More improvements, because lua can do some things:

- Remove manual bookkeeping of array size
- Remove duplicate 'snd' instruction
- Instructions start at 1 now, I guess

2017/day-18/solution.lua

@@ -14,15 +14,11 @@ function split(input, match)
 end
 
 instructions = {}
-icount = 0
 
 while true do
 	line = io.read()
-
 	if line == nil then break end
-
+	table.insert(instructions, split(line, "%S+"))
-	instructions[icount] = split(line, "%S+")
-	icount = icount + 1
 end
 
 registers = {}
@@ -41,8 +37,6 @@ function alu(instr, registers)
 	-- Perform instruction and return offset to iptr
 	if instr[1] == "set" then
 		registers[instr[2]] = argval(instr[3], registers)
-	elseif instr[1] == "snd" then
-		last_played = argval(instr[2], registers)
 	elseif instr[1] == "add" then
 		registers[instr[2]] = argval(instr[2], registers) + argval(instr[3], registers)
 	elseif instr[1] == "mul" then
@@ -61,29 +55,25 @@ function alu(instr, registers)
 	return 1
 end
 
-iptr = 0
+iptr = 1
 
-while true do
+while instructions[iptr][1] ~= "rcv" do
 	instr = instructions[iptr]
 
 	if instr[1] == "snd" then
 		last_played = argval(instr[2], registers)
 		iptr = iptr + 1
-	elseif instr[1] == "rcv" then
-		print("Last played", last_played)
-		iptr = iptr + 1
-		break
 	else
 		iptr = iptr + alu(instr, registers)
 	end
 end
+print("Last played", last_played)
 
 registers = {{p=0}, {p=1}}
 
-iptr = {0, 0} -- instruction pointers
+iptr = {1, 1} -- instruction pointers
 rbuf = {{}, {}} -- receive buffers
-rptr = {0, 0} -- Pointer in the receive buffer
+rptr = {1, 1} -- Pointer in the receive buffer
-rctr = {0, 0} -- Number of entries in each buffer
 waiting = {false, false} -- Whether one thread is waiting
 
 pid = 1
@@ -96,18 +86,12 @@ function nextpid(pid)
 	end
 end
 
-while true do
+while not waiting[pid] do
 	instr = instructions[iptr[pid]]
 
-	if waiting[pid] then
-		print("Deadlocked!")
-		break
-	end
-
 	if instr[1] == "snd" then
 		npid = nextpid(pid)
-		rbuf[npid][rctr[npid]] = argval(instr[2], registers[pid])
+		table.insert(rbuf[npid], argval(instr[2], registers[pid]))
-		rctr[npid] = rctr[npid] + 1
 		waiting[npid] = false
 
 		iptr[pid] = iptr[pid] + 1
@@ -127,4 +111,4 @@ while true do
 	end
 
 end
-print("Final sends from 1:", rctr[1])
+print("Final sends from 1:", #rbuf[1])