might be making a mistake starting 2018...

2019/day16/part2/main.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"adventofcode/util"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	// parse input file into a slice of numbers
+	input := util.ReadFile("../input.txt")
+	// input := "03036732577212944063491565474664" // test should output "84462026"
+	characters := strings.Split(input, "")
+
+	digits := make([]int, len(characters)*10000)
+	for i := 0; i < 10000; i++ {
+		for j, v := range characters {
+			digits[i*len(characters)+j], _ = strconv.Atoi(v)
+		}
+	}
+
+	var offsetIndex int
+	for i := 0; i < 7; i++ {
+		offsetIndex *= 10
+		offsetIndex += digits[i]
+	}
+	fmt.Println("offsetIndex", offsetIndex)
+
+	// run through 100 phases, overwriting digits
+	for i := 0; i < 100; i++ {
+		digits = getNextOutputNumber(digits)
+		// output a time to make sure this is running fast enough
+		fmt.Printf("output received at %v, %v to go\n", time.Now(), 100-i-1)
+	}
+
+	// Transform into AoC output
+	var firstEightDigits int
+	for i := 0; i < 8; i++ {
+		firstEightDigits *= 10
+		firstEightDigits += digits[i+offsetIndex]
+	}
+	fmt.Printf("\nOffset 8 digits after 100 phases: %v\n", firstEightDigits)
+	fmt.Println("Expect 84462026 for test, 36265589 for actual input")
+}
+
+// takes in digits and all patterns, generates next set of digits
+func getNextOutputNumber(digits []int) []int {
+	// ONE INDEX THE PARTIAL SUMS, so partials[0] = 0, partials[1] = digits[0]
+	partials := make([]int, len(digits)+1)
+	for i := range digits {
+		// add previous partial subset if i is not zero
+		partials[i+1] += partials[i]
+		// add digit on as well
+		partials[i+1] += digits[i]
+	}
+
+	output := make([]int, len(digits))
+	for i := range digits {
+		chunkLength := i + 1
+
+		adding := true
+		for start := i; start < len(digits); start += chunkLength * 2 {
+			// calculate chunk sum
+			startOfChunkIndex := chunkLength - 1
+			endOfChunkIndex := start + chunkLength - 1
+			if endOfChunkIndex >= len(digits) {
+				endOfChunkIndex = len(digits) - 1
+			}
+			chunkSum := partials[endOfChunkIndex] - partials[startOfChunkIndex]
+
+			// increment or decrements output index
+			if adding {
+				output[i] += chunkSum
+			} else {
+				output[i] -= chunkSum
+			}
+			adding = !adding
+		}
+	}
+
+	// make all output digits positive & single digits
+	// NOTE this is not equivalent to taking the mod of a negative number!
+	// This has to be done because of the problem's spec to just take the 1's digit
+	for i, v := range output {
+		if v < 0 {
+			output[i] *= -1
+		}
+		output[i] %= 10
+	}
+
+	return output
+}