# Day 20

Start: 1/6/2016

Finish 1/7/2017

Language: PHP

SPOILER ALERT: If you have any inkling, any whatsoever, to work on the Advent of Code…. DO NOT READ THIS BLOG POST.  DO NOT LOOK AT MY GITHUB PROJECT.  It is no fun if you’re not solving it yourself, and you’ll feel so much better about yourself if you can figure it out without looking up an answer.  This blog post is not to give away the answer, but instead, is there for people to learn from.

As always, the following code is in my GitHub: https://github.com/pviafore/AdventOfCode2016

## The Challenge

This challenge was to take a string, and apply a series of transformations on it (Swapping letters, rotating, reversing, etc.)

My idea was to keep an array where the head pointer moves around based on rotations, and operating based on that (similar to the idea of a circular array)

## Part 1

Code Code Code ……

``````
<?php

\$arr = array("a", "b", "c", "d", "e", "f", "g", "h");

function convertIndex(\$index){
while(\$newIndex < 0){
\$newIndex += sizeof(\$arr);
}
return  \$newIndex%sizeof(\$arr);
}
function getCharAt(\$index)
{
\$newIndex =convertIndex(\$index);
return \$arr[\$newIndex];
}

function setCharAt(\$index, \$value)
{
\$arr[convertIndex(\$index)] = \$value;
}

function swapPos(\$pos1, \$pos2) {
\$tmp1 = getCharAt(\$pos1);
\$tmp2 = getCharAt(\$pos2);
setCharAt(\$pos1, \$tmp2);
setCharAt(\$pos2, \$tmp1);
}

function swapLetters(\$letter1, \$letter2) {
global \$arr;
\$index1 = array_search(\$letter1, \$arr);
\$index2 = array_search(\$letter2, \$arr);
\$arr[\$index2] = \$letter1;
\$arr[\$index1] = \$letter2;
}

function reverse(\$pos1, \$pos2) {
for (\$i = \$pos1, \$j=\$pos2; \$i < \$j; \$i++, \$j--){
swapPos(\$i, \$j);
}
}

function printArray() {
global \$arr;
for (\$i = 0; \$i < sizeof(\$arr); \$i++){
echo getCharAt(\$i);
}
echo "\n";
}

function rotateLeft(\$steps)
{
}

function rotateRight(\$steps)
{
}

function move(\$from, \$to) {
global \$arr;

if (\$to =\$to; \$i--){
\$newChar = getCharAt(\$i);
setCharAt(\$i, \$char);
setCharAt(\$i+1, \$newChar);
}
}
else {
\$char = getCharAt(\$from);
for (\$i = \$from+1; \$i<=\$to; \$i++){
\$newChar = getCharAt(\$i);
setCharAt(\$i, \$char);
setCharAt(\$i-1, \$newChar);
}
}
}

function rotatePos(\$letter) {
\$index = 0;
for (\$i = 0; \$i = 4) {
\$index += 1;
}
rotateRight(\$index+1);
}

function getLines() {
\$f = fopen("../day21.txt", "r");
\$lines = array();
while ((\$buffer = fgets(\$f)) !== false) {
\$lines[] = trim(\$buffer);
}
fclose(\$f);
return \$lines;
}

function processLine(\$line) {
\$words = explode(" ", \$line);
if(\$words[0] == "swap" && \$words[1] =="position"){
swapPos(\$words[2], \$words[5]);
}
if(\$words[0] == "swap" && \$words[1] =="letter"){
swapLetters(\$words[2], \$words[5]);
}
if(\$words[0] == "rotate" && \$words[1] =="left"){
rotateLeft(\$words[2]);
}
if(\$words[0] == "rotate" && \$words[1] =="right"){
rotateRight(\$words[2]);
}
if(\$words[0] == "rotate" && \$words[1] =="based"){
rotatePos(\$words[6]);
}
if(\$words[0] == "reverse"){
reverse(\$words[2], \$words[4]);
}
if(\$words[0] == "move"){
move(\$words[2], \$words[5]);
}
}

\$lines = getLines();
foreach(\$lines as \$line){
processLine(\$line);
}
printArray();
?>
``````

This was my first foray in PHP in quite some time.  It was a bit more verbose than I remembered, but I still got this one relatively easy.

The main loop was easy, as it mostly took things line by line by line, and then chose a transformation based on the text.

Most transformations are straight forward.  I keep a global array and head pointer, along with a way to convert indices, get characters and set characters in the array.

``````
\$arr = array("a", "b", "c", "d", "e", "f", "g", "h");

function convertIndex(\$index){
while(\$newIndex < 0){
\$newIndex += sizeof(\$arr);
}
return  \$newIndex%sizeof(\$arr);
}
function getCharAt(\$index)
{
\$newIndex =convertIndex(\$index);
return \$arr[\$newIndex];
}

function setCharAt(\$index, \$value)
{
\$arr[convertIndex(\$index)] = \$value;
}
``````

Swapping based on positions and swapping based on letters are pretty straight forward

``````

function swapPos(\$pos1, \$pos2) {
\$tmp1 = getCharAt(\$pos1);
\$tmp2 = getCharAt(\$pos2);
setCharAt(\$pos1, \$tmp2);
setCharAt(\$pos2, \$tmp1);
}

function swapLetters(\$letter1, \$letter2) {
global \$arr;
\$index1 = array_search(\$letter1, \$arr);
\$index2 = array_search(\$letter2, \$arr);
\$arr[\$index2] = \$letter1;
\$arr[\$index1] = \$letter2;
}

``````

Reversing and Rotating were also pretty easy.

``````
function reverse(\$pos1, \$pos2) {
for (\$i = \$pos1, \$j=\$pos2; \$i < \$j; \$i++, \$j--){
swapPos(\$i, \$j);
}
}

function rotateLeft(\$steps)
{
}

function rotateRight(\$steps)
{
}
``````

Once we get into moving (removing from one index and inserting in the other index), it took me a little while longer to get this.  I treat moving differently based on which direction I’m going to, and then I effectively “bubble” the values to the appropriate position.  I originally tried to do array slicing and recombining, but my wonky index recalculations made it tricky.

``````
function move(\$from, \$to) {
global \$arr;

if (\$to =\$to; \$i--){
\$newChar = getCharAt(\$i);
setCharAt(\$i, \$char);
setCharAt(\$i+1, \$newChar);
}
}
else {
\$char = getCharAt(\$from);
for (\$i = \$from+1; \$i<=\$to; \$i++){
\$newChar = getCharAt(\$i);
setCharAt(\$i, \$char);
setCharAt(\$i-1, \$newChar);
}
}
}

``````

Lastly, I worked on the rotate based on the position of a letter.  I pretty much searched for the letter in my array, then figured out how many rotations needed to happen.

``````
function rotatePos(\$letter) {
\$index = 0;
for (\$i = 0; \$i = 4) {
\$index += 1;
}
rotateRight(\$index+1);
}

``````

## Part 2

This part was to unscramble a password rather than scramble a password.  I reversed the lines of code, and reversed the relevant sections (such as rotating left when the command is rotate right)

``````
function processLine(\$line) {
\$words = explode(" ", \$line);
if(\$words[0] == "swap" && \$words[1] =="position"){
swapPos(\$words[2], \$words[5]);
}
if(\$words[0] == "swap" && \$words[1] =="letter"){
swapLetters(\$words[2], \$words[5]);
}
if(\$words[0] == "rotate" && \$words[1] =="left"){
rotateRight(\$words[2]);
}
if(\$words[0] == "rotate" && \$words[1] =="right"){
rotateLeft(\$words[2]);
}
if(\$words[0] == "rotate" && \$words[1] =="based"){
rotatePos(\$words[6]);
}
if(\$words[0] == "reverse"){
reverse(\$words[2], \$words[4]);
}
if(\$words[0] == "move"){
move(\$words[5], \$words[2]);
}
}

``````

The tricky part of this one was figuring out how the rotate based on the position of a letter was handled.  Based on some pen and paper calculations, I found a way to derive how many rotations were needed to get to the next password

``````
function rotatePos(\$letter) {
\$index = 0;
for (\$i = 0; \$i < sizeof(\$arr); \$i++){
if (getCharAt(\$i) == \$letter) {
\$index = \$i;
break;
}
}
\$numRotations=0;
for (\$j = 0; \$j < sizeof(\$arr); \$j++) {
if (\$j = 4 && (\$j + \$j + 2) % sizeof(\$arr) == \$index) {
\$numRotations = \$j+2;
break;
}
}
rotateLeft(\$numRotations);
}

``````

## Wrap-up

I had a lot of stupid mistakes in my part 2.  I kept not thinking through the problem.  Also PHP’s verbosity was weighing me down.  I felt like the array functions were clunky, and my code endd up all over the place.

I give myself a B- on this one.  I could have done better, and thought through the problems better.  My PHP code isn’t great either, so I wasn’t satisfied with this, but it’s better than some other ones.

Next up, is Day 22 with Elm.