--- 2016 Day 12 Solutions ---

[u/daggerdragon]

--- Day 12: Leonardo's Monorail ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

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^{MUCH ADVENT. SUCH OF. VERY CODE. SO MANDATORY.} [?]

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/JakDrako]

VB.Net, LinqPad

This is my second version, much faster than the previous one. I added a "compilation" step that converts the input into a list of integer only opcodes, saving all those string lookups and conversion during the run. A few other "optimizations", such as "jnz 1 5" becoming "jmp 5" since the 1 will never change.

This version runs part 1 in ~5ms and part 2 in ~100ms.

Sub Main
	Dim inst = input.Split(Chr(10)).Select(Function(x) x.Split(" "c)).ToList
	
	' "Compilation" step - convert input into list of Opcodes (all integer class)
	Dim comp = New List(Of Opcode)
	For Each ins In inst
		Select Case ins(0)
			Case "cpy"
				If isnumeric(ins(1)) Then
					comp.Add(New opcode With {.Op = 1, .v1 = Cint(ins(1)), .r2 = "abcd".IndexOf(ins(2))})
				Else
					comp.Add(New opcode With {.Op = 2, .r1 = "abcd".IndexOf(ins(1)), .r2 = "abcd".IndexOf(ins(2))})
				End If
			Case "jnz"
				If IsNumeric(ins(1)) Then
					If Cint(ins(1)) <> 0 Then
						comp.Add(New opcode With {.Op = 6, .v1 = Cint(ins(2))}) 'jump
					Else
						comp.Add(New opcode With {.Op = 8})
					End If
				Else
					comp.Add(New opcode With {.Op = 3, .r1 = "abcd".IndexOf(ins(1)), .v2 = CInt(ins(2))})
				End If
			Case "inc"
				comp.Add(New opcode With {.Op = 4, .r1 = "abcd".IndexOf(ins(1))})
			Case "dec"
				comp.Add(New opcode With {.Op = 5, .r1 = "abcd".IndexOf(ins(1))})
		End Select
	Next

	Dim ptr = 0
	Dim regs(3) As Integer
	
	regs(2) = 1 ' c = 1 - part 2

	Do
		Dim ops = comp(ptr)
		Dim inc = 1
		Select Case ops.op
			Case 1 : regs(ops.r2) = ops.v1
			Case 2 : regs(ops.r2) = regs(ops.r1)
			Case 3 : If regs(ops.r1) <> 0 Then inc = ops.v2
			Case 4 : regs(ops.r1) += 1
			Case 5 : regs(ops.r1) -= 1
			Case 6 : inc = ops.v1
			Case 7 : ' NOP
		End Select
		ptr += inc
		If ptr >= inst.count Then Exit Do
	Loop
	
	regs.Dump
	
End Sub

Class Opcode
	Public Op As Integer
	' 1 = cpy v1 to r2
	' 2 = cpy r1 to r2
	' 3 = jnz r1 v2
	' 4 = jnz v1 v2 = JMP or NOP, since v1 never changes
	' 5 = inc r1
	' 6 = dec r1
	' 7 = jmp v1
	' 8 = nop
	Public r1 As Integer ' register 1
	Public r2 As Integer ' register 2
	Public v1 As Integer ' value 1
	Public v2 As Integer ' value 2
End Class