Day 16 solution

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.

days/16.go

@@ -0,0 +1,104 @@
+package days
+
+import (
+	"fmt"
+	"math"
+
+	u "parnic.com/aoc2019/utilities"
+)
+
+type Day16 struct {
+	numberSet []int8
+}
+
+func (d *Day16) Parse() {
+	numberSequence := u.GetStringContents("16p")
+	d.numberSet = make([]int8, len(numberSequence))
+	for i, numRune := range numberSequence {
+		d.numberSet[i] = int8(numRune - '0')
+	}
+}
+
+func (d Day16) Num() int {
+	return 16
+}
+
+func (d *Day16) Part1() string {
+	transformed := make([]int8, len(d.numberSet))
+	copy(transformed, d.numberSet)
+
+	transformPattern := []int8{0, 1, 0, -1}
+
+	phases := 100
+	workingSet := make([]int8, len(transformed))
+	for i := 0; i < phases; i++ {
+		copy(workingSet, transformed)
+
+		// fmt.Printf("Phase %d. Input signal: %v\n", (i + 1), transformed)
+		for destIdx := range transformed {
+			repeated := 0
+			patternIdx := 0
+			workingVal := int64(0)
+			for idx := range transformed {
+				if repeated >= destIdx {
+					repeated = 0
+					patternIdx++
+					if patternIdx == len(transformPattern) {
+						patternIdx = 0
+					}
+				} else {
+					repeated++
+				}
+
+				// fmt.Printf("%d*%d", transformed[idx], transformPattern[patternIdx])
+				// if idx < len(transformed)-1 {
+				// 	fmt.Print(" + ")
+				// }
+				workingVal += int64(transformed[idx] * transformPattern[patternIdx])
+			}
+
+			workingSet[destIdx] = int8(int64(math.Abs(float64(workingVal))) % 10)
+			// fmt.Printf(" = %d\n", workingSet[destIdx])
+		}
+
+		copy(transformed, workingSet)
+	}
+
+	finalVal := 0
+	for i := range transformed[0:8] {
+		finalVal += int(transformed[i]) * int(math.Pow10(8-1-i))
+	}
+	return fmt.Sprintf("First 8 digits of the final output list: %s%d%s", u.TextBold, finalVal, u.TextReset)
+}
+
+func (d *Day16) Part2() string {
+	transformed := make([]int8, len(d.numberSet)*10000)
+	for i := 0; i < 10000; i++ {
+		copy(transformed[i*len(d.numberSet):(i*len(d.numberSet))+len(d.numberSet)], d.numberSet)
+	}
+
+	finalMsgOffset := 0
+	for i := 0; i < 7; i++ {
+		finalMsgOffset += int(d.numberSet[i]) * int(math.Pow10(7-1-i))
+	}
+
+	if finalMsgOffset < len(transformed)/2 {
+		panic("offset must be in the back half of the message for this solution to work")
+	}
+
+	phases := 100
+	for p := 0; p < phases; p++ {
+		rollingTotal := int8(0)
+		for i := len(transformed) - 1; i >= finalMsgOffset; i-- {
+			rollingTotal += transformed[i]
+			rollingTotal = rollingTotal % 10
+			transformed[i] = rollingTotal
+		}
+	}
+
+	finalVal := 0
+	for i := range transformed[finalMsgOffset : finalMsgOffset+8] {
+		finalVal += int(transformed[finalMsgOffset+i]) * int(math.Pow10(8-1-i))
+	}
+	return fmt.Sprintf("Embedded message in the final output list: %s%d%s", u.TextBold, finalVal, u.TextReset)
+}

inputs/16p.txt

@@ -0,0 +1 @@
+59791875142707344554745984624833270124746225787022156176259864082972613206097260696475359886661459314067969858521185244807128606896674972341093111690401527976891268108040443281821862422244152800144859031661510297789792278726877676645835805097902853584093615895099152578276185267316851163313487136731134073054989870018294373731775466754420075119913101001966739563592696702233028356328979384389178001923889641041703308599918672055860556825287836987992883550004999016194930620165247185883506733712391462975446192414198344745434022955974228926237100271949068464343172968939069550036969073411905889066207300644632441054836725463178144030305115977951503567

inputs/16s1.txt

@@ -0,0 +1 @@
+12345678

inputs/16s2.txt

@@ -0,0 +1 @@
+80871224585914546619083218645595

inputs/16s3.txt

@@ -0,0 +1 @@
+19617804207202209144916044189917

inputs/16s4.txt

@@ -0,0 +1 @@
+69317163492948606335995924319873

main.go

@@ -46,6 +46,7 @@ var dayMap = []day{
 	&days.Day13{},
 	&days.Day14{},
 	&days.Day15{},
+	&days.Day16{},
 }
 
 func main() {