2021 Day 10

2021/day10/main.go

@@ -0,0 +1,81 @@
+package main
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	lines := aoc.Input().ReadLines("day10.txt")
+	fmt.Println(part1(lines))
+	fmt.Println(part2(lines))
+}
+
+func part1(lines []string) (score int) {
+	for _, l := range lines {
+		points, _ := parseLine(l)
+		score += points
+	}
+	return
+}
+
+func part2(lines []string) int {
+	var scores []int
+	for _, l := range lines {
+		points, stack := parseLine(l)
+		if points == 0 {
+			scores = append(scores, autocomplete(stack))
+		}
+	}
+	sort.Ints(scores)
+	return scores[(len(scores)-1)/2]
+}
+
+func parseLine(line string) (score int, stack []rune) {
+	for _, c := range line {
+		switch c {
+		case '(', '[', '{', '<':
+			stack = append([]rune{c}, stack...)
+		case ')':
+			if stack[0] != '(' {
+				return 3, stack
+			}
+			stack = stack[1:]
+		case ']':
+			if stack[0] != '[' {
+				return 57, stack
+			}
+			stack = stack[1:]
+		case '}':
+			if stack[0] != '{' {
+				return 1197, stack
+			}
+			stack = stack[1:]
+		case '>':
+			if stack[0] != '<' {
+				return 25137, stack
+			}
+			stack = stack[1:]
+		}
+	}
+	return 0, stack
+}
+
+func autocomplete(stack []rune) (score int) {
+	for _, c := range stack {
+		score *= 5
+		switch c {
+		case '(':
+			score += 1
+		case '[':
+			score += 2
+		case '{':
+			score += 3
+		case '<':
+			score += 4
+		}
+	}
+	return
+}