Day 10

README.md

@@ -16,3 +16,4 @@
 | 7   | Lua      | 230ms                 |
 | 8   | Lua      | 3.31ms                |
 | 9   | Lua      | 458ms                 |
+| 10  | Lua      | 8.94ms                |

src/day10/day10.lua

@@ -0,0 +1,133 @@
+-- Author: Michael Adler
+local M = {}
+
+---@class Point2D
+---@field x number
+---@field y number
+local Point2D = {}
+Point2D.__index = Point2D
+
+--- @return Point2D
+function Point2D.new(_, x, y)
+    local instance = setmetatable({}, Point2D)
+    instance.x = x or 0
+    instance.y = y or 0
+    return instance
+end
+
+--- @return Point2D
+function Point2D.__add(a, b)
+    return Point2D:new(a.x + b.x, a.y + b.y)
+end
+
+--- @return Point2D
+function Point2D.__sub(a, b)
+    return Point2D:new(a.x - b.x, a.y - b.y)
+end
+
+--- @return boolean
+function Point2D.__eq(a, b)
+    return a.x == b.x and a.y == b.y
+end
+
+--- @return string
+function Point2D:hash()
+    return string.format("%d,%d", self.x, self.y)
+end
+
+--- @return string
+function Point2D:__tostring()
+    return string.format("Point2D(%d, %d)", self.x, self.y)
+end
+
+local function contains(t, value)
+    for _, v in pairs(t) do
+        if v == value then
+            return true
+        end
+    end
+    return false
+end
+
+--- @param input string
+M.solve = function(input)
+    local part1, part2 = 0, 0
+    local map = {}
+    local trailheads = {}
+
+    local y = 0
+    for line in input:gmatch("[^\r\n]+") do
+        y = y + 1
+
+        local row = {}
+        local x = 0
+        for i = 1, string.len(line) do
+            x = x + 1
+            local c = tonumber(string.sub(line, i, i))
+            if c == 0 then
+                table.insert(trailheads, Point2D:new(x, y))
+            end
+            table.insert(row, c)
+        end
+        table.insert(map, row)
+    end
+
+    local row_count = y
+    local col_count = #map[1]
+
+    local all_directions = { Point2D:new(1, 0), Point2D:new(-1, 0), Point2D:new(0, 1), Point2D:new(0, -1) }
+    --- @param p Point2D
+    local neighbors = function(p)
+        local height = map[p.y][p.x]
+        local result = {}
+        for _, d in ipairs(all_directions) do
+            local candidate = p + d
+            -- check out-of-bounds
+            if candidate.y >= 1 and candidate.y <= row_count and candidate.x >= 1 and candidate.x <= col_count then
+                if map[candidate.y][candidate.x] - height == 1 then
+                    table.insert(result, candidate)
+                end
+            end
+        end
+        return result
+    end
+
+    local nines_reached = {}
+
+    local function dfs(start, path)
+        path = path or {}
+        table.insert(path, start)
+        if map[start.y][start.x] == 9 then
+            nines_reached[start:hash()] = true
+            return { path }
+        end
+        local all_paths = {}
+        for _, neighbor in ipairs(neighbors(start)) do
+            if not contains(path, neighbor) then
+                local path_copy = {}
+                for _, v in ipairs(path) do
+                    table.insert(path_copy, v)
+                end
+                local new_paths = dfs(neighbor, path_copy)
+                for _, new_path in pairs(new_paths) do
+                    table.insert(all_paths, new_path)
+                end
+            end
+        end
+        return all_paths
+    end
+
+    for _, p in ipairs(trailheads) do
+        local all_paths = dfs(p)
+        for _, _ in pairs(nines_reached) do
+            part1 = part1 + 1
+        end
+        nines_reached = {}
+
+        part2 = part2 + #all_paths
+    end
+
+    return part1, part2
+end
+
+return M

src/day10/day10_spec.lua

@@ -0,0 +1,28 @@
+local day10 = require("day10")
+
+describe("day10", function()
+    it("solves example 1", function()
+        local input = [[0123
+1234
+8765
+9876
+]]
+        local part1, _ = day10.solve(input)
+        assert.are.equal(1, part1)
+    end)
+
+    it("solves example 2", function()
+        local input = [[89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732
+]]
+        local part1, part2 = day10.solve(input)
+        assert.are.equal(36, part1)
+        assert.are.equal(81, part2)
+    end)
+end)

src/day10/main.lua

@@ -0,0 +1,8 @@
+#!/usr/bin/env luajit
+local day10 = require("day10")
+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 = day10.solve(input)
+print(string.format("Part 1: %d\nPart 2: %d", part1, part2))