might be making a mistake starting 2018...

2019/day22/part2/main.go

@@ -0,0 +1,90 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"math/big"
+	"strconv"
+	"strings"
+)
+
+const CmdDeal string = "deal into new stack"
+const CmdCutN string = "cut"
+const CmdDealN string = "deal with increment"
+
+func MatchCmd(cmd, cmdToMatch string) (bool, int) {
+	if len(cmd) < len(cmdToMatch) || cmd[:len(cmdToMatch)] != cmdToMatch {
+		return false, 0
+	}
+
+	if len(cmd) == len(cmdToMatch) {
+		return true, 0
+	}
+
+	i := strings.LastIndex(cmd, " ")
+	x := cmd[i+1:]
+	n, err := strconv.Atoi(x)
+	if err != nil {
+		panic(err)
+	}
+	return true, n
+}
+
+func ReadCommands(path string) []string {
+	dat, err := ioutil.ReadFile(path)
+	if err != nil {
+		panic(err)
+	}
+	dat = dat[:len(dat)]
+
+	txt := string(dat)
+	txt = strings.TrimRight(txt, "\n")
+	lines := strings.Split(txt, "\n")
+
+	return lines
+}
+
+func IndexShuffle(index int64, l int64, times int64, cmds []string) int64 {
+	v := big.NewInt(index)
+	m := big.NewInt(l)
+	t := big.NewInt(times)
+
+	i := big.NewInt(0)
+	d := big.NewInt(1)
+
+	for _, cmd := range cmds {
+		if cmd == CmdDeal {
+			d.Neg(d)
+			i.Add(i, d)
+		} else if ok, n := MatchCmd(cmd, CmdCutN); ok {
+			x := big.NewInt(int64(n))
+			i.Add(i, x.Mul(x, d))
+		} else if ok, n := MatchCmd(cmd, CmdDealN); ok {
+			x := big.NewInt(int64(n))
+			x.ModInverse(x, m)
+			d.Mul(d, x)
+		}
+	}
+
+	// (1-d)**(m-2) % m
+	a := big.NewInt(1)
+	a.Sub(a, d).ModInverse(a, m)
+
+	// d = d**t % m
+	d.Exp(d, t, m)
+	// it = (1-d) * a * i
+	it := big.NewInt(1)
+	it.Sub(it, d).Mul(it, a).Mul(it, i)
+	v.Mul(v, d).Add(v, it).Mod(v, m)
+
+	return v.Int64()
+}
+
+func main() {
+	cmds := ReadCommands("../input.txt")
+
+	l := int64(119315717514047)
+	times := int64(101741582076661)
+	index := IndexShuffle(2020, l, times, cmds)
+	fmt.Println(index)
+}