day 3 part 2

day03/part2.lua

@@ -1,33 +1,55 @@
 local ZERO = string.byte("0", 1)
 local ONE = string.byte("1", 1)
-local total = 0
-local counts = {}
+local lines = {}
 for line in io.lines(arg[3]) do
+	local ones = {}
 	for char = 1, #line do
-		if line:byte(char) == ONE then
-			counts[char] = (counts[char] or 0) + 1
-		end
+		table.insert(ones, line:byte(char) == ONE)
 	end
-	total = total + 1
+	table.insert(lines, ones)
 end
 
-local least, most, lmax, mmax = nil, nil, 0, 0
-for line in io.lines(arg[3]) do
-	local lcur, mcur, lin, min = 0, 0, true, true
-	for char = 1, #line do
-		if line:byte(char) == ONE then
-			if counts[char] >= total / 2 then
-				if min then mcur = mcur + 1 end
-			else
-				min = false
-			end
-		else
-			if counts[char] < total / 2 then
-				if min then mcur = mcur + 1 end
-			else
-				min = false
+function reduce(left, keep)
+	local char = 1
+	while #left > 1 and char <= #lines[1] do
+		local ones = 0
+		for k, line in pairs(left) do
+			if lines[line][char] then
+				ones = ones + 1
 			end
 		end
+		local nleft = {}
+		for k, line in pairs(left) do
+			if keep(lines[line][char], ones, #left) then
+				table.insert(nleft, line)
+			end
+		end
+		left = nleft
+		char = char + 1
 	end
-	print(line, mcur)
+	return left[1]
 end
+
+function bin2dec(t)
+	local v = 0
+	for i = 1, #t do
+		v = v * 2
+		if t[i] then v = v + 1 end
+	end
+	return v
+end
+
+local indices = {}
+for line = 1, #lines do
+	table.insert(indices, line)
+end
+
+local oxygen = reduce(indices, function (bit, ones, total)
+	return (not bit or ones >= total/2) and (bit or ones < total/2)
+end)
+
+local co2 = reduce(indices, function (bit, ones, total)
+	return (not bit or ones < total/2) and (bit or ones >= total/2)
+end)
+
+print(bin2dec(lines[oxygen]) * bin2dec(lines[co2]))