packR←{                             ⍝ Run Length Encoding (RLE packing).

    cmp←{                           ⍝ compress:
        shape←⍴⍵ ⋄ vect←,⍵          ⍝ shape and items of array.
        runs←1,2≠/vect              ⍝ mask of start of runs.
        repls←¯2-/⍸runs,1           ⍝ replication count.
        solosruns/vect             ⍝ non-repeated items.
        shape repls solos           ⍝ compressed structure.
    }

    exp←{                           ⍝ expand:
        shape repls solos←⍵         ⍝ compressed structure.
        shaperepls/solos           ⍝ reconstituted array.
    }

    ⍺←1 ⋄ ⍺:cmp ⍵ ⋄ exp ⍵           ⍝ compress or expand.
}
code_colours

test script

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