2020 day 11

2020/11/day11.lua

@@ -0,0 +1,120 @@
+FLOOR, EMPTY, OCCUPIED = 0, 1, 2
+
+-- Table of tables of integers representing states.
+local states = {}
+
+for line in io.lines() do
+    local row = {}
+    local state_mappings = {["."]=FLOOR, L=EMPTY, ["#"]=OCCUPIED}
+    line:gsub(".", function (c) table.insert(row, state_mappings[c]) end)
+    table.insert(states, row)
+end
+
+-- Deep copy of the table to keep it for part 2
+local initial = {}
+for x, row in ipairs(states) do
+    initial[x] = {}
+    for y, tile in ipairs(row) do
+        initial[x][y] = tile
+    end
+end
+
+local function iteration(func)
+    local modified = false
+    local new_states = {}
+    for x, row in ipairs(states) do
+        new_states[x] = {}
+        for y, tile in ipairs(row) do
+            local result = func(x, y, tile)
+            modified = modified or tile ~= result
+            new_states[x][y] = result
+        end
+    end
+    states = new_states
+    return modified
+end
+
+local function is_occupied(x, y)
+    if x < 1 or y < 1 then
+        return 0
+    end
+    if states[x] ~= nil and states[x][y] == OCCUPIED then
+        return 1
+    else
+        return 0
+    end
+end
+
+local function count_occupied()
+    local total = 0
+    for x, row in ipairs(states) do
+        for y = 1, #row do
+            total = total + is_occupied(x, y)
+        end
+    end
+    return total
+end
+
+local function part1(x, y, tile)
+    if tile == FLOOR then
+        return FLOOR
+    end
+    local adjacent =
+        is_occupied(x-1, y-1)
+        + is_occupied(x, y-1)
+        + is_occupied(x+1, y-1)
+        + is_occupied(x-1, y)
+        + is_occupied(x+1, y)
+        + is_occupied(x-1, y+1)
+        + is_occupied(x, y+1)
+        + is_occupied(x+1, y+1)
+    if tile == EMPTY and adjacent == 0 then
+        return OCCUPIED
+    elseif tile == OCCUPIED and adjacent >= 4 then
+        return EMPTY
+    end
+    return tile
+end
+
+-- Find a seat iteratively in a given direction
+local function find_occupied_seat(pos_x, pos_y, dir_x, dir_y)
+    local i, j = pos_x + dir_x, pos_y + dir_y
+    while i >= 1 and j >= 1 and i <= #states and j <= #states[i] do
+        local tile = states[i][j]
+        if tile == OCCUPIED then
+            return 1
+        elseif tile == EMPTY then
+            return 0
+        end
+        i, j = i + dir_x, j + dir_y
+    end
+    return 0
+end
+
+local function part2(x, y, tile)
+    if tile == FLOOR then
+        return tile
+    end
+    local adjacent =
+        find_occupied_seat(x, y, 1, 1)
+        + find_occupied_seat(x, y, 0, 1)
+        + find_occupied_seat(x, y, -1, 1)
+        + find_occupied_seat(x, y, 1, 0)
+        + find_occupied_seat(x, y, -1, 0)
+        + find_occupied_seat(x, y, 1, -1)
+        + find_occupied_seat(x, y, 0, -1)
+        + find_occupied_seat(x, y, -1, -1)
+    if tile == EMPTY and adjacent == 0 then
+        return OCCUPIED
+    elseif tile == OCCUPIED and adjacent >= 5 then
+        return EMPTY
+    end
+    return tile
+end
+
+repeat until iteration(part1) == false
+print(count_occupied())
+
+states = initial
+repeat until iteration(part2) == false
+print(count_occupied())

README.md

@@ -21,7 +21,7 @@ is acceptable; anything goes as long as I solve it myself!
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