Trying to finish up 2019

2019/day22/main.go

@@ -0,0 +1,101 @@
+package main
+
+import (
+	"fmt"
+	"math/big"
+	"strconv"
+
+	helpers "git.bullercodeworks.com/brian/adventofcode/helpers"
+)
+
+func main() {
+	pt := helpers.GetArgNumber(1)
+	inp := helpers.StdinToStringSlice()
+	if pt == "1" {
+		part1(inp)
+	} else {
+		part2(inp)
+	}
+}
+
+func NewDeck(n int) []int {
+	s := make([]int, n)
+	for i := 0; i < n; i++ {
+		s[i] = i
+	}
+	return s
+}
+
+func deal(s []int) []int {
+	s2 := make([]int, len(s))
+	for i := range s {
+		s2[len(s)-1-i] = s[i]
+	}
+	return s2
+}
+
+func cut(s []int, n int) []int {
+	if n >= 0 {
+		return append(s[n:], s[:n]...)
+	}
+	n = -n
+	return append(s[len(s)-n:], s[:len(s)-n]...)
+}
+
+func dealn(s []int, n int) []int {
+	s2 := make([]int, len(s))
+	for i := range s {
+		s2[(i*n)%len(s)] = s[i]
+	}
+	return s2
+}
+
+func part1(inp []string) {
+	s := NewDeck(10007)
+	for _, line := range inp {
+		if line == "deal into new stack" {
+			s = deal(s)
+		} else if line[:3] == "cut" {
+			i, _ := strconv.Atoi(line[4:])
+			s = cut(s, i)
+		} else if line[:19] == "deal with increment" {
+			i, _ := strconv.Atoi(line[20:])
+			s = dealn(s, i)
+		}
+	}
+	for i, v := range s {
+		if v == 2019 {
+			fmt.Println(i)
+			return
+		}
+	}
+}
+
+func part2(inp []string) {
+	deckSize := big.NewInt(119315717514047)
+	shuffles := big.NewInt(101741582076661)
+	off, inc := big.NewInt(0), big.NewInt(1)
+	for _, line := range inp {
+		if line == "deal into new stack" {
+			inc.Mul(inc, big.NewInt(-1))
+			off.Add(off, inc)
+		} else if line[:3] == "cut" {
+			i, _ := strconv.Atoi(line[4:])
+			off.Add(off, big.NewInt(0).Mul(big.NewInt(int64(i)), inc))
+		} else if line[:19] == "deal with increment" {
+			i, _ := strconv.Atoi(line[20:])
+			inc.Mul(inc, big.NewInt(0).Exp(big.NewInt(int64(i)), big.NewInt(0).Sub(deckSize, big.NewInt(2)), deckSize))
+		}
+	}
+	lastInc := big.NewInt(0).Exp(inc, shuffles, deckSize)
+	lastOff := big.NewInt(0).Exp(inc, shuffles, deckSize)
+	lastOff.Sub(big.NewInt(1), lastOff)
+	mod := big.NewInt(0).Exp(big.NewInt(0).Sub(big.NewInt(1), inc), big.NewInt(0).Sub(deckSize, big.NewInt(2)), deckSize)
+	lastOff.Mul(lastOff, mod)
+	lastOff.Mul(lastOff, off)
+
+	res := big.NewInt(0).Mul(big.NewInt(2020), lastInc)
+	res.Add(res, lastOff)
+	res.Mod(res, deckSize)
+	fmt.Println("Result:", res)
+}