2020 day 17

What have I done…

2020/17/day17.lua

@@ -0,0 +1,173 @@
+-- Three-dimensional table of cubes.
+-- {x={y={z=true}}}
+local cubes = {}
+-- Four-dimensional!
+local hypercube = {}
+
+local i = 0
+for line in io.lines() do
+    i = i + 1
+    -- Create deep copies immediately
+    local row, row2 = {}, {}
+    line:gsub(".", function (c) table.insert(row, {c == '#'}) end)
+    line:gsub(".", function (c) table.insert(row2, {{c == '#'}}) end)
+    cubes[i] = row
+    hypercube[i] = row2
+end
+
+
+local function list_neighbors(x, y, z, w)
+    local neighbors = {}
+    for dx = -1, 1 do
+        for dy = -1, 1 do
+            for dz = -1, 1 do
+                if w then
+                    for dw = -1, 1 do
+                        if dx ~= 0 or dy ~= 0 or dz ~= 0 or dw ~= 0 then
+                            table.insert(neighbors, {x+dx, y+dy, z+dz, w+dw})
+                        end
+                    end
+                else
+                    if dx ~= 0 or dy ~= 0 or dz ~= 0 then
+                        table.insert(neighbors, {x+dx, y+dy, z+dz})
+                    end
+                end
+            end
+        end
+    end
+    return neighbors
+end
+
+local function count_active(x, y, z)
+    local total = 0
+    for _, neighbor in ipairs(list_neighbors(x, y, z)) do
+        local nx, ny, nz = table.unpack(neighbor)
+        if cubes[nx] and cubes[nx][ny] and cubes[nx][ny][nz] then
+            total = total + 1
+        end
+    end
+    return total
+end
+
+local function cycle()
+    local new_cubes = {}
+    -- Set of new neighboring cubes that will be checked after the first loop
+    local new_neighbors = {}
+    for x, row in pairs(cubes) do
+        new_cubes[x] = {}
+        for y, column in pairs(row) do
+            new_cubes[x][y] = {}
+            for z, state in pairs(column) do
+                local active = count_active(x, y, z)
+                new_cubes[x][y][z] = (state and active == 2) or active == 3
+                -- Load the new neighbors if the cube was active
+                if state then
+                    for _, neighbor in ipairs(list_neighbors(x, y, z)) do
+                        new_neighbors[neighbor] = true
+                    end
+                end
+            end
+        end
+    end
+    for neighbor, _ in pairs(new_neighbors) do
+        local x, y, z = table.unpack(neighbor)
+        local active = count_active(x, y, z)
+        if active == 3 then
+            if not new_cubes[x] then
+                new_cubes[x] = {}
+            end
+            if not new_cubes[x][y] then
+                new_cubes[x][y] = {}
+            end
+            new_cubes[x][y][z] = true
+        end
+    end
+    cubes = new_cubes
+end
+
+for _ = 1, 6 do
+    cycle()
+end
+local part1 = 0
+for _, row in pairs(cubes) do
+    for _, column in pairs(row) do
+        for _, state in pairs(column) do
+            if state then
+                part1 = part1 + 1
+            end
+        end
+    end
+end
+print(part1)
+
+-- I'm lazy as always: copy-pasting half of the code to add the fourth dimension.
+local function count_hyper_active(x, y, z, w)
+    local total = 0
+    for _, neighbor in ipairs(list_neighbors(x, y, z, w)) do
+        local nx, ny, nz, nw = table.unpack(neighbor)
+        if hypercube[nx] and hypercube[nx][ny] and hypercube[nx][ny][nz] and hypercube[nx][ny][nz][nw] then
+            total = total + 1
+        end
+    end
+    return total
+end
+
+-- Mmmhhh… Tasty O(n⁴)
+local function hyper_cycle()
+    local new_hypercube = {}
+    -- Set of new neighboring cubes that will be checked after the first loop
+    local new_neighbors = {}
+    for x, plan in pairs(hypercube) do
+        new_hypercube[x] = {}
+        for y, row in pairs(plan) do
+            new_hypercube[x][y] = {}
+            for z, column in pairs(row) do
+                new_hypercube[x][y][z] = {}
+                for w, state in pairs(column) do
+                    local active = count_hyper_active(x, y, z, w)
+                    new_hypercube[x][y][z][w] = (state and active == 2) or active == 3
+                    -- Load the new neighbors if the cube was active
+                    if state then
+                        for _, neighbor in ipairs(list_neighbors(x, y, z, w)) do
+                            new_neighbors[neighbor] = true
+                        end
+                    end
+                end
+            end
+        end
+    end
+    for neighbor, _ in pairs(new_neighbors) do
+        local x, y, z, w = table.unpack(neighbor)
+        local active = count_hyper_active(x, y, z, w)
+        if active == 3 then
+            if not new_hypercube[x] then
+                new_hypercube[x] = {}
+            end
+            if not new_hypercube[x][y] then
+                new_hypercube[x][y] = {}
+            end
+            if not new_hypercube[x][y][z] then
+                new_hypercube[x][y][z] = {}
+            end
+            new_hypercube[x][y][z][w] = true
+        end
+    end
+    hypercube = new_hypercube
+end
+
+for _ = 1, 6 do
+    hyper_cycle()
+end
+local part2 = 0
+for _, plan in pairs(hypercube) do
+    for _, row in pairs(plan) do
+        for _, column in pairs(row) do
+            for _, state in pairs(column) do
+                if state then
+                    part2 = part2 + 1
+                end
+            end
+        end
+    end
+end
+print(part2)

README.md

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