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Task/Gamma-function/Oforth/gamma-function.oforth

@@ -0,0 +1,17 @@
+import: math
+
+[
+   676.5203681218851,  -1259.1392167224028, 771.32342877765313,
+  -176.61502916214059, 12.507343278686905, -0.13857109526572012,
+   9.9843695780195716e-6, 1.5056327351493116e-7
+] const: Gamma.Lanczos
+
+: gamma(z)
+| i t |
+   z 0.5 < ifTrue: [ Pi dup z * sin 1.0 z - gamma * / return ]
+   z 1.0 - ->z
+   0.99999999999980993 Gamma.Lanczos size loop: i [ i Gamma.Lanczos at z i + / + ]
+   z Gamma.Lanczos size + 0.5 - ->t
+   2 Pi * sqrt *
+   t z 0.5 + powf *
+   t neg exp * ;