Day 6, Part 1

src/day06/day06.lua

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+-- Author: Michael Adler
+local M = {}
+
+local Direction = {
+    NORTH = 1,
+    EAST = 2,
+    SOUTH = 3,
+    WEST = 4,
+}
+
+local function rotate_right(dir)
+    if dir == Direction.NORTH then
+        return Direction.EAST
+    elseif dir == Direction.EAST then
+        return Direction.SOUTH
+    elseif dir == Direction.SOUTH then
+        return Direction.WEST
+    elseif dir == Direction.WEST then
+        return Direction.NORTH
+    end
+end
+
+local function hash(x, y)
+    return string.format("%d,%d", x, y)
+end
+
+-- The table keeps track of the number of unique keys added to it.
+local function create_smart_table()
+    local count = 0
+    return setmetatable({}, {
+        __index = function(t, k)
+            return rawget(t, k)
+        end,
+        __newindex = function(t, k, v)
+            if rawget(t, k) == nil then
+                count = (count or 0) + 1
+            end
+            rawset(t, k, v)
+        end,
+        __len = function(_)
+            return count
+        end,
+    })
+end
+
+--- @param input string
+M.solve = function(input)
+    local map = {}
+
+    local y, x -- current position
+    local row_count = 0
+    for line in input:gmatch("[^\r\n]+") do
+        row_count = row_count + 1
+        local n = string.len(line)
+        local row = {}
+        for i = 1, n do
+            local c = string.sub(line, i, i)
+            if c == "^" then
+                y = row_count
+                x = i
+            end
+            table.insert(row, c)
+        end
+        table.insert(map, row)
+    end
+    local col_count = #map[1]
+
+    local visited = create_smart_table()
+    local dir = Direction.NORTH
+    while true do
+        -- mark current position as visited
+        visited[hash(x, y)] = true
+        -- try advance
+        local new_x, new_y
+        if dir == Direction.NORTH then
+            new_y, new_x = y - 1, x
+        elseif dir == Direction.EAST then
+            new_x, new_y = x + 1, y
+        elseif dir == Direction.SOUTH then
+            new_y, new_x = y + 1, x
+        elseif dir == Direction.WEST then
+            new_x, new_y = x - 1, y
+        end
+
+        -- is OOB?
+        if new_y < 1 or new_x < 1 or new_y > row_count or new_x > col_count then
+            break
+        end
+
+        -- is obstacle?
+        local c = map[new_y][new_x]
+        if c == "#" then
+            -- go back and rotate
+            new_y, new_x = y, x
+            dir = rotate_right(dir)
+        end
+        y, x = new_y, new_x
+    end
+
+    -- in Lua >= 5.2, we could call #visited
+    local part1 = getmetatable(visited).__len()
+    local part2 = 0
+
+    return part1, part2
+end
+
+return M

src/day06/day06_spec.lua

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+local day06 = require("day06")
+
+describe("day06", function()
+    it("solves the example", function()
+        local input = [[....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...
+]]
+        local part1, part2 = day06.solve(input)
+        assert.are.equal(41, part1)
+        assert.are.equal(0, part2)
+    end)
+end)

src/day06/main.lua

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+#!/usr/bin/env luajit
+local day06 = require("day06")
+local fname = arg[1] or "input.txt"
+local f = assert(io.open(fname, "r"), fname .. " missing")
+local input = f:read("*a")
+f:close()
+local part1, part2 = day06.solve(input)
+print(string.format("Part 1: %d\nPart 2: %d", part1, part2))