Day 8, Part 2

README.md

@@ -14,3 +14,4 @@
 | 5   | Lua      | 3.81ms                |
 | 6   | Lua      | 5400ms                |
 | 7   | Lua      | 230ms                 |
+| 8   | Lua      | 3.31ms                |

src/day08/day08.lua

@@ -2,61 +2,83 @@
 local M = {}
 
 local Point2D = require("point2d")
-local pprint = require("pprint")
 
 --- @param input string
 M.solve = function(input)
-    local matrix = {}
     --- @type table<number, Point2D[]>
     local antennas = {}
-    local col_count
+    local col_count = input:find("\n") - 1
     local row_count = 0
     for line in input:gmatch("[^\r\n]+") do
         row_count = row_count + 1
-        if not col_count then
-            col_count = string.len(line)
-        end
-        local row = {}
         for y = 1, col_count do
             local c = string.sub(line, y, y)
-            if c:match("[0-9A-Za-z]") then
+            if c ~= "." then
                 local t = antennas[c] or {}
                 table.insert(t, Point2D:new(row_count, y))
                 antennas[c] = t
             end
-            table.insert(row, c)
         end
-        table.insert(matrix, row)
     end
 
     local is_within_bounds = function(p)
         return p.x >= 1 and p.x <= col_count and p.y >= 1 and p.y <= row_count
     end
 
-    local antinodes = {}
-    for name, points in pairs(antennas) do
-        local n = #points
-        for i = 1, n do
-            local p = points[i]
-            for j = 1, n do
-                if j ~= i then
-                    local q = points[j]
-                    local delta = q - p
-                    local anti = q + delta
-                    if is_within_bounds(anti) then
-                        antinodes[anti:hash()] = true
+    local part1 = function()
+        local antinodes = {}
+        for _, points in pairs(antennas) do
+            local n = #points
+            for i = 1, n do
+                local p = points[i]
+                for j = 1, n do
+                    if j ~= i then
+                        local q = points[j]
+                        local delta = q - p
+                        local anti = q + delta
+                        if is_within_bounds(anti) then
+                            antinodes[anti:hash()] = true
+                        end
                     end
                 end
             end
         end
+
+        local count = 0
+        for _, _ in pairs(antinodes) do
+            count = count + 1
+        end
+        return count
     end
 
-    local part1, part2 = 0, 0
-    for k, _ in pairs(antinodes) do
-        part1 = part1 + 1
+    local part2 = function()
+        local antinodes = {}
+        for _, points in pairs(antennas) do
+            local n = #points
+            for i = 1, n do
+                local p = points[i]
+                for j = 1, n do
+                    if j ~= i then
+                        local q = points[j]
+                        local delta = q - p
+                        local anti = q
+                        repeat
+                            antinodes[anti:hash()] = true
+                            anti = anti + delta
+                        until not is_within_bounds(anti)
+                    end
+                end
+            end
+        end
+
+        local count = 0
+        for _, _ in pairs(antinodes) do
+            count = count + 1
+        end
+        return count
     end
 
-    return part1, part2
+    return part1(), part2()
 end
 
 return M

src/day08/day08_spec.lua

@@ -17,6 +17,6 @@ describe("day08", function()
 ]]
         local part1, part2 = day08.solve(input)
         assert.are.equal(14, part1)
-        assert.are.equal(0, part2)
+        assert.are.equal(34, part2)
     end)
 end)