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teakaysea's solution

to Rail Fence Cipher in the Go Track

Published at Jan 28 2021 · 0 comments
Instructions
Test suite
Solution

Implement encoding and decoding for the rail fence cipher.

The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded. It was already used by the ancient Greeks.

In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag). Finally the message is then read off in rows.

For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Then reads off:

WECRLTEERDSOEEFEAOCAIVDEN

To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.

? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

The first row has seven spots that can be filled with "WECRLTE".

W . . . E . . . C . . . R . . . L . . . T . . . E
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Now the 2nd row takes "ERDSOEEFEAOC".

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Leaving "AIVDEN" for the last row.

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

If you now read along the zig-zag shape you can read the original message.

Coding the solution

Look for a stub file having the name rail_fence_cipher.go and place your solution code in that file.

Running the tests

To run the tests run the command go test from within the exercise directory.

If the test suite contains benchmarks, you can run these with the --bench and --benchmem flags:

go test -v --bench . --benchmem

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

Further information

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.

Source

Wikipedia https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

cases_test.go

package railfence

// Source: exercism/problem-specifications
// Commit: 88db37b rail-fence-cipher: apply "input" policy
// Problem Specifications Version: 1.1.0

type testCase struct {
	description string
	message     string
	rails       int
	expected    string
}

// encode
var encodeTests = []testCase{

	{"encode with two rails",
		"XOXOXOXOXOXOXOXOXO",
		2,
		"XXXXXXXXXOOOOOOOOO"},

	{"encode with three rails",
		"WEAREDISCOVEREDFLEEATONCE",
		3,
		"WECRLTEERDSOEEFEAOCAIVDEN"},

	{"encode with ending in the middle",
		"EXERCISES",
		4,
		"ESXIEECSR"},
}

// decode
var decodeTests = []testCase{

	{"decode with three rails",
		"TEITELHDVLSNHDTISEIIEA",
		3,
		"THEDEVILISINTHEDETAILS"},

	{"decode with five rails",
		"EIEXMSMESAORIWSCE",
		5,
		"EXERCISMISAWESOME"},

	{"decode with six rails",
		"133714114238148966225439541018335470986172518171757571896261",
		6,
		"112358132134558914423337761098715972584418167651094617711286"},
}

rail_fence_cipher_test.go

package railfence

import "testing"

func testCases(op func(string, int) string, cases []testCase, t *testing.T) {
	for _, tc := range cases {
		if actual := op(tc.message, tc.rails); actual != tc.expected {
			t.Fatalf("FAIL: %s\nExpected: %q\nActual: %q", tc.description, tc.expected, actual)
		}
		t.Logf("PASS: %s", tc.description)
	}
}

func TestEncode(t *testing.T) { testCases(Encode, encodeTests, t) }
func TestDecode(t *testing.T) { testCases(Decode, decodeTests, t) }
package railfence

func Encode(input string, nOfRails int) string {
	rails := make([][]rune, nOfRails)
	charsInCycle := 2 * (nOfRails - 1)
	bottomIndex := charsInCycle / 2
	for i, r := range input {
		// to calculate railNo to put the current character(r)
		remainder := i % charsInCycle
		railNo := remainder
		if remainder > bottomIndex {
			railNo = bottomIndex - (remainder - bottomIndex)
		}
		rails[railNo] = append(rails[railNo], r)
	}
	var result string
	for _, line := range rails {
		result += string(line)
	}
	return result
}

func Decode(input string, numOfRails int) string {
	numOfChars := len(input)
	numOfCharsInCycle := 2 * (numOfRails - 1)
	numOfCycles := numOfChars / numOfCharsInCycle
	remainder := numOfChars % numOfCharsInCycle
	numsOfCharsInRail := make([]int, numOfRails)
	// set number of characters in each rail
	for i := 0; i < numOfRails; i++ {
		if i == 0 || i == numOfRails-1 {
			// 1 char in 1 cycle
			numsOfCharsInRail[i] = numOfCycles
		} else {
			// 2 chars in 1 cycle
			numsOfCharsInRail[i] = numOfCycles * 2
		}
		if remainder > 0 {
			numsOfCharsInRail[i]++
			remainder--
		}
	}
	split := split(input, numsOfCharsInRail)
	// top and bottom have 1 char for 1 cycle while others have 2 chars
	result := make([]rune, numOfChars)
	bottomIndex := numOfCharsInCycle / 2
	var railNo int
	for i := 0; i < numOfChars; i++ {
		result[i] = split[railNo][0]
		if len(split[railNo]) > 0 {
			split[railNo] = split[railNo][1:]
		}
		// railNo setting
		if (i % numOfCharsInCycle) < bottomIndex {
			railNo++
		} else {
			railNo--
		}
	}
	return string(result)
}

func split(str string, lengths []int) [][]rune {
	split := make([][]rune, len(lengths))
	var start, end int
	for i := 0; i < len(split); i++ {
		start = end
		end += lengths[i]
		split[i] = []rune(str[start:end])
	}
	return split
}

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