day 11, part 1

11-dumbo-octopus/code.scm

@@ -0,0 +1,88 @@
+(load "../common.scm")
+
+; matrix visitor
+; calls function with all coordinates of matrix and value at corresponding point
+; value returned by function is set at the visiting coordinate
+(define (visit-matrix! matrix function)
+  (let [(data (matrix-data matrix))
+	(width-1 (- (matrix-width matrix) 1))
+	(height-1 (- (matrix-height matrix) 1))]
+    (let y-loop [(y 0)]
+      (let x-loop [(x 0)]
+	(let [(value (vector-ref (vector-ref data y) x))]
+	  (vector-set! (vector-ref data y) x
+		       (function x y value))
+	  (when (< x width-1)
+	    (x-loop (+ x 1)))))
+      (when (< y height-1)
+	(y-loop (+ y 1))))))
+
+; increase by 1 value at x,y in given matrix
+; return that value too
+(define (matrix-inc! matrix x y)
+  (let [(v (+ 1 (matrix-get matrix x y 0)))]
+    (matrix-set! matrix x y v)
+    v))
+
+(define-record-type vec2i (fields x y))
+
+; flash given octopus
+; ie. increase energy of neighbor ones
+; returns list of octopuses that now need to flash too
+(define (flash! matrix x y)
+  (let [(extra '())]
+    (let y-loop [(yy (- y 1))]
+      (let x-loop [(xx (- x 1))]
+	(when (not (and (= xx x) (= yy y)))
+	  (when (< (matrix-get matrix xx yy 0) 10)
+	    (when (= (matrix-inc! matrix xx yy) 10)
+	      (set! extra (cons (make-vec2i xx yy) extra)))))
+	(when (< xx (+ x 1))
+	  (x-loop (+ xx 1))))
+      (when (< yy (+ y 1))
+	(y-loop (+ yy 1))))
+    extra))
+
+(define (step! matrix)
+  (let [(flashes '())]
+    ; increase energy of all octopuses
+    (visit-matrix! 
+      matrix
+      (lambda (x y energy)
+	(when (= energy 9)
+	  (set! flashes (cons (make-vec2i x y) flashes)))
+	(+ 1 energy)))
+    ; recursively flash octopuses
+    (let loop [(flash flashes)]
+      (when (not (null? flash))
+	(loop (append (cdr flash)
+		      (flash! matrix (vec2i-x (car flash))
+			      (vec2i-y (car flash)))))))
+    ; drain all octopuses that flashed
+    (visit-matrix! 
+      matrix
+      (lambda (x y energy)
+	(if (< energy 10) energy 0)))
+    ; count octopuses that flashed
+    (vector-fold
+      (lambda (y sum line)
+	(+ sum
+	   (vector-fold
+	     (lambda (x sum value)
+	       (if (= value 0) (+ sum 1) sum))
+	     0 line)))
+      0 (matrix-data matrix))))
+
+(call-with-input-file
+  "input"
+  (lambda (file)
+    (let [(matrix (load-matrix file))
+	  (flash-count 0)]
+      (let loop [(i 0)]
+	(set! flash-count (+ flash-count
+			     (step! matrix)))
+	(when (< i 99)
+	  (loop (+ i 1))))
+      (printf "part 1:~% ~a flashes after a hundred steps."
+	      flash-count))))
+

11-dumbo-octopus/input

@@ -0,0 +1,10 @@
+1326253315
+3427728113
+5751612542
+6543868322
+4422526221
+2234325647
+1773174887
+7281321674
+6562513118
+4824541522