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+;;;
+;;; Problem 14: Longest Collatz Sequence
+;;;
+
+;;; https://projecteuler.net/problem=14
+
+(use-modules (srfi srfi-1))
+
+(define (collatz n)
+  (if (even? n)
+      (/ n 2)
+      (+ (* 3 n) 1)))
+
+(define (collatz-fold n init proc)
+  (let loop ((n      n)
+             (result init))
+    (if (= n 1)
+        result
+        (let ((c (collatz n)))
+          (loop c (proc c result))))))
+
+(define (collatz-length n)
+  "Return the length of the Collatz sequence for N"
+  (collatz-fold n 1 (lambda (n length) (1+ length))))
+
+;; Just for fun.
+(define (collatz-sequence n)
+  "Return the Collatz sequence for N."
+  (reverse (collatz-fold n (list n) cons)))
+
+(define (longest-collatz up-to)
+  (let loop ((n      13)
+             (m      13)
+             (length 10))
+    (if (>= n up-to)
+        m
+        (let ((l (collatz-length n)))
+          (if (> l length)
+              (loop (1+ n) n l)
+              (loop (1+ n) m length))))))
+
+(longest-collatz (expt 10 6))