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Brainfuck Self Interpreter: by Clive Gifford
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Version 1: 01 December 2006 (one trip between code and data per op)
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Version 2: 16 December 2006 (virtual codes for various combinations)
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Credits
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A large section of code to load the input to the interpreter is copied
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from the the 423 byte dbfi interpreter as described in the November 2003
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paper "A Very Small Interpeter" by Oleg Mazonka and Daniel B Cristofani
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Goals
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The goal for this interpreter was to be efficient rather than small and
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particularly to allow as many copies of itself as possible to be "stacked"
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up with something else on top / In other words to achieve a low "eigenratio"
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(See http eigenratios dot blogspot dot com for more information)
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The main idea of the first version was to only make one round trip between
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the emulated code and emulated data for each instruction executed instead
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of multiple round trips which is what Daniel and Oleg's version does
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The second version does more pre processing of the guest program in order
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to map several common sequences to virtual codes thus reducing the memory
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footprint and also further reducing the number of round trips between the
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emulated code and data
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Other info:
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The input must consist of valid brainfuck code (to be interpreted) which
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must always be followed by an exclamation mark and then any associated data
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Input can also include "comments" (if desired) except for exclamation mark
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If you are stacking multiple copies of this interpreter then each additional
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level also has to appear in the input with a trailing exclamation mark and
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then we finally have the input for the very top level to finish things off
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The underlying brainfuck machine determines the possible range of values in
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the data cell values and what happens if an attempt is made to go outside the
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supported range but this interpreter does not more than 8 bit data itself
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Loops in the emulated code can be nested up to the maximum cell value in the
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underlying machine and this interpreter requires that at least 17 levels of
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nesting is supported
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Behaviour on end of input is also inherited from the next level down
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>>>> leave a little extra space before the program code
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+ start in left hand cell of first program code pair
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[
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->>> clear flag and move to the starting position
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++>+>+++++++ setup differences and read char as per dbfi
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[<++++>>++<-]++>>+>+> (by Daniel B Cristofani)
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+++++[>++>++++++<<-]
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+>>>,<++
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[
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[>[->>]<[>>]<<-] see section 3 of dbfi paper
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<[<]<+>>[>]>
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[ see section 4 of dbfi paper
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<+>-
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[[<+>-]>] see section 5 of dbfi paper
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< see section 6 of dbfi paper
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[
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[[-]<] scan left and zero the differences
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++<- see section 7 of dbfi paper
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[
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<+++++++++>[<->-]>>
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]
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>>
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]
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]
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<<
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]
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a three way switch to handle possibilities and adjust positioning
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>[-]+<< set "done" flag and position to value
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[
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-- 2 means last decode was for a valid instruction
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[ so if we still have something left now it was a 9
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[-] originally (meaning input should be ignored)
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>>- <<<+> so all we do is set the "more input" flag
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]
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>>
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[
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-
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<<<<[>+<-] for valid input move everything right one
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start of processing to find and recode certain combinations of codes
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+<<+ set flags to indicate we want to process
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the last two instructions in the loop below
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[
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-> clear flag and move to instruction
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[<+>>>>>>+<<<<<-] double copy instruction and then
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<[>+<-]>>>>>> restore original before moving to copy
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map relevant codes to values giving unique pairwise sums and also
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set a flag if we see the end of a loop
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>+<
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[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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[-]
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>-<
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]>[-<<+++++++>>] <
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]
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]
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]>[-]<
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]>[-<<+++>>]<
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]>[-<<+>>]<
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]>[-]<
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]>[-<<<<<<<+>>>>>>>]< set flag if it is end of loop
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]>[-]<
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<<<< goto next instruction flag
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]
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add values from above together to get unique sum
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>>>[<<+>>-]
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<+<
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setup a switch to figure out what it means
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[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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[-]
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>-<<<[-]<<+>> change code 8 (add 1) to 9 (add 2)
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]
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]
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]
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]>[-]<
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]>[-<<<[-]<<+++++++>>>]< change code 4 (left) to 11 (left 2)
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]
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]
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]>[-]<
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]>[-<<<[-]<<+++++++>>>]< change code 3 (right) to 10 (right 2)
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]
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]>[-]<
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clear flag set if second to last was end of loop
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and go to similar flag for last instruction
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<<<<<[-]>>
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[
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-
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<<<[>+>>+<<<-]>[<+>-]>> copy third to last instruction
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if it is the start of a loop then we can (in some cases)
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collapse the last three instructions to single virtual code
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[
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-[
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-[
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[-]
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>>
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]
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>
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[ Now we are ready to check what code is in the loop (must be at least 1)
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<<[<+>>+<-]>[<+>-]+<<
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-
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[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-
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<+> fall through & code as 16 (double skip left)
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]<+++++++++++++>>[-]>->-<< code as 15 (double skip right)
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]
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]
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]>>[->>>>>]<<
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]>>[-<<<++++++++++++>>[-]>>-]<< code as 14 (zero)
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]>>[->>>>>]<<
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]>>[-<<<+++++++++++>>[-]>>-]<< code as 13 (skip left)
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]>>[-<<<++++++++++>>[-]>>-]<< code as 12 (skip right)
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]
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]>>[->>>>>]<<
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]
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<
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]
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]>[>>]<
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<<
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]
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>>+>>>
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]
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<<
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]
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>> [->+>] << end of input so clear "done" and set data mark and
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finally position to a zero cell ready for next phase
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< move to "more input" flag
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]
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<<[<<]>> go to the first instruction
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******** MAIN INTERPRETER LOOPS STARTS HERE ********
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[ start on current instruction code
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setup a big switch statement to decode instructions
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[<+>>+<-] move/copy instruction code and set "done" flag to
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+<- start with '(i less 1) (1) (i) for i = instruction
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[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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-[
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- can't be anything but 1 so bracketing not needed
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>->> [>>] >> [>>]< [<-<<-<] <[<<] << [<<]< double skip left (code 16)
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]
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>[->> [>>] >> [>>]< [>+>>+>] <[<<] << [<<]]< double skip right (code 15)
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]
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>[->> [>>] >> [>>] <[-]< [<<] << [<<]]< zero (code 14)
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]
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>[->> [>>] >> [>>]< [<-<] <[<<] << [<<]]< skip left (code 13)
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]
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>[->> [>>] >> [>>]< [>+>] <[<<] << [<<]]< skip right (code 12)
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]
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>[->> [>>] >> [>>]< <-<<-< <[<<] << [<<]]< double move left (code 11)
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]
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>[->> [>>] >> [>>] +>>+ [<<] << [<<]]< double move right (code 10)
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]
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>[->> [>>] >> [>>]< ++ <[<<] << [<<]]< add 2 (code 9)
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]
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>[->> [>>] >> [>>]< + <[<<] << [<<]]< increment
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]
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>[->> [>>] >> [>>]< , <[<<] << [<<]]< input
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]
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>[->> [>>] >> [>>]< - <[<<] << [<<]]< decrement
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]
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>[->> [>>] >> [>>]< . <[<<] << [<<]]< output
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]
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>[->> [>>] >> [>>] <<-<< [<<] << [<<]]< move left
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]
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>[->> [>>] >> [>>] + [<<] << [<<]]< move right
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]
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>
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[- left hand bracket
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>> [>>] >> [>>]< move to data cell
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[>+>>+<<<-]> make double copy and move to first
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[<+>-] restore original data cell value
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>>[<<+>>[-]]+ This and the following achieves
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<<[>>-<<-] x = not x
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>> go to flag cell (0 or 1)
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Some tricky stuff here: set up (not flag) also so we can later choose
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appropriate instruction sequence to get back to code area in one pass
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In one case we set flags at the other end (data greater than 0) but
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for the other we just go back without setting any flags (data equals 0)
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[<<+>>>>+<<-] make two copies of flag
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>>[<<+>>-]
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<<[>>+<<-]+ This and the following achieves
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>>[<<->>-]<< x = not x
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<< so we now have (data) '(flag) (?) (not flag)
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[ if flag set then
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-<< [<<] << [<<]< clear and return to code section where we save
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<< << ++ a 2 meaning we need (later) to match left bracket
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>> stop in zero cell for now
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]
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>> if we executed code above then now at switch flag
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else it will put us ready to return from data area
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[-<<<<<<[<<]<<[<<]<] move back to switch flag without setting anything
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>
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]
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<
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]
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>
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[- right hand bracket
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>> [>>] >> [>>]< move to data cell
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[>+>>+<<<-]> make double copy and move to first
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[[<+>-]>>[-]+<<] restore data from one then zero second and set flag
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>> go to flag cell (0 or 1)
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Some tricky stuff here: set up (not flag) also so we can later choose
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appropriate instruction sequence to get back to code area in one pass
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In one case we set flags at the other end (data greater than 0) but
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for the other we just go back without setting any flags (data equals 0)
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[<<+>>>>+<<-] make two copes of flag
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>>[<<+>>-]
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<<[>>+<<-]+ This and the following achieves
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>>[<<->>-]<< x = not x
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<< so we now have (data) '(flag) (?) (not flag)
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[ if flag set then
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-<< [<<] << [<<]< clear and return to code section where we save
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<< << + a 1 meaning we need (later) to match right bracket
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>> stop in zero cell for now
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]
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>> if we executed code above then now at switch flag
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else it will put us ready to return from data area
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[-<<<<<<[<<]<<[<<]<] move back to switch flag without setting anything
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>
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]
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>[<+>-] restore original instruction code
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*** We are positioned in the cell immediately to the right of the ***
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*** instruction that has just been "executed" in the switch above ***
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*** The following code is to handle finding matching brackets ***
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*** because code above has only set a cell value to 1 or 2 to show ***
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*** what kind of loop scanning is required (1=scan left 2=right) ***
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<< << << position to cell showing if matching required
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[ if non zero we need to find a matching bracket
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>> + set up "done" flag for switch and
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<< - decrement switch value so now is 0 or 1
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[ if 1 we are looking for matching right bracket
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- >> - >> + clear switch value & "done" & set level to 1
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[ while level is more than 0
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>>>[-<+>>+<] make double copy of instruction code
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+<- set flag and prepare for switch
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[
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-[
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[-] clear whatever is left of code
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> - < do nothing except clear flag
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]
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> [- <<< + >>>] < increment level
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]
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> [- <<< - >>>] decrement level
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>[-<+>]<< restore instruction code
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<< go to level
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[>>+<<-] if level then move right one instruction
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>>
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]
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<< << << go back to switch value cell
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]
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>> go to switch done flag and if still set then
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[ we must be looking for a matching left bracket
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- << + clear switch value & "done" & set level to 1
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[ repeat while level is more than 0
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>>>[-<+>>+<] make double copy of instruction code
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+<- set flag and prepare for switch
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[
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-[
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[-] clear whatever is left of code
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> - < do nothing except clear flag
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]
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> [- <<< - >>>] < decrement level
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]
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> [- <<< + >>>] increment level
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>[-<+>]<< restore instruction code
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<< go to level
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[<<+>>-] if level then move left one instruction
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<<
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]
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]
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]
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>> >> >>
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> move forward to next instruction
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]

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