griffin-balance.clj

(ns griffin.spec.journal-entry
  (:require [clojure.spec.alpha :as s]))

(s/def ::id uuid?)
(s/def ::account-id uuid?)
(s/def ::type #{:credit :debit})
(s/def ::amount pos-int?)
(s/def ::line-item (s/keys :req-un [::account-id ::amount ::type]))
(s/def ::line-items (s/coll-of ::line-item :min-count 2))
(s/def ::journal-entry (s/keys :req-un [::id ::line-items]))

(def line-item-cred {
  :account-id (java.util.UUID/fromString "f81d4fae-7dec-11d0-a765-00a0c91e6bf6")
  :amount 100
  :type :credit})

(def line-item-deb {
  :account-id (java.util.UUID/fromString "f81d4fae-7dec-11d0-a765-00a0c91e6bf6")
  :amount 100
  :type :debit})

(def unbalanced-entry {
  :id (java.util.UUID/fromString "4fe5d828-6444-11e8-8222-720007e40350")
  :line-items [line-item-cred, line-item-cred, line-item-deb]})

(def balanced-entry {
  :id (java.util.UUID/fromString "4fe5d828-6444-11e8-8222-720007e40350")
  :line-items [line-item-cred, line-item-deb]})


;;
;; I'm assuming the following:
;; Each line-item we need to have a 'counter' line-item. Example: credit 100, debit 100.
;; Works only with :type is always credit or debit
;; I'm not correlating the account-id inside the items, but it would not be hard to do so
;; Could increase the test coverage for corner cases (empty collections, etc)
;; Could use a test framework :)
;; Probably this is not idiomatic, has been years since I wrote any Clojure

(defn total-amount [items] (reduce + (map :amount items)))
(defn credits-and-debits [entry] (partition-by :type (:line-items entry)))
(defn is-balanced [entry]
  (let [[credits debits] (credits-and-debits entry)]
  (and (= (count credits) (count debits))
       (= 0 (- (total-amount credits) (total-amount debits))))))


(println "Balanced entry should return true: ")
(println (is-balanced balanced-entry))

(println)

(println "Unbalanced entry should return false: ")
(println (is-balanced unbalanced-entry))

;; Each line-item we need to have one or more 'counter' line-items. Example: credit 100, debit 100 or credit 100, debit 50, debit 50

Reviewing I found that for a ledger we must have the same number of counter transactions. So this is a bug