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Task/Execute-Brain-/REXX/execute-brain-.rexx
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Task/Execute-Brain-/REXX/execute-brain-.rexx
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/*REXX program implements the Brainf*ck (self─censored) language. */
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@.=0 /*initialize the infinite "tape". */
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p =0 /*the "tape" cell pointer. */
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! =0 /* ! is the instruction pointer (IP).*/
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parse arg $ /*allow user to specify a BrainF*ck pgm*/
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/* ┌──◄── No program? Then use default;*/
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if $='' then $=, /* ↓ it displays: Hello, World! */
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"++++++++++ initialize cell #0 to 10; then loop: ",
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"[ > +++++++ add 7 to cell #1; final result: 70 ",
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" > ++++++++++ add 10 to cell #2; final result: 100 ",
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" > +++ add 3 to cell #3; final result 30 ",
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" > + add 1 to cell #4; final result 10 ",
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" <<<< - ] decrement cell #0 ",
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"> ++ . display 'H' which is ASCII 72 (decimal) ",
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"> + . display 'e' which is ASCII 101 (decimal) ",
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"+++++++ .. display 'll' which is ASCII 108 (decimal) {2}",
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"+++ . display 'o' which is ASCII 111 (decimal) ",
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"> ++ . display ' ' which is ASCII 32 (decimal) ",
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"<< +++++++++++++++ . display 'W' which is ASCII 87 (decimal) ",
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"> . display 'o' which is ASCII 111 (decimal) ",
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"+++ . display 'r' which is ASCII 114 (decimal) ",
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"------ . display 'l' which is ASCII 108 (decimal) ",
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"-------- . display 'd' which is ASCII 100 (decimal) ",
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"> + . display '!' which is ASCII 33 (decimal) "
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/* [↑] note the Brainf*ck comments.*/
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do !=1 while !\==0 & !<=length($) /*keep executing BF as long as IP ¬ 0*/
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parse var $ =(!) x +1 /*obtain a Brainf*ck instruction (x),*/
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/*···it's the same as x=substr($,!,1) */
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select /*examine the current instruction. */
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when x=='+' then @.p=@.p + 1 /*increment the "tape" cell by 1 */
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when x=='-' then @.p=@.p - 1 /*decrement " " " " " */
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when x=='>' then p= p + 1 /*increment " instruction ptr " " */
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when x=='<' then p= p - 1 /*decrement " " " " " */
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when x=='[' then != forward() /*go forward to ]+1 if @.P = 0. */
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when x==']' then !=backward() /* " backward " [+1 " " ¬ " */
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when x== . then call charout , d2c(@.p) /*display a "tape" cell to terminal. */
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when x==',' then do; say 'input a value:'; parse pull @.p; end
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otherwise iterate
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end /*select*/
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end /*forever*/
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exit /*stick a fork in it, we're all done. */
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/*──────────────────────────────────────────────────────────────────────────────────────*/
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forward: if @.p\==0 then return !; c=1 /*C: ◄─── is the [ nested counter.*/
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do k=!+1 to length($); ?=substr($, k, 1)
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if ?=='[' then do; c=c+1; iterate; end
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if ?==']' then do; c=c-1; if c==0 then leave; end
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end /*k*/
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return k
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/*──────────────────────────────────────────────────────────────────────────────────────*/
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backward: if @.p==0 then return !; c=1 /*C: ◄─── is the ] nested counter.*/
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do k=!-1 to 1 by -1; ?=substr($, k, 1)
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if ?==']' then do; c=c+1; iterate; end
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if ?=='[' then do; c=c-1; if c==0 then return k+1; end
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end /*k*/
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return k
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