Parnic
c788813cd2
Following the formula for part 1 was straightforward enough, but finding the pattern for part 2 and deducing the shortcut formula took me a while. That first 0 1 propagate enough that by the time we get halfway through applying the formula, it's all 0s and 1s, so sort of like an addition with a mask on what numbers we're adding.
105 lines
2.5 KiB
Go
105 lines
2.5 KiB
Go
package days
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import (
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"fmt"
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"math"
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u "parnic.com/aoc2019/utilities"
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)
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type Day16 struct {
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numberSet []int8
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}
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func (d *Day16) Parse() {
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numberSequence := u.GetStringContents("16p")
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d.numberSet = make([]int8, len(numberSequence))
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for i, numRune := range numberSequence {
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d.numberSet[i] = int8(numRune - '0')
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}
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}
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func (d Day16) Num() int {
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return 16
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}
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func (d *Day16) Part1() string {
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transformed := make([]int8, len(d.numberSet))
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copy(transformed, d.numberSet)
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transformPattern := []int8{0, 1, 0, -1}
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phases := 100
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workingSet := make([]int8, len(transformed))
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for i := 0; i < phases; i++ {
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copy(workingSet, transformed)
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// fmt.Printf("Phase %d. Input signal: %v\n", (i + 1), transformed)
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for destIdx := range transformed {
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repeated := 0
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patternIdx := 0
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workingVal := int64(0)
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for idx := range transformed {
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if repeated >= destIdx {
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repeated = 0
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patternIdx++
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if patternIdx == len(transformPattern) {
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patternIdx = 0
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}
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} else {
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repeated++
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}
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// fmt.Printf("%d*%d", transformed[idx], transformPattern[patternIdx])
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// if idx < len(transformed)-1 {
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// fmt.Print(" + ")
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// }
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workingVal += int64(transformed[idx] * transformPattern[patternIdx])
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}
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workingSet[destIdx] = int8(int64(math.Abs(float64(workingVal))) % 10)
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// fmt.Printf(" = %d\n", workingSet[destIdx])
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}
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copy(transformed, workingSet)
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}
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finalVal := 0
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for i := range transformed[0:8] {
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finalVal += int(transformed[i]) * int(math.Pow10(8-1-i))
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}
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return fmt.Sprintf("First 8 digits of the final output list: %s%d%s", u.TextBold, finalVal, u.TextReset)
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}
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func (d *Day16) Part2() string {
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transformed := make([]int8, len(d.numberSet)*10000)
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for i := 0; i < 10000; i++ {
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copy(transformed[i*len(d.numberSet):(i*len(d.numberSet))+len(d.numberSet)], d.numberSet)
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}
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finalMsgOffset := 0
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for i := 0; i < 7; i++ {
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finalMsgOffset += int(d.numberSet[i]) * int(math.Pow10(7-1-i))
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}
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if finalMsgOffset < len(transformed)/2 {
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panic("offset must be in the back half of the message for this solution to work")
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}
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phases := 100
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for p := 0; p < phases; p++ {
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rollingTotal := int8(0)
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for i := len(transformed) - 1; i >= finalMsgOffset; i-- {
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rollingTotal += transformed[i]
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rollingTotal = rollingTotal % 10
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transformed[i] = rollingTotal
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}
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}
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finalVal := 0
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for i := range transformed[finalMsgOffset : finalMsgOffset+8] {
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finalVal += int(transformed[finalMsgOffset+i]) * int(math.Pow10(8-1-i))
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}
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return fmt.Sprintf("Embedded message in the final output list: %s%d%s", u.TextBold, finalVal, u.TextReset)
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}
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