Day 4

04/answers.txt

@@ -0,0 +1,2 @@
+979
+635

04/input.txt

@@ -0,0 +1 @@
+256310-732736

04/main.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"github.com/csmith/aoc-2019/common"
+	"strings"
+)
+
+func matchesRules(input int) (hasAnyRun bool, hasTwoRun bool, next int) {
+	last := 10
+	run := 0
+
+	for input > 0 {
+		digit := input % 10
+
+		if digit > last {
+			// Simple optimisation: if we hit a digit that's larger than the previous (running right-to-left) then
+			// we can predict the next possible number that might match. e.g. 1234111 => 1234444.
+			for input < 100000 {
+				input = 10*input + digit
+			}
+			return false, false, input
+		} else if digit == last {
+			run++
+		} else {
+			hasTwoRun = hasTwoRun || run == 1
+			hasAnyRun = hasAnyRun || run > 0
+			last = digit
+			run = 0
+		}
+
+		input = input / 10
+	}
+
+	hasTwoRun = hasTwoRun || run == 1
+	hasAnyRun = hasAnyRun || run > 0
+	return
+}
+
+func main() {
+	var (
+		input = strings.Split(common.ReadFileAsStrings("04/input.txt")[0], "-")
+		from  = common.MustAtoi(input[0])
+		to    = common.MustAtoi(input[1])
+		part1 = 0
+		part2 = 0
+	)
+
+	for i := from; i <= to; i++ {
+		p1, p2, n := matchesRules(i)
+		if p1 {
+			part1++
+		}
+		if p2 {
+			part2++
+		}
+		if n > 0 {
+			i = n - 1
+		}
+	}
+
+	println(part1)
+	println(part2)
+}