Day 13 solution.

p13.dyalog

@@ -0,0 +1,17 @@
+⎕IO←0
+sort←{(⊂⍋⍵)⌷⍵}
+p←↑⊃⎕NGET'p13.txt'1
+g←'|'@(∊∘'^v')'-'@(∊∘'<>')⊢p ⍝ grid
+m←4>t←'^>v<'⍳p
+
+x←↑⍸m
+c←0×d←(,m)/,t
+turn←{⍵='/':(⍺⊃d)←4|1-⍺⊃d ⋄ ⍵='\':(⍺⊃d)←3-⍺⊃d ⋄ ⍵='+':(⍺⊃d)←4|(⍺⊃d)+¯1+3|⍺⊃c ⋄ 0}
+move←{z←(⍵⌷x)+(⍵⊃d)⌷4 2⍴¯1 0 0 1 1 0 0 ¯1 ⋄ (⊂z)∊↓x:r,←z ⋄ (⍵⌷x)←z ⋄ _←⍵ turn (a←z⌷g) ⋄ (⍵⊃c)+←'+'=a}
+tick←{move¨⍋x}
+r←⍬ ⋄ {}tick⍣{×≢r}⊢0 ⋄ ⌽2↑r ⍝ part 1
+
+x←↑⍸m
+v←1+c←0×d←(,m)/,t
+move←{z←(⍵⌷x)+(⍵⊃d)⌷4 2⍴¯1 0 0 1 1 0 0 ¯1 ⋄ i←(↓x)⍳⊂z ⋄ v[⍵ i/⍨1 1≡v[⍵ i/⍨i<≢v]]←0 ⋄ (⍵⌷x)←z ⋄ _←⍵ turn (a←z⌷g) ⋄ (⍵⊃c)+←'+'=a}
+{}tick⍣{1=+/v}⊢0 ⋄ ⌽v⌿x ⍝ part 2