Comment on A Linear Algebra Trick for Computing Fibonacci Numbers Fast

• IAm_A_Complete_Idiot@sh.itjust.works ⁨1⁩ ⁨year⁩ ago

  According to the benchmark in the article it’s already way faster at n = 1000. I think you’re overestimating the cost of multiplication relative to just cutting down n logarithmically.

  log_2(1000) = roughly 10, which is a hundred times less than 1000. 2000 would be 11, and 4000 would be 12. Logs are crazy.

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  • cbarrick@lemmy.world ⁨1⁩ ⁨year⁩ ago

    The article is comparing to the dynamic programming algorithm, which requires reading and writing to an array or hash table (the article uses a hash table, which is slower).

    The naive algorithm is way faster than the DP algorithm.

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    • t_veor@sopuli.xyz ⁨1⁩ ⁨year⁩ ago

      It’s not that hard to check yourself. Running the following code on my machine, I get that the linear algebra algorithm is already faster than the naive algorithm at around n = 100 or so. I’ve written a more optimised version of the naive algorithm, which is beaten somewhere between n = 200 and n = 500.

      Try running this Python code on your machine and see what you get:

      import timeit
      
      def fib_naive(n):
          a = 0
          b = 1
          while 0 < n:
              b = a + b
              a = b - a
              n = n - 1
          return a
      
      def fib_naive_opt(n):
          a, b = 0, 1
          for _ in range(n):
              a, b = b + a, b
          return a
      
      def matmul(a, b):
          return (
              (a[0][0] * b[0][0] + a[0][1] * b[1][0], a[0][0] * b[0][1] + a[0][1] * b[1][1]),
              (a[1][0] * b[0][0] + a[1][1] * b[1][0], a[1][0] * b[0][1] + a[1][1] * b[1][1]),
          )
      
      def fib_linear_alg(n):
          z = ((1, 1), (1, 0))
          y = ((1, 0), (0, 1))
          while n > 0:
              if n % 2 == 1:
                  y = matmul(y, z)
              z = matmul(z, z)
              n //= 2
      
          return y[0][0]
      
      def time(func, n):
          times = timeit.Timer(lambda: func(n)).repeat(repeat=5, number=10000)
          return min(times)
      
      for n in (50, 100, 200, 500, 1000):
          print("========")
          print(f"n = {n}")
          print(f"fib_naive:\t{time(fib_naive, n):.3g}")
          print(f"fib_naive_opt:\t{time(fib_naive_opt, n):.3g}")
          print(f"fib_linear_alg:\t{time(fib_linear_alg, n):.3g}")
      

      Here’s what it prints on my machine:

      ========
      n = 50
      fib_naive:      0.0296
      fib_naive_opt:  0.0145
      fib_linear_alg: 0.0701
      ========
      n = 100
      fib_naive:      0.0652
      fib_naive_opt:  0.0263
      fib_linear_alg: 0.0609
      ========
      n = 200
      fib_naive:      0.135
      fib_naive_opt:  0.0507
      fib_linear_alg: 0.0734
      ========
      n = 500
      fib_naive:      0.384
      fib_naive_opt:  0.156
      fib_linear_alg: 0.112
      ========
      n = 1000
      fib_naive:      0.9
      fib_naive_opt:  0.347
      fib_linear_alg: 0.152
      

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      • cbarrick@lemmy.world ⁨1⁩ ⁨year⁩ ago

        Nice.

        I have successfully stuck my foot in my own mouth.

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