Add Problem 16 solution.HEADmaster

1 files changed, 22 insertions, 0 deletions

diff --git a/problem-16.scm b/problem-16.scm
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+;;;
+;;; Problem 16: Power digit sum
+;;;
+
+;;; https://projecteuler.net/problem=16
+
+(define (digit-sum n)
+  (let loop ((m   1)
+             (sum 0))
+    (if (> (expt 10 (1- m)) n)
+        sum
+        (loop (1+ m) (+ sum (digit m n))))))
+
+(define (digit m n)
+  "Return the Mth digit of N."
+  ;; Tenths place, hundreds place, etc.
+  (let ((place (expt 10 (1- m))))
+    (/ (- (modulo n (expt 10 m))
+          (modulo n place))
+       place)))
+
+(digit-sum (expt 2 1000))