Vigenère cipher

General Usage
3

I think the table should be an result of the shifting alphabet concept! I do like the idea of being more robust to input strings.

Here’s version 2:

number(letter) = letter-'A'         # number of letters between `letter` and 'A'
letter(number) = 'A'+(number+26)%26 # the `number`th letter after 'A' (negative `number` means it should be before 'A')
# Loop around if you run out of letters in the alphabet. So "A-1=Z" and "A+27=B".

→(a::Char,b::Char) = letter(number(a)+number(b)) # encode using +
←(a::Char,b::Char) = letter(number(a)-number(b)) # decode using -

isletter(a) = a in 'A':'Z' # is `a` inside the letters from 'A' to 'Z'?
cleanup(str::String) = filter(isletter, collect(uppercase.(str))) # make all letters uppercase and filter out non-letters
match_length(word,len) = repeat(word,len)[1:len]  # repeat the codeword until you match the length

function →(message::String,codeword::String, ↔ = →)
    plaintext = cleanup(message)
    key = match_length(cleanup(codeword),length(plaintext))
    return String(plaintext .↔ key)
end
←(message::String,codeword::String) = →(message,codeword,←)

Trying to slow down a little and add comments to help her (she’s 12 and hasn’t taken any coding yet).

julia> number('A'),number('L')
(0, 11)

julia> letter(2),letter(-2)
('C', 'Y')

julia> alpha = 'A':'Z';
julia> alpha .→ permutedims(alpha)
26×26 Matrix{Char}:
 'A'  'B'  'C'  'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'  'W'  'X'  'Y'  'Z'
 'B'  'C'  'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'  'W'  'X'  'Y'  'Z'  'A'
 'C'  'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'  'W'  'X'  'Y'  'Z'  'A'  'B'
 'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'  'W'  'X'  'Y'  'Z'  'A'  'B'  'C'
 ⋮                        ⋮                        ⋮                        ⋮                        ⋮                        ⋮
 'W'  'X'  'Y'  'Z'  'A'  'B'  'C'  'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'
 'X'  'Y'  'Z'  'A'  'B'  'C'  'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'  'W'
 'Y'  'Z'  'A'  'B'  'C'  'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'  'W'  'X'
 'Z'  'A'  'B'  'C'  'D'  'E'  'F'  'G'  'H'  'I'  'J'  'K'  'L'  'M'  'N'  'O'  'P'  'Q'  'R'  'S'  'T'  'U'  'V'  'W'  'X'  'Y'

julia> cleanup("a & b & z")
3-element Vector{Char}:
 'A': ASCII/Unicode U+0041 (category Lu: Letter, uppercase)
 'B': ASCII/Unicode U+0042 (category Lu: Letter, uppercase)
 'Z': ASCII/Unicode U+005A (category Lu: Letter, uppercase)

julia> match_length("funrun",10)
"funrunfunr"

julia> cyphertext = "I want spearmint gum" → "Gimme!"
"OEMZXYXQMVSQZFKAU"

julia> cyphertext ← "Gimme!"
"IWANTSPEARMINTGUM"
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