2020 day14 - circling around a slice, matching patterns, simple-ish

2018/day14/main.go

@@ -0,0 +1,124 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	if part == 1 {
+		ans := part1(util.ReadFile("./input.txt"))
+		fmt.Println("Output:", ans)
+	} else {
+		ans := part2(util.ReadFile("./input.txt"))
+		fmt.Println("Output:", ans)
+	}
+}
+
+func part1(input string) int {
+	recipesWanted := parseInput(input)
+
+	recipes := []int{3, 7}
+	elf1, elf2 := 0, 1
+
+	for len(recipes) < recipesWanted+10 {
+		recipes, elf1, elf2 = step(recipes, elf1, elf2)
+		// fmt.Println("new recipes", recipes)
+		// fmt.Println("new elf indices", elf1, elf2)
+	}
+
+	ans := 0
+	for i := 0; i < 10; i++ {
+		ans *= 10
+		ans += recipes[i+recipesWanted]
+	}
+
+	return ans
+}
+
+func part2(input string) int {
+	patternToFind := parseInput(input)
+
+	recipes := []int{3, 7}
+	elf1, elf2 := 0, 1
+
+	var recipesToLeft int
+	for recipesToLeft == 0 {
+		recipes, elf1, elf2 = step(recipes, elf1, elf2)
+
+		// check patterns
+		recipesToLeft = patternMatch(recipes, patternToFind)
+	}
+
+	return recipesToLeft
+}
+
+func parseInput(input string) int {
+	lines := strings.Split(input, "\n")
+
+	return util.StrToInt(lines[0])
+}
+
+func step(recipes []int, elf1, elf2 int) ([]int, int, int) {
+	recipe1, recipe2 := recipes[elf1], recipes[elf2]
+	newRecipe := recipe1 + recipe2
+
+	// add new recipes onto slice
+	if newRecipe >= 10 {
+		recipes = append(recipes, 1)
+	}
+	recipes = append(recipes, newRecipe%10)
+
+	// get new elf indices
+	elf1 += recipe1 + 1
+	elf2 += recipe2 + 1
+	elf1 %= len(recipes)
+	elf2 %= len(recipes)
+
+	return recipes, elf1, elf2
+}
+
+func patternMatch(recipes []int, patternToFind int) int {
+	patternLength := len(strconv.Itoa(patternToFind))
+	// not enough recipes to compare
+	if len(recipes) < patternLength {
+		return 0
+	}
+
+	// check last two recipes in recipes slice (because it will grow by at most
+	// 2 recipes per step
+	var pattern int
+	// get first pattern
+	for i := len(recipes) - patternLength; i < len(recipes); i++ {
+		pattern *= 10
+		pattern += recipes[i]
+	}
+
+	if pattern == patternToFind {
+		return len(recipes) - patternLength
+	}
+
+	// check second pattern
+	// removes one's digit
+	pattern /= 10
+	// add new 10^6 digit
+	if index := len(recipes) - patternLength - 1; index >= 0 {
+		pattern += 100000 * recipes[index]
+	}
+
+	if pattern == patternToFind {
+		return len(recipes) - patternLength - 1
+	}
+
+	// return zero when no pattern is found
+	return 0
+}