gocollatz/collatz/calculator.go

package collatz

import (
	"collatz/parsing"
	"fmt"
	"math/big"
)

type Calculator struct {
	parser   parsing.Parser
	three    *big.Int
	two      *big.Int
	one      *big.Int
	sequence Sequence
}
type Sequence struct {
	Iterations int
	Sequence   []*big.Int
}

func NewCalculator(parser parsing.Parser) *Calculator {
	r := &Calculator{
		parser: parser,
		three:  big.NewInt(3),
		two:    big.NewInt(2),
		one:    big.NewInt(1),
	}
	return r
}

func (calc *Calculator) CalculateCollatz() (Sequence, error) {
	number, err := calc.parser.Parse()
	if err != nil {
		return Sequence{}, fmt.Errorf("couldn't parse input: %w", err)
	}
	calc.sequence = Sequence{Iterations: 0}
	calc.sequence.Sequence = append(calc.sequence.Sequence, new(big.Int).Set(number))
	err = calc.calculateNextCollatzNumber(*number)
	if err != nil {
		return Sequence{}, err
	}
	return calc.sequence, nil
}

func (calc *Calculator) calculateNextCollatzNumber(in big.Int) error {
	if in.Cmp(calc.one) < 0 {
		return fmt.Errorf("must be greater than 0")
	}
	if in.Cmp(calc.one) == 0 {
		return nil
	}
	var newNumber *big.Int
	if new(big.Int).And(&in, calc.one).Cmp(calc.one) == 0 {
		newNumber = new(big.Int).Add(new(big.Int).Mul(&in, calc.three), calc.one)
	} else {
		newNumber = new(big.Int).Div(&in, calc.two)
	}
	calc.sequence.Iterations++
	calc.sequence.Sequence = append(calc.sequence.Sequence, newNumber)
	return calc.calculateNextCollatzNumber(*newNumber)
}