2020 day 8

2020/8/day8.lua

@@ -0,0 +1,66 @@
+local commands = {}
+
+for line in io.lines() do
+    local name, arg = line:match('^(%l+) ([+-]%d+)$')
+    local intarg = tonumber(arg:gsub('%+', ''), 10)
+    table.insert(commands, {name=name, arg=intarg, ran=false})
+end
+
+local function nop_cmd(arg, acc, pos)
+    return acc, pos + 1
+end
+local function acc_cmd(arg, acc, pos)
+    return acc + arg, pos + 1
+end
+local function jmp_cmd(arg, acc, pos)
+    return acc, pos + arg
+end
+
+local functions = {
+    nop=nop_cmd,
+    acc=acc_cmd,
+    jmp=jmp_cmd,
+}
+
+local function reset()
+    for _, command in ipairs(commands) do
+        command.ran = false
+    end
+end
+
+--[[
+    This function runs the program and returns two values:
+    - true if the program ends because of its last instruction
+    - false if the program ends because a command was already ran
+--]]
+local function run()
+    local acc, pos = 0, 1
+    while true do
+        if pos > #commands then
+            return true, acc
+        end
+        local command = commands[pos]
+        if command.ran then
+            return false, acc
+        end
+        acc, pos = functions[command.name](command.arg, acc, pos)
+        command.ran = true
+    end
+end
+
+-- Part 1
+print(select(2, run()))
+
+-- Part 2
+for i, command in ipairs(commands) do
+    if (command.name == "nop" or command.name == "jmp") and command.arg ~= 0 then
+        command.name = command.name == "nop" and "jmp" or "nop"
+        reset()
+        local ended, acc = run()
+        if ended then
+            print(acc)
+            return
+        end
+        command.name = command.name == "nop" and "jmp" or "nop"
+    end
+end

README.md

@@ -13,7 +13,7 @@ My solutions to the Advent of Code puzzles.
  5 ██    ██ ██    ██
  6 ██    ██       ██
  7 ██    ██       ██
- 8 ██    ██
+ 8 ██    ██       ██
  9       ██
 10 ██    ██
 11 ██    ██