Day 3. It's ugly don't look at it.
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R1000,D940,L143,D182,L877,D709,L253,U248,L301,U434,R841,U715,R701,U92,R284,U115,R223,U702,R969,U184,L992,U47,L183,U474,L437,D769,L71,U96,R14,U503,R144,U432,R948,U96,L118,D696,R684,U539,L47,D851,L943,U606,L109,D884,R157,U946,R75,U702,L414,U347,R98,D517,L963,D177,R467,D142,L845,U427,R357,D528,L836,D222,L328,U504,R237,U99,L192,D147,L544,D466,R765,U845,L267,D217,L138,U182,R226,U466,R785,U989,R55,D822,L101,U292,R78,U962,R918,U218,L619,D324,L467,U885,L658,U890,L764,D747,R369,D930,L264,D916,L696,U698,R143,U537,L922,U131,R141,D97,L76,D883,R75,D657,R859,U503,R399,U33,L510,D318,L455,U128,R146,D645,L147,D651,L388,D338,L998,U321,L982,U150,R123,U834,R913,D200,L455,D479,L38,U860,L471,U945,L946,D365,L377,U816,R988,D597,R181,D253,R744,U472,L345,U495,L187,D443,R924,D536,R847,U430,L145,D827,L152,D831,L886,D597,R699,D751,R638,D580,L488,D566,L717,D220,L965,D587,L638,D880,L475,D165,L899,U388,R326,D568,R940,U550,R788,D76,L189,D641,R629,D383,L272,D840,L441,D709,L424,U158,L831,D576,R96,D401,R425,U525,L378,D907,L645,U609,L336,D232,L259,D280,L523,U938,R190,D9,L284,U941,L254,D657,R572,U443,L850,U508,L742,D661,L977,U910,L190,U626,R140,U762,L673,U741,R317,D518,R111,U28,R598,D403,R465,D684,R79,U725,L556,U302,L367,U306,R632,D550,R89,D292,R561,D84,L923,D109,L865,D880,L387,D24,R99,U934,L41,U29,L225,D12,L818,U696,R652,U327,L69,D773,L618,U803,L433,D467,R840,D281,R161,D400,R266,D67,L205,D94,R551,U332,R938,D759,L437,D515,L480,U774,L373,U478,R963,D863,L735,U138,L580,U72,L770,U968,L594
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L990,D248,L833,U137,L556,U943,R599,U481,R963,U812,L825,U421,R998,D847,R377,D19,R588,D657,R197,D354,L548,U849,R30,D209,L745,U594,L168,U5,L357,D135,R94,D686,R965,U838,R192,U428,L861,U354,R653,U543,L633,D508,R655,U575,R709,D53,L801,D709,L92,U289,L466,D875,R75,D448,R576,D972,L77,U4,L267,D727,L3,D687,R743,D830,L803,D537,L180,U644,L204,U407,R866,U886,R560,D848,R507,U470,R38,D652,R806,D283,L836,D629,R347,D679,R609,D224,L131,D616,L687,U181,R539,D829,L598,D55,L806,U208,R886,U794,L268,D365,L145,U690,R50,D698,L140,D512,L551,U845,R351,U724,R405,D245,L324,U181,L824,U351,R223,D360,L687,D640,L653,U158,R786,D962,R931,D151,R939,D34,R610,U684,L694,D283,R402,D253,R388,D195,R732,U809,R246,D571,L820,U742,L507,U967,L886,D693,L273,U558,L914,D122,R146,U788,R83,U149,R241,U616,R326,U40,L192,D845,L577,U803,L668,D443,R705,D793,R443,D883,L715,U757,R767,D360,L289,D756,R696,D236,L525,U872,L332,U203,L152,D234,R559,U191,R340,U926,L746,D128,R867,D562,L100,U445,L489,D814,R921,D286,L378,D956,L36,D998,R158,D611,L493,U542,R932,U957,R55,D608,R790,D388,R414,U670,R845,D394,L572,D612,R842,U792,R959,U7,L285,U769,L410,D940,L319,D182,R42,D774,R758,D457,R10,U82,L861,D901,L310,D217,R644,U305,R92,U339,R252,U460,R609,D486,R553,D798,R809,U552,L183,D238,R138,D147,L343,D597,L670,U237,L878,U872,R789,U268,L97,D313,R22,U343,R907,D646,L36,D516,L808,U622,L927,D982,L810,D149,R390,U101,L565,U488,L588,U426,L386,U305,R503,U227,R969,U201,L698,D850,R800,D961,R387,U632,R543,D541,R750,D174,R543,D237,R487,D932,R220
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#lang racket
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(require "../lib.rkt")
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;; wire : (listof path)
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;; path : (symbol . number)
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(define-values (wire1 wire2)
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(let* ([input (problem-input 3)]
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[string->path (λ (str) (cons (string->symbol (substring str 0 1))
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(string->number (substring str 1))))]
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[string->wire (λ (str) (map string->path (string-split str ",")))]
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[wire1 (string->wire (first input))]
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[wire2 (string->wire (second input))])
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(values wire1 wire2)))
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;; step-path : boolean -> path -> (number . number) -> hashtable -> procedure -> (number . number)
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;; first-wire? : Indicates whether we're stepping for the first or second wire.
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;; path : A pair (s: symbol . n: number), where s indicates the direction we step
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;; (R, L, U, D for right (+x), left (-x), up (+y), down (-y))
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;; and n indicates the number of units we step in that direction
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;; position : A pair (x . y) that indicates the current location
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;; hashtable : A map from positions to ((boolean . number) . (boolean . number))that indicate
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;; the total number of steps to get to that position and
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;; whether the first or second wire has visited that position
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;; Returns the new position after stepping
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(define (step-path first-wire? path pos-count ht)
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(let* ([position (car pos-count)]
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[count (cdr pos-count)]
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[x (car position)]
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[y (cdr position)]
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[update (λ (dir)
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(λ (i count)
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(let* ([key (match dir
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['x (cons i y)]
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['y (cons x i)])]
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[value (hash-ref ht key '((#f . 0) . (#f . 0)))]
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[new-count (add1 count)]
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[new-value
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(if first-wire?
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(cons `(#t . ,new-count) (cdr value))
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(cons (car value) `(#t . ,new-count)))])
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(hash-set! ht key new-value)
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new-count)))])
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(match path
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[`(R . ,(? number? n))
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(cons (cons (+ x n) y)
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(foldl (update 'x) count (range (add1 x) (add1 (+ x n)))))]
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[`(L . ,(? number? n))
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(cons (cons (- x n) y)
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(foldl (update 'x) count (reverse (range (- x n) x))))]
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[`(U . ,(? number? n))
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(cons (cons x (+ y n))
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(foldl (update 'y) count (range (add1 y) (add1 (+ y n)))))]
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[`(D . ,(? number? n))
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(cons (cons x (- y n))
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(foldl (update 'y) count (reverse (range (- y n) y))))])))
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(define (step-wire first-wire? wire ht)
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(foldl (λ (path pos-count) (step-path first-wire? path pos-count ht)) '((0 . 0) . 0) wire))
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(define-values (part1 intersections)
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(let* ([hashtable (make-hash '(((0 . 0) . ((#f . 0) . (#f . 0)))))]
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[_ (step-wire #t wire1 hashtable)]
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[_ (step-wire #f wire2 hashtable)]
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[hashlist (hash->list hashtable)]
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[intersections
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(filter (λ (kv)
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(let ([v (cdr kv)])
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(and (caar v) (cadr v))))
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hashlist)]
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[distances
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(map (λ (kv)
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(let ([k (car kv)])
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(+ (abs (car k)) (abs (cdr k)))))
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intersections)])
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(values (apply min distances)
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intersections)))
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(define part2
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(let* ([steps
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(map (λ (kv)
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(let ([v (cdr kv)])
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(+ (cdar v) (cddr v))))
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intersections)])
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(apply min steps)))
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(show-solution part1 part2)
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