Calculate Pi using Chudnovsky's Algorithm Synchronously and Concurrently Using Clojure

core.clj

(ns clojure-chudnovskys.core)

(require '[clojure.math.numeric-tower :as math :refer [expt]])
(import '(java.util.concurrent Executors))

(defn factorial [n]
  (reduce *' (range 1 n))) ; *' automatically handles bigints for us

(defn calc-denominator [i]
  (*' (factorial (*' 3 i)) (expt (factorial i) 3) (expt 262537412640768000 (+' 0.5 i))))

(defn calc-numerator [i]
  (*' (expt -1 i) (factorial (*' 6 i)) (+' (*' 545140134 i) 13591409)))

(defn -main [& args]
  (println "Enter number of iterations: ")
  (def iterations (Integer/parseInt (read-line)))
  ; Synchronous calculation
  (def sum
    (reduce +'
      (for [i (range iterations)]
        (/ (calc-numerator i)
        (calc-denominator i))
      )
    )
  )
  (println (/ 1 (*' 12 sum)))
  ; Concurrent calculation using STM
  (def csum (ref 0))
  (def pool (Executors/newFixedThreadPool 8))
  ; Our tasks are functions that take the current iteration number and return a closure that does
  ; the work
  (def tasks
    (map
      (fn [i]
        (fn []; this is what the map function returns -- another function to be worked by thread exc
          (def itervalue (/ (calc-numerator i) (calc-denominator i)))
            (dosync ; Start our transaction
            (alter csum + itervalue)
           ) ; End our transaction
         )
       ), (range iterations) ; -> (0..iterations)
    ) ;; basically, for i in 0..iterations, return a closure processing i and += csum
  )
  (doseq [future (.invokeAll pool tasks)]
    (.get future))
  (.shutdown pool)
  (println (/ 1 (*' 12 (deref csum))))
)

project.clj

(defproject clojure-chudnovskys "0.1.0-SNAPSHOT"
  :description "Calculate Pi"
  :license {:name "EPL-2.0 OR GPL-2.0-or-later WITH Classpath-exception-2.0"
            :url "https://www.eclipse.org/legal/epl-2.0/"}
  :dependencies [[org.clojure/clojure "1.10.1"], [org.clojure/math.numeric-tower "0.0.4"]]
  :repl-options {:init-ns clojure-chudnovskys.core}
  :main clojure-chudnovskys.core
 )