base64←{⎕IO ⎕ML←0 1             ⍝ Base64 encoding and decoding as used in MIME.

    chars←'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    bits←{,⍉(⍺⍴2)⊤⍵}                   ⍝ encode each element of ⍵ in ⍺ bits,
                                       ⍝   and catenate them all together
    part←{((⍴⍵)⍴⍺↑1)⊂⍵}                ⍝ partition ⍵ into chunks of length ⍺

    0=2|⎕DR ⍵:2∘⊥∘(8∘↑)¨8 part{(-8|⍴⍵)↓⍵}6 bits{(⍵≠64)/⍵}chars⍳⍵
                                       ⍝ decode a string into octets

    four←{                             ⍝ use 4 characters to encode either
        8=⍴⍵:'=='∇ ⍵,0 0 0 0           ⍝   1,
        16=⍴⍵:'='∇ ⍵,0 0               ⍝   2
        chars[2∘⊥¨6 part ⍵],⍺          ⍝   or 3 octets of input
    }
    cats←⊃∘(,/)∘((⊂'')∘,)              ⍝ catenate zero or more strings
    cats''∘four¨24 part 8 bits ⍵
}

code_colours

test script

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