Day 15

input/15.txt

@@ -0,0 +1,26 @@
+Sensor at x=2483411, y=3902983: closest beacon is at x=2289579, y=3633785
+Sensor at x=3429446, y=303715: closest beacon is at x=2876111, y=-261280
+Sensor at x=666423, y=3063763: closest beacon is at x=2264411, y=2779977
+Sensor at x=3021606, y=145606: closest beacon is at x=2876111, y=-261280
+Sensor at x=2707326, y=2596893: closest beacon is at x=2264411, y=2779977
+Sensor at x=3103704, y=1560342: closest beacon is at x=2551409, y=2000000
+Sensor at x=3497040, y=3018067: closest beacon is at x=3565168, y=2949938
+Sensor at x=1708530, y=855013: closest beacon is at x=2551409, y=2000000
+Sensor at x=3107437, y=3263465: closest beacon is at x=3404814, y=3120160
+Sensor at x=2155249, y=2476196: closest beacon is at x=2264411, y=2779977
+Sensor at x=3447897, y=3070850: closest beacon is at x=3404814, y=3120160
+Sensor at x=2643048, y=3390796: closest beacon is at x=2289579, y=3633785
+Sensor at x=3533132, y=3679388: closest beacon is at x=3404814, y=3120160
+Sensor at x=3683790, y=3017900: closest beacon is at x=3565168, y=2949938
+Sensor at x=1943208, y=3830506: closest beacon is at x=2289579, y=3633785
+Sensor at x=3940100, y=3979653: closest beacon is at x=2846628, y=4143786
+Sensor at x=3789719, y=1225738: closest beacon is at x=4072555, y=1179859
+Sensor at x=3939775, y=578381: closest beacon is at x=4072555, y=1179859
+Sensor at x=3880152, y=3327397: closest beacon is at x=3404814, y=3120160
+Sensor at x=3280639, y=2446475: closest beacon is at x=3565168, y=2949938
+Sensor at x=2348869, y=2240374: closest beacon is at x=2551409, y=2000000
+Sensor at x=3727441, y=2797456: closest beacon is at x=3565168, y=2949938
+Sensor at x=3973153, y=2034945: closest beacon is at x=4072555, y=1179859
+Sensor at x=38670, y=785556: closest beacon is at x=311084, y=-402911
+Sensor at x=3181909, y=2862960: closest beacon is at x=3565168, y=2949938
+Sensor at x=3099490, y=3946226: closest beacon is at x=2846628, y=4143786

src/15.rkt

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+#lang curly-fn racket
+
+(require "../lib.rkt"
+         data/integer-set)
+
+(define input
+  (for/list ([positions (problem-input 15)])
+    (match-define (pregexp "Sensor at x=(-?\\d+), y=(-?\\d+): closest beacon is at x=(-?\\d+), y=(-?\\d+)" sb)
+      positions)
+    (map string->number (rest sb))))
+
+(define y 2000000)
+(define bound 4000000)
+
+(define (manhat x1 y1 x2 y2)
+  (+ (abs (- x1 x2)) (abs (- y1 y2))))
+
+(define sensors
+  (for/list ([sb input])
+    (match-define `(,sx ,sy ,bx ,by) sb)
+    (list sx sy (manhat sx sy bx by))))
+
+(define (impossible? x y)
+  (for/or ([sensor sensors])
+    (match-define `(,sx ,sy ,d) sensor)
+    (<= (manhat sx sy x y) d)))
+
+(define part1
+  (for/fold ([impossibles (make-range)]
+             #:result (count impossibles))
+            ([sb input])
+    (match-define `(,sx ,sy ,bx ,by) sb)
+    (define span (- (manhat sx sy bx by) (abs (- sy y))))
+    (if (> span 0)
+        (~> impossibles
+            (union (make-range (- sx span) (+ sx span)))
+            (subtract (if (= by y) (make-range bx) (make-range))))
+        impossibles)))
+
+(define part2
+  (for/or ([sensor sensors])
+    (match-define `(,sx ,sy ,d) sensor)
+    (define d* (add1 d))
+    (for/or ([x (in-inclusive-range (- sx d*) (+ sx d*))]
+             #:when (<= 0 x bound))
+      (define +y (+ sy (- d* (- sx x))))
+      (define -y (- sy (- d* (- sx x))))
+      (define (freq y)
+        (and (<= 0 y bound)
+             (not (impossible? x y))
+             (+ (* x 4000000) y)))
+      (or (freq +y) (freq -y)))))
+
+(show-solution part1 part2)