adventofcode2025/day10/sol2.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
	"strconv"
	"strings"
)

func die(err error) {
	log.Fatal(err)
}

func check(err error) {
	if err != nil {
		die(err)
	}
}

func main() {
	solve("sample")
	solve("input")
}

// Brilliant insight from a redditor:
// suppose we only want to match the parity of the required joltages.
// we can use our solution for part 1 to find the minimum number of button presses
// to make that happen. now, consider what happens if we press a button twice:
// it won't change the parity at all, only add 2 to some joltages. ok.
// take the 2nd bit of each joltage. what's the minimum number of double-presses
// required to match that pattern? and so on.

func solve(filename string) {
	input, err := os.Open(filename)
	check(err)

	scanner := bufio.NewScanner(input)
	scanner.Split(bufio.ScanLines)
	total := 0
	for scanner.Scan() {
		line := scanner.Text()
		parts := strings.Fields(line)
		parts = parts[1:]
		var buts []uint32
		var target Jolts
		for _, p := range parts {
			j, err := parseJolt(p)
			check(err)
			if p[0] == '{' {
				target = j
			} else {
				buts = append(buts, j.uint32())
			}
		}
		fmt.Printf("%v %b %v\n", target, buts, parts)
		n := solveJolts(target, buts)
		fmt.Printf("%s = %v\n", line, n)
		total += n
		//fmt.Println(n)
	}
	check(scanner.Err())
	fmt.Println(total)

}

type Jolts [10]int16

func parseJolt(s string) (Jolts, error) {
	if s[0] == '(' {
		return parseJoltByIndex(s)
	} else if s[0] == '{' {
		return parseJoltByValue(s)
	}
	return Jolts{}, fmt.Errorf("invalid jolts: %q", s)
}

func parseJoltByIndex(s string) (Jolts, error) {
	var j Jolts
	s = strings.Trim(s, "()")
	for _, part := range strings.Split(s, ",") {
		idx, err := strconv.ParseInt(part, 10, 16)
		if err != nil {
			return j, err
		}
		j[idx] = 1
	}
	return j, nil
}

func parseJoltByValue(s string) (Jolts, error) {
	var j Jolts
	s = strings.Trim(s, "{}")
	for i, part := range strings.Split(s, ",") {
		val, err := strconv.ParseInt(part, 10, 16)
		if err != nil {
			return j, err
		}
		j[i] = int16(val)
	}
	return j, nil
}

func (j *Jolts) uint32() (u uint32) {
	for i, v := range j {
		if v != 0 {
			u |= uint32(1) << i
		}
	}
	return u
}

func (j Jolts) Sub(m uint32, value int) Jolts {
	for i := range j {
		if m>>i&1 != 0 {
			j[i] -= int16(value)
		}
	}
	return j
}

func (j Jolts) Valid() bool {
	for i := range j {
		if j[i] < 0 {
			return false
		}
	}
	return true
}

// TODO: this isn't quite enough. we need to know not only how many button presses
// a mask can be solved in, but also *the specific buttons* -- because even though
// each button is pressed at most once, two buttons can be connected to the same
// output so the joltages can be between like 0-10. this needs to be subtracted
// from the target, and two different button patterns can have different effects
// on the target.
//
// the good news is that we can cache and reuse the cost map computed in best(),
// since it depends only on the button wiring. all that's left after that is a
// graph search through the state space, using masks to prune each level.

func solveJolts(target Jolts, pool []uint32) int {
	var bits uint32
	for _, x := range target {
		bits |= uint32(x)
	}
	var answer int
	for bit := uint(0); bits>>bit > 0; bit++ {
		var mask uint32
		for i := range target {
			mask |= uint32(target[i]) >> bit << i
		}

		if mask == 0 {
			continue
		}

		n := best(mask, pool)
		answer += n << bit

		//for i, p := range pool {
		//	if mask>>i&1 != 0 {
		//		target.Sub(p, 1)
		//	}
		//}
	}
	return answer
}

func best(target uint32, pool []uint32) int {
	var cost = make(map[uint32]int)
	var states = []uint32{0}
	cost[0] = 0
	for _, p := range pool {
		// enumerate all the states that can be reached by toggling button p
		for _, s := range states {
			t := s ^ p
			c := cost[t] + 1

			if cost_t, ok := cost[t]; ok {
				if c < cost_t {
					cost[t] = c
				}
			} else {
				cost[t] = c
				states = append(states, t)
			}
		}
	}

	return cost[target]
}