might be making a mistake starting 2018...

2019/day16/part1/main.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"adventofcode/util"
+	"fmt"
+	"log"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	// parse input file into a slice of numbers
+	input := util.ReadFile("../input.txt")
+	characters := strings.Split(input, "")
+	digits := make([]int, len(characters))
+	for i, v := range characters {
+		digits[i], _ = strconv.Atoi(v)
+	}
+
+	// generate all patterns once at start
+	patterns := make([][]int, len(digits))
+	for i := range digits {
+		patterns[i] = generatePattern(i+1, len(digits))
+	}
+
+	// run through 100 phases, overwriting digits
+	for i := 0; i < 100; i++ {
+		digits = getNextOutputNumber(digits, patterns)
+	}
+
+	// Transform into AoC output
+	var firstEightDigits int
+	for i := 0; i < 8; i++ {
+		firstEightDigits *= 10
+		firstEightDigits += digits[i]
+	}
+	fmt.Printf("First 8 digits after 100 phases: %v\n", firstEightDigits)
+}
+
+// generatePattern takes in the index (one indexed, not zero) that is being considered, and returns the pattern to multiply
+// digits by
+func generatePattern(oneIndex, lengthNeeded int) []int {
+	if oneIndex < 1 {
+		log.Fatal("Input to generatePattern must be a positive int")
+	}
+	basePattern := []int{0, 1, 0, -1}
+	pattern := make([]int, 0, 4*(oneIndex+1))
+	for len(pattern)-1 < lengthNeeded {
+		for _, v := range basePattern {
+			for i := 0; i < oneIndex; i++ {
+				pattern = append(pattern, v)
+			}
+		}
+	}
+	return pattern[1 : lengthNeeded+1]
+}
+
+// takes in digits and all patterns, generates next set of digits
+func getNextOutputNumber(digits []int, patterns [][]int) []int {
+	output := make([]int, len(digits))
+	for index := range digits {
+		// for this index, sum up all products of digits and this index's pattern
+		var sum int
+		for i := 0; i < len(digits); i++ {
+			sum += patterns[index][i] * digits[i]
+		}
+
+		// ensure the sum is positive & take the single's place digit
+		if sum < 0 {
+			sum *= -1
+		}
+		sum %= 10
+
+		// assign to output slice for the same index
+		output[index] = sum
+	}
+
+	return output
+}