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.

Also, fun fact: changing the numberSet from an array of ints (my initial implementation) to an array of int8's (what's here now) reduced the memory footprint of the application substantially (from something like 50mb to closer to 8).

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() {