day 10: knot

10dec/input_test

@@ -0,0 +1 @@
+3,4,1,5

10dec/main.go

@@ -0,0 +1,76 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+const (
+	//SZ = 5
+	SZ = 256
+)
+
+var (
+	list    [SZ]int
+	lengths []int
+	pos     int
+	skip    int
+)
+
+func main() {
+	lengths = parseLengths()
+	//fmt.Println(lengths)
+	for i, _ := range list {
+		list[i] = i
+	}
+	for _, l := range lengths {
+		//fmt.Println(list, pos, skip, l)
+		reverse(l)
+		pos = (pos + l + skip) % SZ
+		skip++
+	}
+	//fmt.Println(list)
+
+	fmt.Println("Result:", list[0]*list[1])
+}
+
+func reverse(l int) {
+	if l < 2 {
+		return
+	}
+	rev := make([]int, l, l)
+	to := (pos + l) % SZ
+	if to > pos {
+		for i, n := range list[pos:to] {
+			rev[l-i-1] = n
+		}
+	} else {
+		var i, j, n int
+		for i, n = range list[pos:SZ] {
+			rev[l-i-1] = n
+		}
+		//fmt.Println("    ", rev)
+		for j, n = range list[0:to] {
+			rev[l-j-i-2] = n
+		}
+	}
+
+	//fmt.Println("  ", rev)
+	for i, n := range rev {
+		list[(pos+i)%SZ] = n
+	}
+}
+
+func parseLengths() []int {
+	l := make([]int, 0, 16)
+	sc := bufio.NewScanner(os.Stdin)
+	sc.Scan()
+	for _, num := range strings.Split(sc.Text(), ",") {
+		n, _ := strconv.Atoi(num)
+		l = append(l, n)
+	}
+	return l
+}