt);
 FUNCTION(gte);
 
 FUNCTION(negate);
+FUNCTION(factorial);
 
 FUNCTION(ln);
 FUNCTION(exp);
diff --git a/src/main.cpp b/src/main.cpp
index 6c5e57a..6c9bf97 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -55,6 +55,7 @@ int main() {
     ab.add_function("divide", function_divide, "(num, num)->num");
     ab.add_function("pow", function_pow, "(num, num)->num");
     ab.add_function("negate", function_negate, "(num)->num");
+    ab.add_function("factorial", function_factorial, "(num)->num");
 
     ab.add_function("ln", function_ln, "(num)->num");
     ab.add_function("exp", function_exp, "(num)->num");
@@ -78,6 +79,7 @@ int main() {
     ab.add_operator_infix("/", "divide", -1, 3);
     ab.add_operator_infix("^", "pow", 1, 3);
     ab.add_operator_prefix("-", "negate");
+    ab.add_operator_postfix("!", "factorial");
 
     while(!close_requested) {
         char* data = readline(" > ");
diff --git a/src/operator_functions.cpp b/src/operator_functions.cpp
index d13db0c..acc31ad 100644
--- a/src/operator_functions.cpp
+++ b/src/operator_functions.cpp
@@ -7,6 +7,22 @@ FUNCTION_MPFR2(divide, div)
 FUNCTION_MPFR2(pow, pow);
 
 FUNCTION_MPFR(negate, neg);
+FUNCTION(factorial) {
+    number* left = (number*) libab_unwrap_param(params, 0);   
+    mpfr_t plus_one;
+    mpfr_t gamma;
+
+    mpfr_init2(plus_one, PRECISION);
+    mpfr_init2(gamma, PRECISION);
+    mpfr_add_ui(plus_one, left->value, 1, MPFR_RNDN);
+    mpfr_gamma(gamma, plus_one, MPFR_RNDN);
+
+    number* to_return = new number(std::move(gamma));
+    ref return_value = create_value<number>(ab, to_return);
+    libab_ref_copy(return_value, into);
+
+    return LIBAB_SUCCESS;
+}
 
 FUNCTION_COMPARE(lt, <);
 FUNCTION_COMPARE(lte, <=);