Day 14

days/day14.st

@@ -0,0 +1,108 @@
+Object subclass: Robot [
+    | pos vel |
+    parse: line
+    [ line scanf: 'p=%d,%d v=%d,%d' with: [
+              :px :py :vx :vy |
+              pos := Posn x: px y: py.
+              vel := Posn x: vx y: vy ]]
+
+    printOn: st [ st << '{p=' << pos << ' v=' << vel << '}' ]
+
+    posAt: time in: br
+    [ ^Posn
+           x: (pos x + (vel x * (time \\ br w))) \\ br w
+           y: (pos y + (vel y * (time \\ br h))) \\ br h ]
+]
+
+Array extend [
+    incAt: pos [ self at: pos put: 1 + (self at: pos) ]
+]
+
+Posn subclass: Bathroom [
+    | midx midy |
+    w [^x] h [^y]
+
+    x: n [super x: n. midx := x // 2]
+    y: n [super y: n. midy := y // 2]
+
+    countQuadrants: posns
+    [ | qs |
+      qs := Array new: 4 withAll: 0.
+      posns do: [ :p | self putPosn: p toQuad: qs ].
+      ^qs inject: 1 into: [:l :r | l * r] ]
+
+    putPosn: p toQuad: qs
+    [ | top bot |
+      top := p y < midy.
+      bot := p y > midy.
+      p x < midx ifTrue:
+          [ top ifTrue: [^qs incAt: 1].
+            bot ifTrue: [^qs incAt: 2].
+            ^nil ].
+      p x > midx ifTrue:
+          [ top ifTrue: [^qs incAt: 3].
+            bot ifTrue: [^qs incAt: 4].
+            ^nil ] ]
+
+    findMaxOf: robots inPeriod: p withTrue: filter
+    [ | best bestTime |
+      best := -1.
+      1 to: p do: [
+          :time | | cnt |
+          cnt := robots count: [:it | filter value: (it posAt: time in: self)].
+          cnt > best ifTrue: [best := cnt. bestTime := time] ].
+      ^bestTime ]
+
+    plot: robots at: time
+    [ | grid orig |
+      grid := 1 to: self h collect: [:i | String new: self w withAll: $.].
+      grid := Grid new rows: grid.
+      orig := Posn x: 1 y: 1.
+      robots do: [:r | grid at: orig + (r posAt: time in: self) put: $#].
+      stdout << 'At ' << time << ':'; nl; << grid; nl; nl. ]
+
+    betweenPercentile: min and: max by: getKey
+    [ | mine minV maxV |
+      mine := getKey value: self.
+      minV := mine * min.
+      maxV := mine * max.
+      ^[ :it | | its | its := getKey value: it.
+         minV <= its and: [its <= maxV] ] ]
+
+    inMiddleBy: getKey
+    [ ^self betweenPercentile: 0.4 and: 0.6 by: getKey ]
+
+    findMaxRobots: robots inMiddleBy: getKey
+    [ ^self findMaxOf: robots
+            inPeriod: (getKey value: self)
+            withTrue: (self inMiddleBy: getKey) ]
+
+    findTreeTimeOf: robots
+    [ | bestx besty |
+      "The tree is a little pictogram in the center of the image -- we
+      find the offset with which the robots condense here on both the
+      X and Y axes, then use CRT to compute where these coincide."
+      bestx := self findMaxRobots: robots inMiddleBy: [:it | it x].
+      besty := self findMaxRobots: robots inMiddleBy: [:it | it y].
+
+      (self w gcd: self h) = 1 ifFalse: [self error: 'Not coprime'].
+      ^(self w egcd: self h) letArrayInBlock:
+           [ :cx :cy |
+             (bestx * cy * self h)
+             + (besty * cx * self w)
+             \\ (self w * self h) ] ]
+]
+
+Bathroom class extend [
+    width: w height: h [^self x: w y: h]
+]
+
+theBathroom := Bathroom width: 101 height: 103.
+
+AOC input: [ stdin toLines asArray collect: [ :it | Robot new parse: it ] ];
+    part1: [ :robots | | posns |
+             posns := robots collect: [:it | it posAt: 100 in: theBathroom].
+             theBathroom countQuadrants: posns ];
+    part2: [ :robots | theBathroom findTreeTimeOf: robots ];
+    result: [ :robots :part | part value: robots. ];
+    finish.

days/utils.st

@@ -74,11 +74,15 @@ Object subclass: ScannerState [
     finish [ ^oc asArray ]
 
     parseIntOn: stream
-    [ | d |
+    [ | d sf c |
-      d := 0.
+      d := 0. sf := 1. c := stream peek.
+      (c = $+) | (c = $-) ifTrue: [
+          stream next.
+          c = $- ifTrue: [sf := -1]
+      ].
       stream whilePeek: [ :c | c isDigit ] do:
              [ :c | d := d * 10 + c digitValue ].
-      oc add: d ]
+      oc add: (sf * d) ]
 
     parseStringOn: stream
     [ | s | s := OrderedCollection new.
@@ -224,7 +228,7 @@ Object subclass: Posn [
     printOn: s [ s << '<' << x << ', ' << y << '>'. ]
 ]
 Posn class extend [
-    x: x y: y [ ^Posn new x: x; y: y ]
+    x: x y: y [ ^self new x: x; y: y ]
     up: n [ ^Posn x: 0 y: n negated ]
     down: n [ ^Posn x: 0 y: n ]
     left: n [ ^Posn x: n negated y: 0 ]
@@ -272,7 +276,9 @@ Array extend [ asPosn [ ^Posn x: (self at: 1) y: (self at: 2) ] ]
 Iterable extend [ asPosns [ ^self collect: [:it | it asPosn] ] ]
 
 Object subclass: Grid [
-    | rows | rows: n [rows:=n]
+    | rows |
+    rows: n [rows:=n]
+    rows [^rows]
 
     at: pos [ ^(rows at: pos y ifAbsent: [^nil]) at: pos x ifAbsent: [nil] ]
     at: pos put: elt
@@ -319,9 +325,12 @@ Dictionary extend [
 ]
 
 Integer extend [
-    ceilingDiv: denom [ ^((self - 1) // denom) + 1 ]
+    ceilingDiv: denom [
+        "Answer (self / denom) ceiling."
+        ^((self - 1) // denom) + 1
+    ]
 
-    "The original implementation was incorrect. Here is the correct code:"
+    "Original code was incorrect:"
     ceilingLog: radix [
         "Answer (self log: radix) ceiling. Optimized to answer an integer."
 
@@ -343,4 +352,22 @@ Integer extend [
              answer := answer + 1].
         ^answer
     ]
+
+    egcd: o [
+        "Answer {x . y} such that (self * x) + (o * y) = (self gcd: o)."
+        | q r0 r s0 s t0 t tmp |
+        r0 := self. r := o.
+        s0 := 1. s := 0.
+        t0 := 0. t := 1.
+
+        [r ~= 0] whileTrue: [
+            q := r0 // r.
+
+            tmp := r. r := r0 - (q * r). r0 := tmp.
+            tmp := s. s := s0 - (q * s). s0 := tmp.
+            tmp := t. t := t0 - (q * t). t0 := tmp.
+        ].
+
+        ^{s0 . t0}
+    ]
 ]