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