Day 22 solution

Part 1 was very straightforward. I believe there are ways to calculate this answer without the complicated maths from part 2 and without actually applying each of the instructions, but I'm not concerned with finding it.

Part 2 was a big "nope" from me. I went out and found this answer and I don't understand it. I'm okay with that.

days/22.go

@@ -0,0 +1,155 @@
+package days
+
+import (
+	"fmt"
+	"math"
+	"math/big"
+	"strconv"
+	"strings"
+
+	u "parnic.com/aoc2019/utilities"
+)
+
+type day22Instruction int
+
+const (
+	day22InstructionNewStack day22Instruction = iota
+	day22InstructionCut
+	day22InstructionDealIncrement
+)
+
+type day22Shuffle struct {
+	instruction day22Instruction
+	arg         int
+}
+
+type Day22 struct {
+	shuffles []day22Shuffle
+}
+
+func (d *Day22) Parse() {
+	lines := u.GetStringLines("22p")
+	d.shuffles = make([]day22Shuffle, len(lines))
+
+	for idx, line := range lines {
+		split := strings.Split(line, " ")
+		if split[0] == "deal" {
+			if split[1] == "into" {
+				d.shuffles[idx] = day22Shuffle{instruction: day22InstructionNewStack}
+			} else if split[1] == "with" {
+				arg, err := strconv.Atoi(split[3])
+				if err != nil {
+					panic(err)
+				}
+				d.shuffles[idx] = day22Shuffle{
+					instruction: day22InstructionDealIncrement,
+					arg:         arg,
+				}
+			}
+		} else if split[0] == "cut" {
+			arg, err := strconv.Atoi(split[1])
+			if err != nil {
+				panic(err)
+			}
+			d.shuffles[idx] = day22Shuffle{
+				instruction: day22InstructionCut,
+				arg:         arg,
+			}
+		}
+	}
+}
+
+func (d Day22) Num() int {
+	return 22
+}
+
+func (d Day22) applyShuffle(s day22Shuffle, stack, scratch []int) {
+	switch s.instruction {
+	case day22InstructionNewStack:
+		for i := 0; i < len(stack)/2; i++ {
+			stack[i], stack[len(stack)-1-i] = stack[len(stack)-1-i], stack[i]
+		}
+
+		// there's probably a way to do these two in place...
+	case day22InstructionCut:
+		absArg := int(math.Abs(float64(s.arg)))
+		for i, v := range stack {
+			if s.arg > 0 {
+				if i < absArg {
+					scratch[len(scratch)-absArg+i] = v
+				} else {
+					scratch[i-absArg] = v
+				}
+			} else {
+				if i < absArg {
+					scratch[i] = stack[len(stack)-absArg+i]
+				} else {
+					scratch[i] = stack[i-absArg]
+				}
+			}
+		}
+		copy(stack, scratch)
+
+	case day22InstructionDealIncrement:
+		for i, v := range stack {
+			scratch[(i*s.arg)%len(stack)] = v
+		}
+		copy(stack, scratch)
+	}
+}
+
+func (d *Day22) Part1() string {
+	deckSize := 10007
+	// deckSize := 10
+
+	stack := make([]int, deckSize)
+	for i := range stack {
+		stack[i] = i
+	}
+
+	scratch := make([]int, len(stack))
+
+	for _, s := range d.shuffles {
+		d.applyShuffle(s, stack, scratch)
+	}
+
+	pos := -1
+	for i, v := range stack {
+		if v == 2019 {
+			pos = i
+			break
+		}
+	}
+
+	return fmt.Sprintf("Card 2019 is at position %s%d%s", u.TextBold, pos, u.TextReset)
+}
+
+func (d *Day22) Part2() string {
+	n, iter := big.NewInt(119315717514047), big.NewInt(101741582076661)
+	offset, increment := big.NewInt(0), big.NewInt(1)
+	for _, s := range d.shuffles {
+		switch s.instruction {
+		case day22InstructionNewStack:
+			increment.Mul(increment, big.NewInt(-1))
+			offset.Add(offset, increment)
+		case day22InstructionCut:
+			offset.Add(offset, big.NewInt(0).Mul(big.NewInt(int64(s.arg)), increment))
+		case day22InstructionDealIncrement:
+			increment.Mul(increment, big.NewInt(0).Exp(big.NewInt(int64(s.arg)), big.NewInt(0).Sub(n, big.NewInt(2)), n))
+		}
+	}
+
+	finalIncr := big.NewInt(0).Exp(increment, iter, n)
+
+	finalOffs := big.NewInt(0).Exp(increment, iter, n)
+	finalOffs.Sub(big.NewInt(1), finalOffs)
+	invmod := big.NewInt(0).Exp(big.NewInt(0).Sub(big.NewInt(1), increment), big.NewInt(0).Sub(n, big.NewInt(2)), n)
+	finalOffs.Mul(finalOffs, invmod)
+	finalOffs.Mul(finalOffs, offset)
+
+	answer := big.NewInt(0).Mul(big.NewInt(2020), finalIncr)
+	answer.Add(answer, finalOffs)
+	answer.Mod(answer, n)
+
+	return fmt.Sprintf("Card at position 2020: %s%d%s", u.TextBold, answer, u.TextReset)
+}

inputs/22p.txt

@@ -0,0 +1,100 @@
+deal into new stack
+cut -2732
+deal into new stack
+deal with increment 57
+cut 5974
+deal into new stack
+deal with increment 32
+cut -1725
+deal with increment 24
+cut 6093
+deal with increment 6
+cut -2842
+deal with increment 14
+cut 2609
+deal with increment 12
+cut -6860
+deal with increment 51
+cut -6230
+deal with increment 61
+cut 3152
+deal with increment 28
+cut 2202
+deal into new stack
+deal with increment 60
+cut 433
+deal into new stack
+cut -6256
+deal with increment 13
+deal into new stack
+cut 8379
+deal into new stack
+deal with increment 54
+cut 1120
+deal with increment 16
+cut -5214
+deal with increment 63
+deal into new stack
+cut -8473
+deal with increment 11
+cut 228
+deal with increment 45
+cut -6755
+deal with increment 50
+cut -3391
+deal with increment 44
+cut -1341
+deal with increment 28
+cut -6788
+deal with increment 52
+cut 3062
+deal with increment 41
+cut 4541
+deal with increment 57
+cut -7962
+deal with increment 56
+cut 9621
+deal with increment 57
+cut 3881
+deal with increment 36
+deal into new stack
+deal with increment 45
+cut 522
+deal with increment 9
+deal into new stack
+deal with increment 60
+deal into new stack
+deal with increment 12
+cut -9181
+deal with increment 63
+deal into new stack
+deal with increment 14
+cut -2906
+deal with increment 10
+cut 848
+deal with increment 75
+cut 798
+deal with increment 29
+cut 1412
+deal with increment 10
+deal into new stack
+cut -5295
+deal into new stack
+cut 4432
+deal with increment 72
+cut -7831
+deal into new stack
+cut 6216
+deal into new stack
+deal with increment 7
+cut -1720
+deal into new stack
+cut -5465
+deal with increment 70
+cut -5173
+deal with increment 7
+cut 3874
+deal with increment 65
+cut 921
+deal with increment 8
+cut -3094

inputs/22s1.txt

@@ -0,0 +1,3 @@
+deal with increment 7
+deal into new stack
+deal into new stack

inputs/22s2.txt

@@ -0,0 +1,3 @@
+cut 6
+deal with increment 7
+deal into new stack

inputs/22s3.txt

@@ -0,0 +1,3 @@
+deal with increment 7
+deal with increment 9
+cut -2

inputs/22s4.txt

@@ -0,0 +1,10 @@
+deal into new stack
+cut -2
+deal with increment 7
+cut 8
+cut -4
+deal with increment 7
+cut 3
+deal with increment 9
+deal with increment 3
+cut -1

main.go

@@ -55,6 +55,7 @@ var dayMap = []day{
 	&days.Day19{},
 	&days.Day20{},
 	&days.Day21{},
+	&days.Day22{},
 }
 
 func main() {