Added Rail Fence Cipher implementation (addresses #23)

This is only my attempt at an implementation.  @r0cketr1kky can still have another attempt, or refine my solution, but it is here now for people to use.

TODO: add better docstrings.

src/ClassicalCiphers.jl

@@ -12,6 +12,7 @@ include("affine.jl")
 include("hill.jl")
 include("playfair.jl")
 include("enigma.jl")
+include("railfence.jl")
 
 export encrypt_monoalphabetic, decrypt_monoalphabetic, crack_monoalphabetic,
        encrypt_caesar, decrypt_caesar, crack_caesar,
@@ -22,6 +23,7 @@ export encrypt_monoalphabetic, decrypt_monoalphabetic, crack_monoalphabetic,
        encrypt_hill, decrypt_hill,
        encrypt_playfair, decrypt_playfair,
        encrypt_enigma, decrypt_enigma,
-       string_fitness, index_of_coincidence
+       string_fitness, index_of_coincidence,
+       construct_railfence, encrypt_railfence, decrypt_railfence
 
 end # module

src/railfence.jl

@@ -0,0 +1,49 @@
+function construct_railfence(input, fence::AbstractArray, n_rails::Integer)
+	rails = vcat(1:n_rails, (n_rails - 1):-1:2)
+
+	for (i, letter) in enumerate(input)
+		fence[rails[mod1(i, length(rails))], i] = letter
+	end
+	
+	return fence
+end
+
+"""
+```julia
+construct_railfence(input::AbstractString, n_rails::Integer)
+construct_railfence(input::AbstractArray{T}, n_rails::Integer) where {T <: Number}
+```
+
+See https://en.wikipedia.org/wiki/Rail_fence_cipher.
+"""
+function construct_railfence(input::AbstractString, n_rails::Integer)
+	input = uppercase(replace(input, " " => ""))
+	# the square is my `UndefInitializer()` for `Char`.
+	return construct_railfence(input, fill('□', n_rails, length(input)), n_rails)
+end
+function construct_railfence(input::AbstractArray{T}, n_rails::Integer) where {T <: Number}
+	return construct_railfence(input, zeros(T, n_rails, length(input)), n_rails)
+end
+
+"""
+```julia
+encrypt_railfence(input::AbstractString, n_rails::Integer)
+```
+
+See https://en.wikipedia.org/wiki/Rail_fence_cipher.
+"""
+function encrypt_railfence(input::AbstractString, n_rails::Integer)
+	return join(Char[c for rail in eachrow(construct_railfence(input, n_rails)) for c in rail if c != '□'])
+end
+
+"""
+```julia
+encrypt_railfence(input::AbstractString, n_rails::Integer)
+```
+
+See https://en.wikipedia.org/wiki/Rail_fence_cipher.
+"""
+function decrypt_railfence(input::AbstractString, n_rails::Integer)
+	char_positions = Int[n for row in eachrow(construct_railfence(1:length(input), n_rails)) for n in row if n != 0]
+	return lowercase(join(Char[input[findfirst(==(i), char_positions)] for i in 1:length(input)]))
+end

test/railfence.jl

@@ -0,0 +1,10 @@
+# using ClassicalCiphers
+# using Test
+
+@test encrypt_railfence("WE ARE DISCOVERED. FLEE AT ONCE", 3) == "WECRFACERDSOEE.LETNEAIVDEO"
+@test decrypt_railfence("WECRFACERDSOEE.LETNEAIVDEO", 3) == "wearediscovered.fleeatonce"
+@test encrypt_railfence("Julia is strong", 10) == "JULIAISGSNTOR"
+@test decrypt_railfence("JULIAISGSNTOR", 10) == "juliaisstrong"
+@test encrypt_railfence("This is not a very strong cipher, but indeed it is classical", 2) == "TIINTVRSRNCPE,UIDEIICASCLHSSOAEYTOGIHRBTNEDTSLSIA"
+@test decrypt_railfence("TIINTVRSRNCPE,UIDEIICASCLHSSOAEYTOGIHRBTNEDTSLSIA", 2) == "thisisnotaverystrongcipher,butindeeditisclassical"
+@test construct_railfence("WE ARE DISCOVERED. FLEE AT ONCE", 3) == ['W' '□' '□' '□' 'E' '□' '□' '□' 'C' '□' '□' '□' 'R' '□' '□' '□' 'F' '□' '□' '□' 'A' '□' '□' '□' 'C' '□'; '□' 'E' '□' 'R' '□' 'D' '□' 'S' '□' 'O' '□' 'E' '□' 'E' '□' '.' '□' 'L' '□' 'E' '□' 'T' '□' 'N' '□' 'E'; '□' '□' 'A' '□' '□' '□' 'I' '□' '□' '□' 'V' '□' '□' '□' 'D' '□' '□' '□' 'E' '□' '□' '□' 'O' '□' '□' '□']

test/runtests.jl

@@ -11,6 +11,7 @@ tests = [
          "enigma",
          "hill",
          "solitaire",
+         "railfence"
         ]
 
 println("Running tests:")