adventofcode/2023/day09/main.go

package main

import (
	"fmt"
	"strings"

	h "git.bullercodeworks.com/brian/adventofcode/helpers"
)

func main() {
	inp := h.StdinToStringSlice()
	solve(inp)
}

func solve(input []string) {
	var p1result, p2result int
	for i := range input {
		s := NewSeq(i, input[i])
		s.stabilize()
		s.predict()
		v2, v1 := s.value()
		p1result += v1
		p2result += v2
	}
	fmt.Println("# Part 1")
	fmt.Println(p1result)
	fmt.Println()
	fmt.Println("# Part 2")
	fmt.Println(p2result)
}

type Seq struct {
	row     int
	history [][]int
}

func NewSeq(row int, input string) *Seq {
	s := Seq{row: row}
	pts := strings.Fields(input)
	first := []int{}
	for i := range pts {
		first = append(first, h.Atoi(pts[i]))
	}
	s.history = append(s.history, first)
	return &s
}

func (s *Seq) value() (int, int) {
	return s.history[0][0], s.history[0][len(s.history[0])-1]
}
func (s *Seq) predict() {
	s.history[len(s.history)-1] = append(s.history[len(s.history)-1], 0)
	s.history[len(s.history)-1] = append([]int{0}, s.history[len(s.history)-1]...)
	if len(s.history) > 1 {
		for i := len(s.history) - 2; i >= 0; i-- {
			nV := s.history[i][len(s.history[i])-1] + s.history[i+1][len(s.history[i+1])-1]
			s.history[i] = append(s.history[i], nV)

			// And now go backwards
			pV := s.history[i][0] - s.history[i+1][0]
			s.history[i] = append([]int{pV}, s.history[i]...)
		}
	}
}

func (s *Seq) stabilize() {
	for !s.isStable() {
		s.iterate()
	}
}

func (s *Seq) iterate() {
	idx := len(s.history) - 1
	l := len(s.history[idx]) - 1
	var next []int
	for i := range s.history[idx] {
		if i < l {
			next = append(next, s.history[idx][i+1]-s.history[idx][i])
		}
	}
	s.history = append(s.history, next)
}

func (s *Seq) isStable() bool {
	for _, v := range s.history[len(s.history)-1] {
		if v != 0 {
			return false
		}
	}
	return true
}