Add anagram.lisp.

misc/anagram.lisp

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+;;; anagram.lisp
+;;; 2018-01-31
+;;;
+;;; Lisp implementation of this tweet:
+;;;     https://twitter.com/fermatslibrary/status/958700402647674880.
+;;;
+;;; Fun fact: this uses the prime list generated by
+;;;     (prime-numbers 26 3)
+;;; using prime.rkt in the par/ toplevel directory. Why start at 3? It
+;;; was a typo.
+
+(defpackage #:anagram
+  (:use #:cl)
+  (:export #:anagram-p))
+(in-package #:anagram)
+
+(defun deconstruct (w)
+  "Convert a string into a list of characters (instead of an array of
+characters)."
+  (loop for c across w collect c))
+
+(defparameter *mapping*
+  (pairlis
+   (deconstruct "abcdefghijklmnopqrstuvwxyz")
+   '(3 5 7 11 13 17 19 23 29 31 37 41 43 47
+     53 59 61 67 71 73 79 83 89 97 101 103)))
+
+(defun lookup (c)
+  "A hack to convert a character to its relevant prime."
+  (cdr (assoc c *mapping*)))
+
+(defun word-number (word)
+  "Convert a word to a number by multiplying the associated prime
+number for all the letters in the word."
+  (apply #'* (mapcar #'lookup
+		     (deconstruct (string-downcase word)))))
+
+(defun anagram-p (&rest words)
+  "Given a list of words, return T if they are all anagrams of each
+other."
+   (apply #'= (mapcar #'word-number words)))