aoc2021/eighteen/snail.go

package eighteen

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

type value struct {
	p *pair
	v int
}

type pair struct {
	left  value
	right value
}

type snail struct {
	input []value
}

func (s *snail) load(filename string) error {
	file, err := os.Open(filename)
	if err != nil {
		return err
	}
	defer file.Close()
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		s.input = append(s.input, parse(scanner.Text()))
	}

	return nil
}

func (s *snail) largestMagnitude() int {
	max := 0

	for i := 0; i < len(s.input) - 1; i++ {
		for j := i + 1; j < len(s.input); j++ {
			// i + j
			sum1 := s.input[i].add(s.input[j])
			mag := sum1.magnitude()
			if mag > max {
				max = mag
			}

			// j + i
			sum2 := s.input[j].add(s.input[i])
			mag = sum2.magnitude()
			if mag > max {
				max = mag
			}
		}
	}

	return max
}

func (s *snail) sum() value {
	v := value{}
	for i, n := range s.input {
		if i == 0 {
			v = n
			continue
		}
		v = v.add(n)
	}
	return v
}

func (v *value) add(vb value) value {
	sum := value{
		p: &pair{
			left:  *v,
			right: vb,
		},
	}

	reduced := sum.reduce()
	for reduced != nil {
		r := reduced.reduce()
		if r == nil {
			return *reduced
		}
		reduced = r
	}

	return sum
}

func (v *value) reduce() *value {
	reduced := v.copy()
	// Check for nests deeper than 4
	if reduced.p != nil {
		_, exploded, _, _ := reduced.p.explode(1)
		if exploded {
			return &reduced
		}
	}

	// Check for values greater than 10
	_, split := reduced.split()
	if split {
		return &reduced
	}
	return nil
}

func (v *value) magnitude() int {
	if v.p == nil {
		return v.v
	}

	return (3 * v.p.left.magnitude()) + (2 * v.p.right.magnitude())
}

func (p *pair) explode(depth int) (int, bool, bool, bool) {
	if p == nil || (p.left.p == nil && p.right.p == nil) {
		return 0, false, false, false
	}

	if depth == 4 {
		if p.left.p != nil {
			if p.right.p != nil {
				p.right.p.increaseLeft(p.left.p.right.v, false)
			} else {
				p.right.v += p.left.p.right.v
			}
			rightVal := p.left.p.left.v
			p.left.v = 0
			p.left.p = nil

			return rightVal, true, true, false
		}

		if p.right.p != nil {
			if p.left.p != nil {
				p.left.p.increaseRight(p.right.p.left.v, false)
			} else {
				p.left.v += p.right.p.left.v
			}
			leftVal := p.right.p.right.v
			p.right.v = 0
			p.right.p = nil

			return leftVal, true, false, true
		}

		return 0, false, false, false
	}

	// Check if left has nested pair
	if p.left.p != nil {
		offender, exploded, left, right := p.left.p.explode(depth + 1)

		if exploded {
			if right {
				p.increaseRight(offender, false)
				return offender, exploded, left, false
			}

			return offender, exploded, left, right
		}
	}

	// Check if right has nested pair
	if p.right.p != nil {
		offender, exploded, left, right := p.right.p.explode(depth + 1)

		if exploded {
			if left {
				p.increaseLeft(offender, false)
				return 0, true, false, right
			}
			return offender, true, left, right
		}
	}

	return 0, false, false, false
}

func (p *pair) increaseLeft(amount int, swapped bool) {
	if p.left.p != nil {
		if !swapped {
			p.left.p.increaseRight(amount, true)
		} else {
			p.left.p.increaseLeft(amount, true)
		}
	} else {
		p.left.v += amount
	}
}

func (p *pair) increaseRight(amount int, swapped bool) {
	if p.right.p != nil {
		if !swapped {
			p.right.p.increaseLeft(amount, true)
		} else {
			p.right.p.increaseRight(amount, swapped)
		}
	} else {
		p.right.v += amount
	}
}

func (v *value) split() (*pair, bool) {
	if v.p == nil {
		if v.v < 10 {
			return nil, false
		}

		left := v.v / 2
		right := v.v / 2
		if v.v%2 == 1 {
			right++
		}

		return &pair{
			left: value{
				v: left,
			},
			right: value{
				v: right,
			},
		}, true
	}

	// Check left
	newPair, split := v.p.left.split()
	if split {
		if newPair != nil {
			v.p.left.p = newPair
			v.p.left.v = 0
		}
		return nil, split
	}

	// Check left
	newPair, split = v.p.right.split()
	if split {
		if newPair != nil {
			v.p.right.p = newPair
			v.p.right.v = 0
		}
		return nil, split
	}

	return nil, false
}

func (p *pair) copy() *pair {
	if p == nil {
		return nil
	}

	return &pair{
		left:  p.left.copy(),
		right: p.right.copy(),
	}
}

func (v *value) copy() value {
	return value{
		p: v.p.copy(),
		v: v.v,
	}
}

func (v *value) print() string {
	if v == nil {
		return "No value to print"
	}

	if v.p == nil {
		return fmt.Sprintf("%d", v.v)
	}

	s := "["
	s += v.p.left.print()
	s += ","
	s += v.p.right.print()
	s += "]"

	return s
}

func parse(input string) value {
	v := value{}
	rawValue := ""
	foundPair := false
	bracketCount := 0
	// Look for the comma in outermost pair
	commaIndex := 0
	for i, r := range input {
		if bracketCount == 0 && r == '[' {
			foundPair = true
			rawValue = ""
		}

		switch r {
		case '[':
			bracketCount++
			if bracketCount != 1 {
				rawValue += string(r)
			}
		case ']':
			bracketCount--
			if bracketCount != 0 {
				rawValue += string(r)
			}
		case ',':
			if bracketCount == 1 {
				commaIndex = i
			}
			rawValue += string(r)
		default:
			rawValue += string(r)
		}
	}

	if foundPair {
		v.p = &pair{
			left:  parse(rawValue[:commaIndex-1]),
			right: parse(rawValue[commaIndex:]),
		}
	} else {
		number, err := strconv.Atoi(rawValue)
		if err != nil {
			panic(err)
		}
		v.v = number
	}

	return v
}