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BrainFuck part 9 - Sort arrays with Bubble and QuickSort

DPAmar
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Quicksort

Now, let's implement QuickSort. This sorting solution is known as one of the most efficient ones in general case.

The principle is:

  • Select a pivot value P
  • Move any value lower than P on an array A
  • Move any value higher than P on an array B
  • Sort A and B
  • Merge [A, P, B]

The problem is that we never did something like recursion in BF. Let's try it now

Let's start

  • Memory: 0 array 0 0 0 0
  • Cursor: after last array value
  • Input: some chars to sort

Process

  • Note: we will read chars from the input and compute array length on the fly
  • There will be on the right of the array a processing queue with pairs [startIndex, length]
  • Initialize algorithm with startIndex and length
    • Note: indexes will be 1-based and from the end of the array. Array[1] = last cell
    • Memory : 0 [array] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 [queue]
  • Get current start index and length
    • Move array cells from first to start index - 1 on the far right
    • Then move the next length cells to the right, only one cell
    • Select pivot : let's pick last value and isolate it
    • for each array value, except pivot, compare with pivot:
      • if less than pivot:
        • move to beginning of array1 (easier than end)
        • increase array1 length
      • if greater:
        • move to begining of array2
        • increase array2 length and move to the left
        • move array1, array 1 length and pivot to the left
    • Memory : 0 [out of range array] 0 [array to sort] 0 [array 1] 0 [array 1 length] [pivot value] 0 0 0 0 0 [array 2 length] 0 [array 2] 0 [out of range] 0 0 0 [queue]
    • if array2 length is not null add at end of queue values [start index, array2 length]
    • if array1 length is not null add at end of queue values [start index + array2 length + 1, array1 length]
    • Move arrays back to the original memory configuration
    • remove queue first entries
    • move queue on the left
  • loop

Code

>,[>+[->+<],]                                  read chars and build array length
>[->>>>>>>>>>>>>>>+<<<<<<<<<<<<<<<]            move length
>>>>>>>>>>>>>>+                                set first queue pair : 1 and length
[                                               for each queue pair
  [-<+<+>>]<[->+<]                             copy start index
  <-[-<<[<]<<<<<<<<<<<<[->>>>>>>>>>            move first out of range array
  >>+<<<<<<<<<<<<]>>>>>>>>>>>>[>]>]              ** part 2 **
  >>>[-<<+<+>>>]<<[->>+<<]<                    copy length
  [-<<[<]<<<<<<<<<+>>>>>>>>>>[>]>]             move length before next array block
  <<[<]<<<<<<<<<                               go to copied length
  [-<<[<]<[->+<]>[>]>]                         move next "length" cells
  <<[->>>>+<<<<]                               isolate pivot
  <[                                           for each value in array to sort
    [->+>+<<]>>[-<<+>>]<                       copy
    [->>[>]>>>>+<<<<<[<]<]                     move copy besides pivot
    >>[>]>>[->+<<<+>>]>[-<+>]>[-<+>]<<         copy pivot and move value
    >>+<<[->[->]>[<<[-]>>->>]                  compare cell and pivot
    <<+<<]>[[-]<+>]<[->+<]<<[->>+<<]>>>          ** part 2 ** and restore pivot copy
     [->-<                                      value greater than pivot
      <<<<[<]<<[->>>[>]>>>>>>>>+<<<<<<<<<[<]<<]move value to array 2
      >>>[>]>>>>>>>+[-<+>]                     increase length 2
      <<<<<<<<[<]>[[-<+>]>]>[-<+>]>[-<+>]      move array 1 and length and pivot to the left
    ]>[-                                       or pivot greater than value
      <<<+<<[<]<<[->>+<<]>>[>]>>>>             or move value to array 1 and increase its length
    ]<<<<<[<]<<                                got to sorting array last cell
  ]                                            loop on array cell
  >>>[>]>>>>>>>                                go to array2 length
  [                                            if not null
    [-<+<+>>]<[->+<]>                          copy length
    [->>[>]>[>]>>+<<<[<]<<[<]<]                move beside queue
    >>[>]>[>]>>                                go to copy
    >[-<<<+>>>]<<<[->>>+[>]>+<<[<]<]           copy start index at end of queue
    >>[-<+>]<[->>[>]>>+<<<[<]<]                move array 2 length at end of queue
    >>[>]>[[-<+>]>]<<[<]<<<[<]<[<]<            enqueue items and go back to length 2 location
  ]<<<<<<[                                     go to array length 1; if not null
    [->>>>>>>>[>]>[>]>>>[>]>>+                 move after queue
    <<<[<]<<<[<]<[<]<<<<<<<]                     ** part 2 **
    >>>>[->>>>[>]>[>]>+<<[<]<[<]<<<]           copy length 2 if not null from a copy
    >>>>[>]>[>]>>>[-<+<+>>]<[->+<]<+           compute length 2 plus start index plus 1 (array1 start index)
    [->>[>]>+<<[<]<]                           move new startindex after queue
    >>[>]>[[-<+>]>]<<[<]<<<[<]<[<]<<<<<<<      enqueue items and go back to length 1 location
  ]                                            end of enqueuing
  <<[<]>[[-<<<+>>>]>]                          move array1 to its leftmost position
  >>[-<<<<<+>>>>>]                             move pivot
  >>>[-]                                       clear array2 length copy that might be there
  >>>>[[-<<<<<<<<<<<+>>>>>>>>>>>]>]            move array2 to its leftmost position
  >[[-<<<<<<<<<<<<+>>>>>>>>>>>>]>]             move out of range array to its leftmost position
  >>>[-]>[-]>[[-<<+>>]>]<<<[<]>                move queue to the left
]                                              loop on queued ranges

Minified version

>,[>+[->+<],]>[->>>>>>>>>>>>>>>+<<<<<<<<<<<<<<<]>>>>>>>>>>>>>>+[[-<
+<+>>]<[->+<]<-[-<<[<]<<<<<<<<<<<<[->>>>>>>>>>>>+<<<<<<<<<<<<]>>>>>
>>>>>>>[>]>]>>>[-<<+<+>>>]<<[->>+<<]<[-<<[<]<<<<<<<<<+>>>>>>>>>>[>]
>]<<[<]<<<<<<<<<[-<<[<]<[->+<]>[>]>]<<[->>>>+<<<<]<[[->+>+<<]>>[-<<
+>>]<[->>[>]>>>>+<<<<<[<]<]>>[>]>>[->+<<<+>>]>[-<+>]>[-<+>]<<>>+<<[
->[->]>[<<[-]>>->>]<<+<<]>[[-]<+>]<[->+<]<<[->>+<<]>>>[->-<<<<<[<]<
<[->>>[>]>>>>>>>>+<<<<<<<<<[<]<<]>>>[>]>>>>>>>+[-<+>]<<<<<<<<[<]>[[
-<+>]>]>[-<+>]>[-<+>]]>[-<<<+<<[<]<<[->>+<<]>>[>]>>>>]<<<<<[<]<<]>>
>[>]>>>>>>>[[-<+<+>>]<[->+<]>[->>[>]>[>]>>+<<<[<]<<[<]<]>>[>]>[>]>>
>[-<<<+>>>]<<<[->>>+[>]>+<<[<]<]>>[-<+>]<[->>[>]>>+<<<[<]<]>>[>]>[[
-<+>]>]<<[<]<<<[<]<[<]<]<<<<<<[[->>>>>>>>[>]>[>]>>>[>]>>+<<<[<]<<<[
<]<[<]<<<<<<<]>>>>[->>>>[>]>[>]>+<<[<]<[<]<<<]>>>>[>]>[>]>>>[-<+<+>
>]<[->+<]<+[->>[>]>+<<[<]<]>>[>]>[[-<+>]>]<<[<]<<<[<]<[<]<<<<<<<]<<
[<]>[[-<<<+>>>]>]>>[-<<<<<+>>>>>]>>>[-]>>>>[[-<<<<<<<<<<<+>>>>>>>>>
>>]>]>[[-<<<<<<<<<<<<+>>>>>>>>>>>>]>]>>>[-]>[-]>[[-<<+>>]>]<<<[<]>]

Final state

  • Memory: 0 sorted array 0 0 0 0 0 0 0 0 0 0
  • Cursor: 16 cells after array end
  • Input: empty (read)
  • Output: unchanged

Note : the first lines need to be modified in order to compute length from the array if not read from the input

This algorithm, performed on one hundred chars, takes about 53% operations less than the bubble sort !

But due to the nature of the recursion engine we implemented, the array size is limited to 255 items.

Test programm

This program does the same but also print chars at the end

>,[>+[->+<],]>[->>>>>>>>>>>>>>>+<<<<<<<<<<<<<<<]>>>>>>>>>>>>>>+[[-<
+<+>>]<[->+<]<-[-<<[<]<<<<<<<<<<<<[->>>>>>>>>>>>+<<<<<<<<<<<<]>>>>>
>>>>>>>[>]>]>>>[-<<+<+>>>]<<[->>+<<]<[-<<[<]<<<<<<<<<+>>>>>>>>>>[>]
>]<<[<]<<<<<<<<<[-<<[<]<[->+<]>[>]>]<<[->>>>+<<<<]<[[->+>+<<]>>[-<<
+>>]<[->>[>]>>>>+<<<<<[<]<]>>[>]>>[->+<<<+>>]>[-<+>]>[-<+>]<<>>+<<[
->[->]>[<<[-]>>->>]<<+<<]>[[-]<+>]<[->+<]<<[->>+<<]>>>[->-<<<<<[<]<
<[->>>[>]>>>>>>>>+<<<<<<<<<[<]<<]>>>[>]>>>>>>>+[-<+>]<<<<<<<<[<]>[[
-<+>]>]>[-<+>]>[-<+>]]>[-<<<+<<[<]<<[->>+<<]>>[>]>>>>]<<<<<[<]<<]>>
>[>]>>>>>>>[[-<+<+>>]<[->+<]>[->>[>]>[>]>>+<<<[<]<<[<]<]>>[>]>[>]>>
>[-<<<+>>>]<<<[->>>+[>]>+<<[<]<]>>[-<+>]<[->>[>]>>+<<<[<]<]>>[>]>[[
-<+>]>]<<[<]<<<[<]<[<]<]<<<<<<[[->>>>>>>>[>]>[>]>>>[>]>>+<<<[<]<<<[
<]<[<]<<<<<<<]>>>>[->>>>[>]>[>]>+<<[<]<[<]<<<]>>>>[>]>[>]>>>[-<+<+>
>]<[->+<]<+[->>[>]>+<<[<]<]>>[>]>[[-<+>]>]<<[<]<<<[<]<[<]<<<<<<<]<<
[<]>[[-<<<+>>>]>]>>[-<<<<<+>>>>>]>>>[-]>>>>[[-<<<<<<<<<<<+>>>>>>>>>
>>]>]>[[-<<<<<<<<<<<<+>>>>>>>>>>>>]>]>>>[-]>[-]>[[-<<+>>]>]<<<[<]>]
<<<<<<<<<<<<<<<<[<]>[.>]
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