2025 day 2

Author: lucianoq

2025/2/Makefile

@@ -0,0 +1,20 @@
+input:
+	http "https://adventofcode.com/2025/day/2/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go
+
+main2:
+	go build -o main2 main2.go common.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input
+

2025/2/common.go

@@ -0,0 +1,96 @@
+package main
+
+import (
+	"bufio"
+	"os"
+	"strconv"
+	"strings"
+)
+
+type Range struct {
+	start, stop, digits int
+}
+
+func (r Range) InvalidIDs(chunkLen int) map[int]struct{} {
+	repeat := r.digits / chunkLen
+
+	invalids := make(map[int]struct{})
+
+	for i := r.start / pow10(r.digits-chunkLen); i <= r.stop/(pow10(r.digits-chunkLen)+1); i++ {
+		id := buildID(i, repeat)
+		if id < r.start {
+			continue
+		}
+		if id > r.stop {
+			break
+		}
+		invalids[id] = struct{}{}
+	}
+
+	return invalids
+}
+
+func parseInput() []Range {
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Scan()
+	var list []Range
+	for _, r := range strings.Split(scanner.Text(), ",") {
+		limits := strings.Split(r, "-")
+		start, _ := strconv.Atoi(limits[0])
+		stop, _ := strconv.Atoi(limits[1])
+		startDigits, stopDigits := digits(start), digits(stop)
+
+		// split the range into two ranges
+		// e.g., start->999 and 1000->stop
+		// to work on single digits range later
+		if startDigits == stopDigits {
+			list = append(list, Range{start, stop, startDigits})
+		} else {
+			list = append(list, Range{start, pow10(stopDigits-1) - 1, startDigits})
+			list = append(list, Range{pow10(startDigits), stop, stopDigits})
+		}
+	}
+	return list
+}
+
+// faster than
+// int(math.Pow10(exp))
+func pow10(exp int) int {
+	base := 10
+	result := 1
+	for {
+		if exp&1 == 1 {
+			result *= base
+		}
+		exp >>= 1
+		if exp == 0 {
+			break
+		}
+		base *= base
+	}
+	return result
+}
+
+// faster than
+// len(strconv.Itoa(i))
+func digits(i int) int {
+	if i == 0 {
+		return 1
+	}
+	count := 0
+	for i != 0 {
+		i /= 10
+		count++
+	}
+	return count
+}
+
+// faster than
+// strconv.Atoi(strings.Repeat(strconv.Itoa(chunk), repeat))
+func buildID(chunk, repeat int) int {
+	n := chunk
+	for i := 1; i < repeat; i++ {
+		n = n*pow10(digits(chunk)) + chunk
+	}
+	return n
+}

2025/2/main1.go

@@ -0,0 +1,22 @@
+package main
+
+import "fmt"
+
+func main() {
+	ranges := parseInput()
+	
+	sum := 0
+	for _, r := range ranges {
+
+		// only even digits
+		if r.digits%2 != 0 {
+			continue
+		}
+
+		for n := range r.InvalidIDs(r.digits / 2) {
+			sum += n
+		}
+	}
+
+	fmt.Println(sum)
+}

2025/2/main2.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"fmt"
+	"maps"
+)
+
+func main() {
+	ranges := parseInput()
+
+	invalids := map[int]struct{}{}
+
+	for _, r := range ranges {
+		for chunkLen := 1; chunkLen <= r.digits/2; chunkLen++ {
+
+			// only divisors
+			if r.digits%chunkLen != 0 {
+				continue
+			}
+			
+			maps.Copy(invalids, r.InvalidIDs(chunkLen))
+		}
+	}
+
+	sum := 0
+	for n := range invalids {
+		sum += n
+	}
+	fmt.Println(sum)
+}