Saturday, February 15, 2014

Dynamic Programming

I was going through another project euler problem, and the solution ended up being a example of dynamic programming.  I've gone through project euler solutions in the past, so I thought it might be interesting to walk through another.

In short, dynamic programming is a way of using cached information to optimize choice in a recursive problem.  By eliminating the non-optimal choices, it reduces the complexity (possibly exponentially).  It's actually a bit of an understated concept, since a lot of problems can naturally be expressed with recursion.  A lot of the problems are also generally not feasible without it.  Obviously the larger the problem, the larger the run-time improvement.

The specific problem has to do with optimally traversing a graph.  Since the problem was simple enough, I ended up writing the solution in yasm (amd64) assembly.

"Find the maximum total from top to bottom in triangle.txt, a 15K text file containing a triangle with one-hundred rows."

So, a valid move on the graph follows a directed edge to the left or right child.  An exhaustive search is $\Theta(n2^{n-1})$ with respect  to height ($n$ nodes per path, $2^{n-1}$ paths).  It's estimated there are $10^{80}$ atoms in the known universe.  So given only a few hundred rows, the number of distinct traversals possible is roughly equivalent to the number of atoms in all of known existence.

Obviously, a pointer based tree structure implies the complexity, since it implies an exhaustive search.  However, there are more ways than one to represent a tree.  One possible representation to avoid the complexity is the same layout that's used for heaps (Eytzinger).

Finally, the dynamic programming step is comparing each of the possible traversals, and caching the optimal one as if it had been taken...

   segment .data

a   dq \
0, \
59, \
73, 41, \
l   equ     100

    segment .bss

    segment .text

    global _start


    xor     rsi, rsi
    mov     rdi, 1
    mov     rdx, 1


    inc     rdx
    cmp     rdx, l
    cmovg   rax, [a + rsi*8]
    jg      max

    mov     rcx, rdx
    mov     rbx, rcx
    shl     rbx, 3
    inc     rsi
    inc     rdi

    mov     rax, [a + rsi*8]
    add     [a + rdi*8], rax
    mov     rax, [a + rsi*8 + rbx - 16]
    add     [a + rdi*8 + rbx - 8], rax
    dec     rcx

    inc     rdi


    dec     rcx
    jz      level

    mov     rax, [a + rsi*8]
    inc     rsi
    cmp     rax, [a + rsi*8]
    cmovl   rax, [a + rsi*8]
    add     [a + rdi*8], rax

    inc     rdi

    jmp     next


    dec     rdx
    jz      _end

    inc     rsi
    cmp     rax, [a + rsi*8]
    cmovl   rax, [a + rsi*8]

    jmp     max


    xor     rdi, rdi
    mov     rax, 60


With dynamic programming, run-time becomes $2n^2 - 4n - 4$ or $O(n^2)$ (the math is left as an exercise).

And final execution time is...

real     0m0.001s
user    0m0.000s
sys      0m0.000s

So, other than what would probably be an extremely large scale solution taking the intuitive approach,  the problem is essentially infeasible.  Using dynamic programming, the problem can be solved in under a millisecond.

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