CaesarCipher:适用于Comp122的CaeserCipher Java任务

public class Brutus { static String alphabet = "abcdefghijklmnopqrstuvwxyz"; // Use a string - same as iterating through an array using .charAt(). // Initialise alphabet outside of methods but inside class so it can be used my multiple methods. /* Takes the encrypted string given to it, loops through each possible shift value and utilises other methods to get a final score back and it keeps track of the best score and its corresponding shift value. */ public static void main(String[] args) { String encryptedString = args[0]; String decryptedString = encryptedString; double decryptLowestScore = 10000.0; // flag set as a high value to be beaten. int decryptShiftValue = 0; // flag to remember the best solution. int i; // loop through each shift value from 0-26 (anything outside that range is modulated to be in that range). for(i = 0; i <=26; i++) { // send encrypted text to be shifted String decryptTest = Caesar.rotate(i, encryptedString); // needs to get the parameter that will be sent to chiSquared method. double[] freqValues = frequency(decryptTest); // call chiSquared method with attained parameter above and english array. double decryptScore = chiSquared(english, freqValues); if (decryptScore < decryptLowestScore){ // if there is a better score: decryptLowestScore = decryptScore; // set the better score as current score. decryptShiftValue = i; // and set the decrypt shift value to i as this is current best decryption key. } } decryptedString = Caesar.rotate(decryptShiftValue, encryptedString); //finally call Caesar again to undo the shift with the key. System.out.println(decryptedString); // Output the decrypted message to the user. } // Contains the frequency for each letter in the english language. public static final double[] english = { 0.0855, 0.0160, 0.0316, 0.0387, 0.1210, 0.0218, 0.0209, 0.0496, 0.0733, 0.0022, 0.0081, 0.0421, 0.0253, 0.0717, 0.0747, 0.0207, 0.0010, 0.0633, 0.0673, 0.0894, 0.0268, 0.0106, 0.0183, 0.0019, 0.0172, 0.0011 }; // I have done the exact same thing in frequency and just changed the output - do not need or use this. public static int[] count(String text) { int[] lettersCount = new int[26]; text = text.toLowerCase(); int i; int j; for (i = 0; i < alphabet.length(); i++) { // loops through each char in alphabet. for (j = 0; j < text.length(); j++) { // loops through each letter in the given text. if (alphabet.charAt(i) == text.charAt(j)) { // checks to see if the letter from first loop is the letter at position j. lettersCount[i]++; // if it is, increments the element in the array by 1. alphabet and letters count have equal length. } else { lettersCount[i] = lettersCount[i] + 0; } } } return lettersCount; } /* Gets the amount of each letter of the english alphabet in the string given to it and divides it by the length of the string given to it in order to calculate the letters frequency in the text. */ public static double[] frequency(String text){ double[] letterFrequency = new double[alphabet.length()]; // First I want to strip the text of any punctuation and spacing so I can get the number of letters in the string. // to do this i'm going to implement a regex that replaces anything not within a specified range with "". String strippedText = text.replaceAll("[^A-Za-z]+", "").toLowerCase(); // also make everything lower case. int i; int j; for(i = 0; i < alphabet.length(); i++){ // loops through each value of the alphabet. int count = 0; // resets count to 0 after for loop below is done. for (j = 0; j < text.length(); j++){ // loops through each character in the text. if(alphabet.charAt(i) == text.charAt(j)){ count++; } } double letterfreq = ((double) count / text.length()); // cast count to a double to stop the loss of digits after decimal point to truncation via integer division. letterFrequency[i] += (letterfreq); // add the frequency to the array. } return(letterFrequency); } /* Implements the Chi Squared formula to get a total chi score for the string by summing the chi score of each letter.*/ public static double chiSquared(double[] englishFreq, double[] textFreq) { double chiScore = 0; //outer for loop that will run through each letter: for (int l = 0; l < textFreq.length; l++){ chiScore += (Math.pow((textFreq[l]-englishFreq[l]), 2) / englishFreq[l]); // chiSquared formula applied. // chi score is summed for each letter. } return chiScore; } }