Init commit

cmd/day2/main.go

@@ -0,0 +1,97 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"math"
+	"os"
+	"slices"
+)
+
+func main() {
+	debug := flag.Bool("debug", false, "Debug.")
+	flag.Parse()
+
+	ranges, err := readRanges(os.Stdin)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var sum1, sum2 uint
+	for _, r := range ranges {
+		invalid1 := handleRange(r, scanPart1)
+		invalid2 := handleRange(r, scanPart2)
+
+		if *debug {
+			fmt.Printf("range: %d-%d\n", r.from, r.to)
+		}
+		if *debug && len(invalid1) > 0 {
+			fmt.Println("invalid (part 1):")
+			for _, id := range invalid1 {
+				fmt.Printf("  %d\n", id)
+			}
+		}
+		if *debug && len(invalid2) > 0 {
+			fmt.Println("invalid (part 2):")
+			for _, id := range invalid2 {
+				fmt.Printf("  %d\n", id)
+			}
+		}
+
+		for _, id := range invalid1 {
+			sum1 += id
+		}
+		for _, id := range invalid2 {
+			sum2 += id
+		}
+	}
+
+	fmt.Println("sum (part 1):", sum1)
+	fmt.Println("sum (part 2):", sum2)
+}
+
+func handleRange(r idRange, scan func(id uint) (advance uint, ok bool)) []uint {
+	var invalid []uint
+	for id := r.from; id <= r.to; {
+		advance, ok := scan(id)
+		if !ok {
+			invalid = append(invalid, id)
+		}
+		id += advance
+	}
+	return invalid
+}
+
+func scanPart1(id uint) (advance uint, ok bool) {
+	digits := digits(id)
+	if len(digits)%2 == 1 { // odd is always valid
+		nextSmallestEven := uint(math.Pow10(len(digits)))
+		return nextSmallestEven - id, true
+	}
+	var (
+		x     = uint(math.Pow10(len(digits) / 2))
+		left  = id / x
+		right = id % x
+	)
+	return 1, left != right
+}
+
+func scanPart2(id uint) (advance uint, ok bool) {
+	digits := digits(id)
+	if len(digits) <= 1 {
+		return 1, true
+	}
+
+loop:
+	for i := 1; i <= len(digits)/2; i++ {
+		ref := digits[:i]
+		for chunk := range slices.Chunk(digits, i) {
+			if !slices.Equal(chunk, ref) {
+				continue loop
+			}
+		}
+		return 1, false
+	}
+	return 1, true
+}