aoc2024/day-17/internal/two/solve.go

package two

import (
	"bytes"
	"math"
	"slices"
)

type operand int
type instructionFn func(operand) (int, bool, int64, bool)

func Solve(buf []byte) (int64, error) {
	r := bytes.NewReader(buf)
	_, program, err := parseLines(r)
	if err != nil {
		return 0, err
	}

	computer := newComputer(make(map[string]int64))
	a := lowestA(program, computer)

	return a, nil
}

func lowestA(program []int64, computer *computer) int64 {
	var a int64 = 1
	computer.bumpRegisters(a)
	var results []int64
	for {
		results = computer.run(program)

		if slices.Equal(results, program) {
			return a
		}

		if len(program) > len(results) {
			a *= 2
			computer.bumpRegisters(a)
			continue
		}

		if len(program) == len(results) {
			for j := len(program) - 1; j >= 0; j-- {
				if program[j] != results[j] {
					a += int64(math.Pow(8, float64(j)))
					break
				}
			}
		}

		if len(program) < len(results) {
			a /= 2
		}

		computer.bumpRegisters(a)
	}
}