Some answers in Javascript

1. ツイート分割

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function textToTweets(txt) {
  var num = Math.ceil(txt.length/140);
  var tweets = [];
  for (i=0; i<num; i++) {
    var tweet = txt.substr(i*140, 140);
    tweets.push(tweet);
  }
  return tweets;
}

function bulletPoint(array) {
  var result = document.getElementById('result');
  for (i = 0; i < array.length; i++) {
    var ul = document.createElement('ul');
    ul.textContent = array[i];
    result.appendChild(ul);
  }
}

function submit() {
  var txt = document.getElementById('txt').value;
  var tweets = textToTweets(txt);
  bulletPoint(tweets);
}

2. ポスター分割

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function splitText(txt, n) {
  var num = Math.ceil(txt.length/n);
  var arr = [];
  for (i=0; i<num; i++) {
    var sub = txt.substr(i*n, n);
    arr.push(sub);
  }
  return arr;
}

3. シーザー暗号

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var CHAR_A = 65;
var CHAR_Z = 90;

function rot13(txt){
  txt = txt.toUpperCase();
  var arr = splitText(txt, 1);
  for (i=0; i<arr.length; i++) {
    var ascii = arr[i].charCodeAt(0);
    if (ascii < CHAR_A || ascii > CHAR_Z) {
      arr[i] = String.fromCharCode(ascii);
    } else if (ascii + 13 <= CHAR_Z) {
      arr[i] = String.fromCharCode(ascii + 13);
    } else {
      arr[i] = String.fromCharCode(ascii - 13);
    }
  }
  return arr.join('');
}

4. 色々なROT

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function rot(txt, n){
  txt = txt.toUpperCase();
  var arr = splitText(txt, 1);
  for (i=0; i<arr.length; i++) {
    var ascii = arr[i].charCodeAt(0);
    if (ascii < CHAR_A || ascii > CHAR_Z) {
      arr[i] = String.fromCharCode(ascii);
    } else if (ascii + n <= CHAR_Z) {
      arr[i] = String.fromCharCode(ascii + n);
    } else {
      arr[i] = String.fromCharCode(ascii - 26 + n);
    }
  }
  return arr.join('');
}

ROT
(ROT23)

deciphering
(ROT21)

hacker-style
(ROT16)