From be1b432a67d334b7f9fd19735827c1698b085970 Mon Sep 17 00:00:00 2001
From: rsiddharth <s@ricketyspace.net>
Date: Sun, 5 Apr 2020 16:29:46 -0400
Subject: Add (net ricketyspace sicp two forty).

---
 net/ricketyspace/sicp/two/forty.scm | 55 +++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 net/ricketyspace/sicp/two/forty.scm

(limited to 'net/ricketyspace/sicp/two/forty.scm')

diff --git a/net/ricketyspace/sicp/two/forty.scm b/net/ricketyspace/sicp/two/forty.scm
new file mode 100644
index 0000000..e7c2851
--- /dev/null
+++ b/net/ricketyspace/sicp/two/forty.scm
@@ -0,0 +1,55 @@
+;;;; License: CC0-1.0
+
+(define-module (net ricketyspace sicp two forty)
+  #:use-module (net ricketyspace sicp one twentyseven)
+  #:export (unique-pairs
+            prime-sum-pairs))
+
+
+(define (accumulate op initial sequence)
+  (if (null? sequence)
+      initial
+      (op (car sequence)
+          (accumulate op initial (cdr sequence)))))
+
+
+(define (enumerate-interval low high)
+  (if (> low high)
+      '()
+      (cons low (enumerate-interval (+ low 1) high))))
+
+
+(define (prime-sum? pair)
+  (prime? (+ (car pair) (cadr pair))))
+
+
+(define (make-pair-sum pair)
+  (list (car pair) (cadr pair) (+ (car pair) (cadr pair))))
+
+
+(define (flatmap proc seq)
+  (accumulate append '() (map proc seq)))
+
+
+(define (unique-pairs n)
+  (flatmap (lambda (i)
+             (map (lambda (j) (list i j))
+                  (enumerate-interval 1 (- i 1))))
+           (enumerate-interval 1 n)))
+
+
+(define (prime-sum-pairs n)
+  (map make-pair-sum
+       (filter prime-sum?
+               (unique-pairs n))))
+
+
+;;;
+;;; Guile REPL
+;;;
+;;; scheme@(guile-user)> ,re (net ricketyspace sicp two forty)
+;;; scheme@(guile-user)> (unique-pairs 6)
+;;; $8 = ((2 1) (3 1) (3 2) (4 1) (4 2) (4 3) (5 1) (5 2) (5 3) (5 4) (6 1) (6 2) (6 3) (6 4) (6 5))
+;;; scheme@(guile-user)> (prime-sum-pairs 6)
+;;; $9 = ((2 1 3) (3 2 5) (4 1 5) (4 3 7) (5 2 7) (6 1 7) (6 5 11))
+;;;
-- 
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