Day 14, Part 1

src/day14/day14.lua

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+-- Author: Michael Adler
+local M = {}
+
+local Point2D = require("aoc_helper.point").Point2D
+
+local function draw(robots, width, height)
+    local map = {}
+    for _, rbt in pairs(robots) do
+        local row = map[rbt.y] or {}
+        row[rbt.x] = (row[rbt.x] or 0) + 1
+        map[rbt.y] = row
+    end
+
+    for y = 0, height - 1 do
+        for x = 0, width - 1 do
+            local count = (map[y] or {})[x]
+            if count then
+                io.stdout:write(tostring(count))
+            else
+                io.stdout:write(".")
+            end
+        end
+        io.stdout:write("\n")
+    end
+end
+
+--- @param input string
+M.solve = function(input, width, height)
+    width, height = width or 101, height or 103
+
+    local robots = {}
+    local velos = {}
+    for line in input:gmatch("[^\r\n]+") do
+        local rbt
+        local vel
+        for x, y in line:gmatch("([-]?%d+),([-]?%d+)") do
+            local p = Point2D:new(assert(tonumber(x)), assert(tonumber(y)))
+            if rbt then
+                vel = p
+            else
+                rbt = p
+            end
+        end
+        table.insert(robots, rbt)
+        table.insert(velos, vel)
+    end
+
+    local advance = function()
+        for i, rbt in ipairs(robots) do
+            local vel = velos[i]
+            rbt.x = (rbt.x + vel.x) % width
+            rbt.y = (rbt.y + vel.y) % height
+        end
+    end
+
+    for _ = 1, 100 do
+        advance()
+    end
+
+    -- count robots in quadrants
+    local mid_height = math.floor(height / 2)
+    local mid_width = math.floor(width / 2)
+    local top_left, top_right, lower_left, lower_right = 0, 0, 0, 0
+    for _, rbt in ipairs(robots) do
+        local x, y = rbt.x, rbt.y
+        if x < mid_width and y < mid_height then
+            top_left = top_left + 1
+        elseif x > mid_width and y < mid_height then
+            top_right = top_right + 1
+        elseif x < mid_width and y > mid_height then
+            lower_left = lower_left + 1
+        elseif x > mid_width and y > mid_height then
+            lower_right = lower_right + 1
+        end
+    end
+
+    local part1, part2 = 0, 0
+    part1 = top_left * top_right * lower_left * lower_right
+
+    return part1, part2
+end
+
+return M

src/day14/day14_spec.lua

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+local day14 = require("day14")
+
+describe("day14", function()
+    describe("example 1", function()
+        local input = [[p=0,4 v=3,-3
+p=6,3 v=-1,-3
+p=10,3 v=-1,2
+p=2,0 v=2,-1
+p=0,0 v=1,3
+p=3,0 v=-2,-2
+p=7,6 v=-1,-3
+p=3,0 v=-1,-2
+p=9,3 v=2,3
+p=7,3 v=-1,2
+p=2,4 v=2,-3
+p=9,5 v=-3,-3
+]]
+        local part1, part2 = day14.solve(input, 11, 7)
+        it("part1", function()
+            assert.are.equal(12, part1)
+        end)
+        it("part2", function()
+            assert.are.equal(0, part2)
+        end)
+    end)
+end)

src/day14/main.lua

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+#!/usr/bin/env luajit
+local day14 = require("day14")
+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 = day14.solve(input)
+print(string.format("Part 1: %d\nPart 2: %d", part1, part2))