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to Rotational Cipher in the Delphi Pascal Track

Published at Aug 10 2020 · 0 comments
Instructions
Test suite
Solution

Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.

The Caesar cipher is a simple shift cipher that relies on transposing all the letters in the alphabet using an integer key between 0 and 26. Using a key of 0 or 26 will always yield the same output due to modular arithmetic. The letter is shifted for as many values as the value of the key.

The general notation for rotational ciphers is ROT + <key>. The most commonly used rotational cipher is ROT13.

A ROT13 on the Latin alphabet would be as follows:

Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: nopqrstuvwxyzabcdefghijklm

It is stronger than the Atbash cipher because it has 27 possible keys, and 25 usable keys.

Ciphertext is written out in the same formatting as the input including spaces and punctuation.

Examples

  • ROT5 omg gives trl
  • ROT0 c gives c
  • ROT26 Cool gives Cool
  • ROT13 The quick brown fox jumps over the lazy dog. gives Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
  • ROT13 Gur dhvpx oebja sbk whzcf bire gur ynml qbt. gives The quick brown fox jumps over the lazy dog.

Testing

In order to run the tests for this track, you will need to install DUnitX. Please see the installation instructions for more information.

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When Questions Come Up

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Submitting Exercises

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For example, if you're submitting ubob.pas for the Bob exercise, the submit command would be something like exercism submit <path_to_exercism_dir>/delphi/bob/ubob.pas.

Source

Wikipedia https://en.wikipedia.org/wiki/Caesar_cipher

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you may request help from a mentor.

uRotationalCipherTests.pas

unit uRotationalCipherTests;

interface
uses
  DUnitX.TestFramework;

const
  CanonicalVersion = '1.2.0.1';

type

  [TestFixture('Test rotation from English to ROTn')]
  RotationalCipherTests = class(TObject)
  public
    [Test]
//    [Ignore('Comment the "[Ignore]" statement to run the test')]
    procedure rotate_a_by_0_same_output_as_input;

    [Test]
    [Ignore]
    procedure rotate_a_by_1;

    [Test]
    [Ignore]
    procedure rotate_a_by_26_same_output_as_input;

    [Test]
    [Ignore]
    procedure rotate_m_by_13;

    [Test]
    [Ignore]
    procedure rotate_n_by_13_with_wrap_around_alphabet;

    [Test]
    [Ignore]
    procedure rotate_capital_letters;

    [Test]
    [Ignore]
    procedure rotate_spaces;

    [Test]
    [Ignore]
    procedure rotate_numbers;

    [Test]
    [Ignore]
    procedure rotate_punctuation;

    [Test]
    [Ignore]
    procedure rotate_all_letters;

    [Test]
    [Ignore('Optional: Challenge Test')]
    procedure rotate_m_by_negative_1;

    [Test]
    [Ignore('Optional: Challenge Test')]
    procedure rotate_A_by_negative_2;

    [Test]
    [Ignore('Optional: Challenge Test')]
    procedure rotate_letters_by_negative_26;
  end;

implementation
uses sysutils, uRotationalCipher;

procedure RotationalCipherTests.rotate_a_by_0_same_output_as_input;
begin
  Assert.AreEqual('a', RotationalCipher.rotate('a', 0));
end;

procedure RotationalCipherTests.rotate_a_by_1;
begin
  Assert.AreEqual('b', RotationalCipher.rotate('a', 1));
end;

procedure RotationalCipherTests.rotate_a_by_26_same_output_as_input;
begin
  Assert.AreEqual('a', RotationalCipher.rotate('a', 26));
end;

procedure RotationalCipherTests.rotate_m_by_13;
begin
  Assert.AreEqual('z', RotationalCipher.rotate('m', 13));
end;

procedure RotationalCipherTests.rotate_n_by_13_with_wrap_around_alphabet;
begin
  Assert.AreEqual('a', RotationalCipher.rotate('n', 13));
end;

procedure RotationalCipherTests.rotate_capital_letters;
begin
  Assert.AreEqual('TRL', RotationalCipher.rotate('OMG', 5));
end;

procedure RotationalCipherTests.rotate_spaces;
begin
  Assert.AreEqual('T R L', RotationalCipher.rotate('O M G', 5));
end;

procedure RotationalCipherTests.rotate_numbers;
begin
  Assert.AreEqual('Xiwxmrk 1 2 3 xiwxmrk',
    RotationalCipher.rotate('Testing 1 2 3 testing', 4));
end;

procedure RotationalCipherTests.rotate_punctuation;
begin
  Assert.AreEqual('Gzo''n zvo, Bmviyhv!',
    RotationalCipher.rotate('Let''s eat, Grandma!', 21));
end;

procedure RotationalCipherTests.rotate_all_letters;
begin
  Assert.AreEqual('Gur dhvpx oebja sbk whzcf bire gur ynml qbt.',
    RotationalCipher.rotate('The quick brown fox jumps over the lazy dog.', 13));
end;

{$region 'Bonus Tests'}
procedure RotationalCipherTests.rotate_m_by_negative_1;
begin
  Assert.AreEqual('l',
    RotationalCipher.rotate('m', -1));
end;

procedure RotationalCipherTests.rotate_A_by_negative_2;
begin
  Assert.AreEqual('Y',
    RotationalCipher.rotate('A', -2));
end;

procedure RotationalCipherTests.rotate_letters_by_negative_26;
begin
  Assert.AreEqual('OMG',
    RotationalCipher.rotate('OMG', -26));
end;
{$endregion 'Bonus Tests'}

initialization
  TDUnitX.RegisterTestFixture(RotationalCipherTests);
end.
unit uRotationalCipher;

interface

type
  RotationalCipher = class
  const
    strAlphabet = 'abcdefghijklmnopqrstuvwxyz';
  private
    class function CalculateCipherIndex(aIndex: Integer): Integer;
  public
    class function rotate(aString: string; aKey: Integer): string;
  end;

implementation

uses
  System.SysUtils;

class function RotationalCipher.CalculateCipherIndex(aIndex: Integer): Integer;
begin
  Result := aIndex;
  if aIndex < 1 then
    Result := aIndex + 26
  else
  if aIndex >26 then
    Result := aIndex mod 27 + 1;
end;

class function RotationalCipher.rotate(aString: string; aKey: Integer): string;
var
  lPos: Integer;
  lChar: string;
  Idx: Integer;
begin
  lChar := string.Empty;
  for Idx := 1 to Length(aString) do
  begin
    lPos := ansipos(AnsiLowerCase(aString[Idx]),strAlphabet);
    if lPos = 0 then
      lChar := aString[Idx]
    else
    begin
      lChar := strAlphabet[CalculateCipherIndex(lPos+aKey)];

    if aString[Idx] = AnsiUpperCase(aString[Idx]) then
      lChar := AnsiUpperCase(lChar);
    end;
    Result := Result + lChar;
  end;
end;

end.

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