day 9

day09/part1.lua

@@ -0,0 +1,26 @@
+#!/usr/bin/env luajit
+require("utils")
+
+local hm = {}
+for line in io.lines(arg[3]) do
+	local row = {}
+	for c in chars(line) do
+		table.insert(row, tonumber(c))
+	end
+	table.insert(hm, row)
+end
+
+local function get(r, c)
+	return (hm[r] or {})[c] or math.huge
+end
+
+local lowsum = 0
+for r, row in pairs(hm) do
+	for c, col in pairs(row) do
+		if col < get(r - 1, c) and col < get(r, c - 1) and col < get(r + 1, c) and col < get(r, c + 1) then
+			lowsum = lowsum + col + 1
+		end
+	end
+end
+
+print(lowsum)

day09/part2.lua

@@ -0,0 +1,71 @@
+#!/usr/bin/env luajit
+require("utils")
+
+local hm = {}
+for line in io.lines(arg[3]) do
+	local row = {}
+	for c in chars(line) do
+		table.insert(row, tonumber(c))
+	end
+	table.insert(hm, row)
+end
+
+local function get(r, c)
+	return (hm[r] or {})[c] or 9
+end
+
+local union = {}
+for r, row in pairs(hm) do
+	for c, col in pairs(row) do
+		if col ~= 9 then
+			if get(r - 1, c) ~= 9 then
+				if union[r] == nil then union[r] = {} end
+				union[r][c] = { r - 1, c }
+			end
+			if get(r, c - 1) ~= 9 then
+				if union[r] == nil then union[r] = {} end
+				if union[r][c] ~= nil then
+					local r0, c0 = r, c - 1
+					while type(union[r0][c0]) == "table" do
+						r0, c0 = unpack(union[r0][c0])
+						if union[r0] == nil then union[r0] = {} end
+					end
+					local r1, c1 = r, c
+					while type(union[r1][c1]) == "table" do
+						r1, c1 = unpack(union[r1][c1])
+						if union[r1] == nil then union[r1] = {} end
+					end
+					if r0 ~= r1 or c0 ~= c1 then
+						union[r1][c1] = { r0, c0 }
+					end
+				else
+					union[r][c] = { r, c - 1 }
+				end
+			end
+		end
+	end
+end
+
+local sums = {}
+for r, row in pairs(hm) do
+	for c, col in pairs(row) do
+		if col ~= 9 then
+			local r0, c0 = r, c
+			while type(union[r0][c0]) == "table" do
+				r0, c0 = unpack(union[r0][c0])
+			end
+			if sums[r0] == nil then sums[r0] = {} end
+			sums[r0][c0] = (sums[r0][c0] or 0) + 1
+		end
+	end
+end
+
+local sizes = {}
+for r, row in pairs(sums) do
+	for c, col in pairs(row) do
+		table.insert(sizes, col)
+	end
+end
+
+table.sort(sizes)
+print(sizes[#sizes] * sizes[#sizes - 1] * sizes[#sizes - 2])