commit a9420a1a07acb9ec64093621aeae81cb4bde2655
Author: Lars Mikal Rogne <logiar@pop-os.localdomain>
Date:   Wed Apr 24 19:35:43 2024 +0200

    Initial commit

diff --git a/collatz.go b/collatz.go
new file mode 100644
index 0000000..cab9120
--- /dev/null
+++ b/collatz.go
@@ -0,0 +1,11 @@
+package main
+
+import (
+	"collatz/collatz"
+	"collatz/io"
+)
+
+func main() {
+	calculator := collatz.NewCalculator(new(io.Cliargs))
+	calculator.Collatz()
+}
diff --git a/collatz/calculator.go b/collatz/calculator.go
new file mode 100644
index 0000000..7be8fb4
--- /dev/null
+++ b/collatz/calculator.go
@@ -0,0 +1,54 @@
+package collatz
+
+import (
+	"collatz/io"
+	"fmt"
+	"log"
+	"math/big"
+)
+
+type Calculator struct {
+	parser *io.Cliargs
+	three  *big.Int
+	two    *big.Int
+	one    *big.Int
+}
+
+func NewCalculator(parser *io.Cliargs) Calculator {
+	r := Calculator{
+		parser: parser,
+		three:  new(big.Int).SetInt64(3),
+		two:    new(big.Int).SetInt64(2),
+		one:    new(big.Int).SetInt64(1),
+	}
+	return r
+}
+
+func (o Calculator) Collatz() {
+	err := o.parser.Parse()
+	if err != nil {
+		log.Panicf("Couldn't parse input: %s\n", err)
+	}
+	number := o.parser.GetNumber()
+	fmt.Print(number.Text(10))
+	o.nextNumber(number)
+	fmt.Println()
+}
+
+func (o Calculator) nextNumber(in *big.Int) {
+	if in.Cmp(o.one) < 0 {
+		log.Panicln("Must be greater than 0")
+	}
+	if in.Cmp(o.one) == 0 {
+		return
+	}
+	var newNumber *big.Int
+	if new(big.Int).And(in, o.one).Cmp(o.one) == 0 {
+		newNumber = new(big.Int).Add(new(big.Int).Mul(in, o.three), o.one)
+	} else {
+		newNumber = new(big.Int).Div(in, o.two)
+	}
+
+	fmt.Printf(" -> %s", newNumber.Text(10))
+	o.nextNumber(newNumber)
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..48dad79
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module collatz
+
+go 1.22
diff --git a/io/cliargs.go b/io/cliargs.go
new file mode 100644
index 0000000..f396516
--- /dev/null
+++ b/io/cliargs.go
@@ -0,0 +1,28 @@
+package io
+
+import (
+	"errors"
+	"math/big"
+	"os"
+)
+
+type Cliargs struct {
+	value *big.Int
+}
+
+func (arg *Cliargs) Parse() error {
+	args := os.Args[1:]
+	if len(args) != 1 {
+		return errors.New("only expected 1 arg")
+	}
+	num, success := new(big.Int).SetString(args[0], 10)
+	if !success {
+		return errors.New("argument is not an integer")
+	}
+	arg.value = num
+	return nil
+}
+
+func (arg *Cliargs) GetNumber() *big.Int {
+	return arg.value
+}