2 files changed, 72 insertions(+), 0 deletions(-)

R day05/{day05.lua => day05a.lua}
A day05/day05b.lua

@@ 0,0 1,72 @@
+#! /usr/bin/env lua
+--
+-- day05.lua
+-- Copyright (C) 2022 Adrian Perez de Castro <aperez@igalia.com>
+--
+-- Distributed under terms of the MIT license.
+--
+
+-- Contains the lines of input describing the initial contents of the
+-- stacks, to be parsed *after* knowing how many stacks are there.
+local stacks_lines = {}
+
+while true do
+   local line = io.read()
+   if line == "" then
+      break
+   end
+
+   stacks_lines[#stacks_lines+1] = line
+end
+
+-- Last read line contains the numbered stacks, the last number will be
+-- the same as the amount of stacks being handled. Also remove that last
+-- line from the pending-to-parse input.
+local nstacks = 0
+for n in stacks_lines[#stacks_lines]:gmatch("%s*(%d+)%s*") do
+   nstacks = tonumber(n)
+end
+table.remove(stacks_lines)
+
+-- Table where each element is a stack, initially empty.
+local stacks = {}
+for i = 1, nstacks do
+   stacks[i] = {}
+end
+
+-- Parse the stack contents lines backwards, so we get to pile up the
+-- items from the bottom upwards. We can iterate over each found "["
+-- character, and from their position in the line determine in which
+-- stack they are: positions for the opening bracket are always multiples
+-- of four.
+for i = #stacks_lines, 1, -1 do
+   for bracketpos, label in stacks_lines[i]:gmatch("()%[(%a+)%]") do
+      local stacknum = (bracketpos // 4) + 1
+      table.insert(stacks[stacknum], label)
+   end
+end
+
+for line in io.lines() do
+   local _, _, n, from, to = line:find("move (%d+) from (%d+) to (%d+)")
+   local f = stacks[tonumber(from)]
+   local t = stacks[tonumber(to)]
+
+   -- TODO: It should work using table.move(), but somehow it did not.
+   local m = {}
+   for _ = 1, n do
+      m[#m + 1] = table.remove(f)
+   end
+   for i = #m, 1, -1 do
+      table.insert(t, m[i])
+   end
+end
+
+local result = {}
+for i = 1, nstacks do
+   local thisstack = stacks[i]
+   if #thisstack > 0 then
+      result[#result+1] = thisstack[#thisstack]
+   end
+end
+
+print(table.concat(result, ""))