⍝ Expand/compress HT chars:

    text←'whistles        far     and wee'

    ht←⎕ucs 9                           ⍝ horizontal tab.

    st ← ht '→'∘subs                    ⍝ show tab chars as →.

    st ¯8 xtabs text                    ⍝ 8-tabs for multiple blanks.
whistles→far→and wee

    st ¯4 xtabs text                    ⍝ 4-tabs for multiple blanks.
whistles→→far→→and→wee

    4 xtabs ¯4 xtabs text               ⍝ 4-tabs round trip.
whistles        far     and wee

    trip ← {⍵≡⍺ xtabs(-⍺)xtabs ⍵}       ⍝ check round trip for tabs ⍺, text ⍵.

    trips ← {∧/(0,⍳1+⍴⍵)trip¨⊂⍵}        ⍝ round trips for all ⍺∊0..1+⍴⍵.

    trips text                          ⍝ check round-trips for all tabs ⍺.
1
    trips ,(6 35⍴↑,/(⍳20)↑¨'⎕'),⊃⌽⎕tc   ⍝ text with embedded linefeeds.
1
    4 trip scripts.xtabs                ⍝ round-trip this script.
1
    (2/ht' ')≡¯4 xtabs 10↑''            ⍝ all-blanks case works correctly.
1
    ∧/trips¨1↓,\10↑''                   ⍝ round-trip all-blank cases.
1

⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝ test various codings of runs:

    ones ← 0 1 1 1 0 1 0 1 1 0

    runs ← {s←+\⍵ ⋄ s-⌈\s×~⍵}       ⍝ runs of adjacent 1s.

    runs ones                       ⍝ runs of adjacent 1s.
0 1 2 3 0 1 0 1 2 0

    runs ← {⍵{⍵-⌈\⍵×~⍺}+\⍵}         ⍝ ditto, but with no local variable.

    runs ones                       ⍝ runs of adjacent 1s.
0 1 2 3 0 1 0 1 2 0

⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝⍝ test various codings of onoff:
⍝
⍝ onoff codings must decide two issues:
⍝
⍝ - whether to start in state ON or OFF and
⍝ - whether a clash of 1-bits results in ON or OFF or STAY or FLIP
⍝
⍝ where STAY means to remain in the current state (ignore concident bits),
⍝ and   FLIP means to change state.
⍝
⍝ This gives rise to 8 permutations, labelled:
⍝
⍝   [00]   start OFF, resolve clash to OFF
⍝   [01]     ··  OFF   ··  ··  ··  ··  ON
⍝   [0+]     ··  OFF   ··  ··  ··  ··  STAY
⍝   [0~]     ··  OFF   ··  ··  ··  ··  FLIP
⍝   [10]     ··  ON    ··  ··  ··  ··  OFF
⍝   [11]     ··  ON    ··  ··  ··  ··  ON
⍝   [1+]     ··  ON    ··  ··  ··  ··  STAY
⍝   [1~]     ··  ON    ··  ··  ··  ··  FLIP
⍝
⍝            .------------------------------  switch ON
⍝            | .----------------------------  switch ON while ON
⍝            | |       .--------------------  switch ON
⍝            | |       |   .----------------  clash while ON
     ons ← 0 1 1 0 0 0 1 0 1 0 0 0 1 0
    offs ← 0 0 0 0 1 1 0 0 1 0 1 0 1 0
⍝                  | |         |   '--------  clash while OFF
⍝                  | |         '------------  switch OFF
⍝                  | '----------------------  switch OFF while OFF
⍝                  '------------------------  switch OFF while ON

    op←{↑⍺ ⍵,⊂⍺ ⍺⍺ ⍵}                       ⍝ arg rows followed by result

    ons {1↓⊃,/∨\¨(1,⍵)⊂0,⍺}op offs          ⍝ [01]
0 1 1 0 0 0 1 0 1 0 0 0 1 0
0 0 0 0 1 1 0 0 1 0 1 0 1 0
0 1 1 1 0 0 1 1 1 1 0 0 1 1

    ons {1↓⊃,/∨\¨(1,⍵)⊂1,⍺}op offs          ⍝ [11]
0 1 1 0 0 0 1 0 1 0 0 0 1 0
0 0 0 0 1 1 0 0 1 0 1 0 1 0
1 1 1 1 0 0 1 1 1 1 0 0 1 1

    ons {1↓⊃,/∨\¨(1,⍵)⊂0,⍺}op offs          ⍝ [01]
0 1 1 0 0 0 1 0 1 0 0 0 1 0
0 0 0 0 1 1 0 0 1 0 1 0 1 0
0 1 1 1 0 0 1 1 1 1 0 0 1 1

    ons {1↓⊃,/∧\¨(1,⍺)⊂1,~⍵}op offs         ⍝ [10]
0 1 1 0 0 0 1 0 1 0 0 0 1 0
0 0 0 0 1 1 0 0 1 0 1 0 1 0
1 1 1 1 0 0 1 1 0 0 0 0 0 0

    ons {1↓⊃,/∧\¨(1,⍺)⊂0,~⍵}op offs         ⍝ [00]
0 1 1 0 0 0 1 0 1 0 0 0 1 0
0 0 0 0 1 1 0 0 1 0 1 0 1 0
0 1 1 1 0 0 1 1 0 0 0 0 0 0

    ons {n←⍺≠⍵ ⋄ ≠\n\2≠/¯1,n/⍺-⍵}op offs    ⍝ [0+]
0 1 1 0 0 0 1 0 1 0 0 0 1 0
0 0 0 0 1 1 0 0 1 0 1 0 1 0
0 1 1 1 0 0 1 1 1 1 0 0 0 0

    ons {(⍺≠⍵){≠\⍺\2≠/¯1,⍺/⍵}⍺-⍵}op offs    ⍝ [0+]
0 1 1 0 0 0 1 0 1 0 0 0 1 0
0 0 0 0 1 1 0 0 1 0 1 0 1 0
0 1 1 1 0 0 1 1 1 1 0 0 0 0

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