Move code out of cmd

day3/battery.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+)
+
+func readBatteries(r io.Reader) (banks [][]uint8, err error) {
+	sc := bufio.NewScanner(r)
+	for sc.Scan() {
+		var bank []uint8
+		for _, b := range sc.Bytes() {
+			if b < '0' || b > '9' {
+				return nil, fmt.Errorf("bank %q: invalid joltage %q", sc.Text(), b)
+			}
+			bank = append(bank, b-'0')
+		}
+		banks = append(banks, bank)
+	}
+	return banks, sc.Err()
+}

day3/example.txt

@@ -0,0 +1,4 @@
+987654321111111
+811111111111119
+234234234234278
+818181911112111

day3/main.go

@@ -0,0 +1,86 @@
+package main
+
+import (
+	"cmp"
+	"flag"
+	"fmt"
+	"log"
+	"math"
+	"os"
+)
+
+func main() {
+	debug := flag.Bool("debug", false, "Debug.")
+	flag.Parse()
+
+	f := os.Stdin
+	if flag.NArg() > 0 {
+		var err error
+		f, err = os.Open(flag.Arg(0))
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+	}
+
+	banks, err := readBatteries(f)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var sum1, sum2 uint
+	for _, bank := range banks {
+		power1 := bestPowerPart1(bank)
+		power2 := bestPowerPart2(bank)
+		if *debug {
+			for _, b := range bank {
+				fmt.Print(b)
+			}
+			fmt.Println(":", power1, power2)
+		}
+		sum1 += power1
+		sum2 += power2
+	}
+
+	fmt.Println("sum (part 1):", sum1)
+	fmt.Println("sum (part 2):", sum2)
+}
+
+func bestPowerPart1(bank []uint8) uint {
+	// i := maxIndex(bank[:len(bank)-1])
+	// j := maxIndex(bank[i+1:]) + i + 1
+	// return uint(bank[i])*10 + uint(bank[j])
+	return bestPower(bank, 2)
+}
+
+func bestPowerPart2(bank []uint8) uint {
+	return bestPower(bank, 12)
+}
+
+func bestPower(bank []uint8, batteries int) uint {
+	if len(bank) < batteries {
+		panic(fmt.Sprintf("bank (%d) is smaller than requested batteries (%d)", len(bank), batteries))
+	}
+	var joltage uint
+	var i int
+	for j := batteries - 1; j >= 0; j-- {
+		k := maxIndex(bank[i : len(bank)-j])
+		k += i
+		i = k + 1
+		joltage += uint(bank[k]) * uint(math.Pow10(j))
+	}
+	return joltage
+}
+
+func maxIndex[S ~[]E, E cmp.Ordered](x S) int {
+	if len(x) < 1 {
+		panic("maxIndex: empty list")
+	}
+	i := 0
+	for j := 1; j < len(x); j++ {
+		if x[j] > x[i] {
+			i = j
+		}
+	}
+	return i
+}