How compatible are my strings?

Question

Introduction

Consider two strings A and B of the same length L, and an integer K ≥ 0. For the purposes of this challenge, we say that the strings are K-compatible, if there exists a string C of length K such that A is a contiguous substring of the concatenation BCB. Note that A is a substring of BAB, so A and B are always L-compatible (but may also be K-compatible for some other K < L).

Input

Your inputs are two strings of the same positive length, consisting of upper- and lowercase ASCII letters.

Output

Your output shall be the lowest non-negative integer K such that the inputs are K-compatible.

Example

Consider the inputs

A = HHHHHH
B = HHttHH

They are not 0-compatible, because A is not a substring of HHttHHHHttHH. They are also not 1-compatible, because A is not a substring of HHttHH#HHttHH no matter which letter is placed on the #. However, A is a substring of HHttHHHHHHttHH, where C is the two-letter string HH. Thus the inputs are 2-compatible, and the correct output is 2.

Rules and scoring

You can write a full program or a function. The lowest byte count wins, and standard loopholes are disallowed.

Test cases

The compatibility condition is symmetric, so swapping the two inputs should not change the output.

E G -> 1
E E -> 0
aB Bc -> 1
YY tY -> 1
abcd bcda -> 0
abcXd bxcda -> 4
Hello Hello -> 0
Hello olHel -> 1
aBaXYa aXYaBa -> 1
aXYaBa aBaXYa -> 1
HHHHHH HHttHH -> 2
abcdab cdabcd -> 2
XRRXXXXR XRXXRXXR -> 4
evveeetev tetevevev -> 7
vzzvzJvJJz vJJvzJJvJz -> 10
JJfcfJJcfJfb JcfJfbbJJJfJ -> 5
GhhaaHHbbhGGH HHaaHHbbGGGhh -> 9
OyDGqyDGDOGOGyG yDGqOGqDyyyyOyD -> 12
ffKKBBpGfGKpfGpbKb fGpbKbpBBBffbbbffK -> 9
UZuPPZuPdVdtuDdDiuddUPtUidtVVV dtUPtUidtVVVtDZbZZPuiUZuPPZuPd -> 21

Leaderboard

Here's a Stack Snippet to generate a leaderboard and list of winners by language. To make sure your answer shows up, start it with a header of the form

## Language, N bytes

You can keep old scores in the header by using the strikethrough tags: <s>57</s> will appear as 57.

/* Configuration */

var QUESTION_ID = 78736; // Obtain this from the url
// It will be like https://XYZ.stackexchange.com/questions/QUESTION_ID/... on any question page
var ANSWER_FILTER = "!t)IWYnsLAZle2tQ3KqrVveCRJfxcRLe";
var COMMENT_FILTER = "!)Q2B_A2kjfAiU78X(md6BoYk";
var OVERRIDE_USER = 32014; // This should be the user ID of the challenge author.

/* App */

var answers = [], answers_hash, answer_ids, answer_page = 1, more_answers = true, comment_page;

function answersUrl(index) {
  return "https://api.stackexchange.com/2.2/questions/" +  QUESTION_ID + "/answers?page=" + index + "&pagesize=100&order=desc&sort=creation&site=codegolf&filter=" + ANSWER_FILTER;
}

function commentUrl(index, answers) {
  return "https://api.stackexchange.com/2.2/answers/" + answers.join(';') + "/comments?page=" + index + "&pagesize=100&order=desc&sort=creation&site=codegolf&filter=" + COMMENT_FILTER;
}

function getAnswers() {
  jQuery.ajax({
    url: answersUrl(answer_page++),
    method: "get",
    dataType: "jsonp",
    crossDomain: true,
    success: function (data) {
      answers.push.apply(answers, data.items);
      answers_hash = [];
      answer_ids = [];
      data.items.forEach(function(a) {
        a.comments = [];
        var id = +a.share_link.match(/\d+/);
        answer_ids.push(id);
        answers_hash[id] = a;
      });
      if (!data.has_more) more_answers = false;
      comment_page = 1;
      getComments();
    }
  });
}

function getComments() {
  jQuery.ajax({
    url: commentUrl(comment_page++, answer_ids),
    method: "get",
    dataType: "jsonp",
    crossDomain: true,
    success: function (data) {
      data.items.forEach(function(c) {
        if (c.owner.user_id === OVERRIDE_USER)
          answers_hash[c.post_id].comments.push(c);
      });
      if (data.has_more) getComments();
      else if (more_answers) getAnswers();
      else process();
    }
  });  
}

getAnswers();

var SCORE_REG = /<h\d>\s*([^\n,]*[^\s,]),.*?(-?\d+)(?=[^\n\d<>]*(?:<(?:s>[^\n<>]*<\/s>|[^\n<>]+>)[^\n\d<>]*)*<\/h\d>)/;

var OVERRIDE_REG = /^Override\s*header:\s*/i;

function getAuthorName(a) {
  return a.owner.display_name;
}

function process() {
  var valid = [];
  
  answers.forEach(function(a) {
    var body = a.body;
    a.comments.forEach(function(c) {
      if(OVERRIDE_REG.test(c.body))
        body = '<h1>' + c.body.replace(OVERRIDE_REG, '') + '</h1>';
    });
    
    var match = body.match(SCORE_REG);
    if (match)
      valid.push({
        user: getAuthorName(a),
        size: +match[2],
        language: match[1],
        link: a.share_link,
      });
    
  });
  
  valid.sort(function (a, b) {
    var aB = a.size,
        bB = b.size;
    return aB - bB
  });

  var languages = {};
  var place = 1;
  var lastSize = null;
  var lastPlace = 1;
  valid.forEach(function (a) {
    if (a.size != lastSize)
      lastPlace = place;
    lastSize = a.size;
    ++place;
    
    var answer = jQuery("#answer-template").html();
    answer = answer.replace("{{PLACE}}", lastPlace + ".")
                   .replace("{{NAME}}", a.user)
                   .replace("{{LANGUAGE}}", a.language)
                   .replace("{{SIZE}}", a.size)
                   .replace("{{LINK}}", a.link);
    answer = jQuery(answer);
    jQuery("#answers").append(answer);

    var lang = a.language;
    if (! /<a/.test(lang)) lang = '<i>' + lang + '</i>';
    lang = jQuery(lang).text().toLowerCase();

    languages[lang] = languages[lang] || {lang: a.language, user: a.user, size: a.size, link: a.link, uniq: lang};
  });

  var langs = [];
  for (var lang in languages)
    if (languages.hasOwnProperty(lang))
      langs.push(languages[lang]);

  langs.sort(function (a, b) {
    if (a.uniq > b.uniq) return 1;
    if (a.uniq < b.uniq) return -1;
    return 0;
  });

  for (var i = 0; i < langs.length; ++i)
  {
    var language = jQuery("#language-template").html();
    var lang = langs[i];
    language = language.replace("{{LANGUAGE}}", lang.lang)
                       .replace("{{NAME}}", lang.user)
                       .replace("{{SIZE}}", lang.size)
                       .replace("{{LINK}}", lang.link);
    language = jQuery(language);
    jQuery("#languages").append(language);
  }

}

body { text-align: left !important}

#answer-list {
  padding: 10px;
  width: 290px;
  float: left;
}

#language-list {
  padding: 10px;
  width: 290px;
  float: left;
}

table thead {
  font-weight: bold;
}

table td {
  padding: 5px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link rel="stylesheet" type="text/css" href="//cdn.sstatic.net/Sites/codegolf/all.css?v=617d0685f6f3">
<div id="answer-list">
  <h2>Leaderboard</h2>
  <table class="answer-list">
    <thead>
      <tr><td></td><td>Author</td><td>Language</td><td>Size</td></tr>
    </thead>
    <tbody id="answers">

    </tbody>
  </table>
</div>
<div id="language-list">
  <h2>Winners by Language</h2>
  <table class="language-list">
    <thead>
      <tr><td>Language</td><td>User</td><td>Score</td></tr>
    </thead>
    <tbody id="languages">

    </tbody>
  </table>
</div>
<table style="display: none">
  <tbody id="answer-template">
    <tr><td>{{PLACE}}</td><td>{{NAME}}</td><td>{{LANGUAGE}}</td><td><a href="{{LINK}}">{{SIZE}}</a></td></tr>
  </tbody>
</table>
<table style="display: none">
  <tbody id="language-template">
    <tr><td>{{LANGUAGE}}</td><td>{{NAME}}</td><td><a href="{{LINK}}">{{SIZE}}</a></td></tr>
  </tbody>
</table>

Answer 1

Pyth, 16

lhf}QjT,vzvz+k.:

Find the shortest substring of A that when inserted between two copies of B results in a string that contains A.

This could be two bytes shorter if the second line didn't have quotes, but that feels weird?

Test Suite

Answer 2

Python 3, 155 168 157 bytes

Total is the length of A. Compare the beginning of A to the end of B and subtract that from total. Compare the beginning of B to the end of A and subtract that from total. Return the absolute value of total unless total is equal to the length, in which case return 0.

def f(A,B):
    T=L=len(A)
    C=D=1
    for i in range(L,0,-1):
        if A[:i]==B[-i:]and C:
            T,C=T-i,0
        if A[-i:]==B[:i]and D:
            T,D=T-i,0
    return (0,abs(T))[T!=-L]

Edit: Handle the f("abcdab","cdabcd")==2 case

Answer 3

Retina, 49 bytes

.*?(?<=^(?=(.*)(?<4-3>.)*(.*) \2.*\1$)(.)*).+
$#4

Try it online! (Slightly modified to run all tests at once.)

The trick is that we need to backtrack over the part of A that we don't find in B, and so far I haven't found a way to do this without rather annoying lookarounds and balancing groups.

Answer 4

Jolf, 40 Bytes

Wά)Ζ0W<ζli)? h++i]Iζ+ζniIoά0nΖhζ}onhn}wn

Try it!

I'm quite new to Jolf, learned a lot while figuring this out. Seems a bit awkward, still could probably could be golfed down further. Even knocked off 2 bytes while writing this explanation.

Explanation:

  Wά)                                      While ά (initialized to 16)
     Ζ0                                    Set ζ to 0
       W<ζli)                              While ζ < length(A)
             ? h++i]Iζ+ζniIoά0n            Set ά to 0 if (A + a substring from B of length n + A) contains B
                               Ζhζ         Increment ζ
                                  }onhn    Increment n (initialize to 0
                                       }wn Decrement n and print

Answer 5

05AB1E, 20 bytes

Code:

õ)²Œ«v¹y¹««²åiyg}})ß

Uses CP-1252 encoding. Try it online!.

Answer 6

JavaScript (ES6), 110 bytes

(a,b)=>{for(i=0;;i++)for(j=i;j<=a.length;j++)if(b.startsWith(a.slice(j))&&b.endsWith(a.slice(0,j-i)))return i}

Works by slicing ever longer pieces out of the middle of a until they match the two ends of b. The loop is not infinite as it will stop on or before i == a.length.