2022 day 13

Author: pvainio

2022/go/day13/day13.go

@@ -0,0 +1,97 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"reflect"
+	"sort"
+	"strings"
+)
+
+func main() {
+	input, _ := os.ReadFile("../../input/day13.txt")
+	pairs := readPairs(string(input))
+
+	packets := [][]any{}
+	sum := 0
+	for i, pair := range pairs {
+		if less(pair[0], pair[1]) {
+			sum += i + 1
+		}
+		packets = append(packets, pair[0], pair[1])
+	}
+	fmt.Printf("part1: %v\n", sum)
+
+	packets = append(packets, dividers...)
+	sort.Sort(ByPacket(packets))
+	var key int
+	for i, p := range packets {
+		if reflect.DeepEqual(p, dividers[0]) {
+			key = i + 1
+		} else if reflect.DeepEqual(p, dividers[1]) {
+			key = key * (i + 1)
+		}
+	}
+	fmt.Printf("part2: %v\n", key)
+}
+
+func less(left []any, right []any) (less bool) {
+	return compare(left, right, 0) <= 0
+}
+
+func compare(left []any, right []any, i int) (res int) {
+	if i >= len(left) && i >= len(right) {
+		return 0 // end of both lists
+	} else if i >= len(left) {
+		return -1 // left ended first
+	} else if i >= len(right) {
+		return 1 // right shorter
+	}
+
+	// number comparison
+	li, ri := left[i], right[i]
+	ln, isln := li.(float64)
+	rn, isrn := ri.(float64)
+	if isln && isrn && ln == rn {
+		return compare(left, right, i+1) // same, continue to next item
+	} else if isln && isrn {
+		return int(ln - rn) // different numbers
+	}
+
+	// list comparison
+	llist, isllist := li.([]any)
+	rlist, isrlist := ri.([]any)
+	if !isllist {
+		llist = []any{ln} // mixed, convert to list
+	}
+	if !isrlist {
+		rlist = []any{rn} // mixed, convert to list
+	}
+	res = compare(llist, rlist, 0)
+	if res == 0 { // tie, continue to next item
+		return compare(left, right, i+1)
+	}
+	return res
+}
+
+var dividers = [][]any{{[]any{float64(2)}}, {[]any{float64(6)}}}
+
+type ByPacket [][]any
+
+func (p ByPacket) Len() int           { return len(p) }
+func (p ByPacket) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
+func (p ByPacket) Less(i, j int) bool { return less(p[i], p[j]) }
+
+func readPairs(input string) (res [][2][]any) {
+	pairs := strings.Split(input, "\n\n")
+	for _, pair := range pairs {
+		part := strings.Split(pair, "\n")
+		var left, right []any
+		json.Unmarshal([]byte(part[0]), &left)
+		json.Unmarshal([]byte(part[1]), &right)
+		pair := [2][]any{left, right}
+		res = append(res, pair)
+	}
+	return res
+}