4Clojure Problem #178 - Revised

I got some great feedback on my last post and decided to re-work my solution a bit. I think this turned out much nicer.

The main things I improved:

• My first solution involved “predicates” (technically they may not have been true predicates, since they returned a keyword or nil, instead of true or false) for each possible hand within one big let binding, and then at the end I removed all the nil values and returned the first possible hand in the list. This was fairly readable, but there were two problems:
  1. The code could be more concise if I didn’t have to wrap each “predicate” in an if statement to either return the keyword or nil. Using when instead would help to some extent (since it returns either a value or nil), but a better solution altogether came from the other problem, which was that
  2. Wrapping all of my “predicates” in a let binding forces them all to be evaluated, even if we may only need to evaluate a handful of them. Using or or cond instead allows you to take advantage of short-circuiting and stop evaluating once you reach a predicate that is met, which is better from a performance standpoint. Since the first possibility we check for (i.e. the highest ranking possible hand) is a straight flush, I decided to include the predicates flush? and straight? in my let binding and then refer to them in the main part of my code within a cond statement.
• It turns out that my previous definition of the flush? predicate, (= 1 (count (set suits))), could be simplified to (apply = suits), thanks to = being a variable-arity function in Clojure.
• My first solution repeated this piece of code an awful lot: (some (fn [[r num]] (>= num n)) (frequencies ranks)). This was a classic opportunity to apply the DRY principle, so I added a rank-freqs symbol to my let binding. To make it easier to use, I defined it as (sort (vals (frequencies ranks))), so that the value of rank-freqs is always[2 3] for a full house, for instance.

Without further ado, here is my revised solution for Clojure Problem #178:

(fnbest-hand[card-strings](let[card-parser(fn[[sr]](let[suit({\S:spade,\H:heart,\D:diamond,\C:club}s)rank(if(>(Character/digitr10)-1)(-(Character/digitr10)2)({\T8,\J9,\Q10,\K11,\A12}r))]{:suitsuit,:rankrank}))cards(mapcard-parsercard-strings)suits(map:suitcards)ranks(map:rankcards)rank-freqs(sort(vals(frequenciesranks)))flush?(apply=suits)straight?(let[aces-high(sortranks)aces-low(sort(replace{12-1}ranks))](or(=aces-high(take5(iterateinc(firstaces-high))))(=aces-low(take5(iterateinc(firstaces-low))))))](cond(andflush?straight?):straight-flush(=rank-freqs[14]):four-of-a-kind(=rank-freqs[23]):full-houseflush?:flushstraight?:straight(some#{3}rank-freqs):three-of-a-kind(=rank-freqs[122]):two-pair(some#{2}rank-freqs):pair:else:high-card)))