examples/shellmath/faster_e_demo.sh

   1 #!/usr/bin/env bash
   2
   3 ###############################################################################
   4 # This script performs the same task as "slower_e_demo.sh" but with a major
   5 # performance optimization. The speedup is especially noticeable on GNU
   6 # emulation layers for Windows such as Cygwin and minGW, where the overhead
   7 # of subshelling is quite significant.
   8 #
   9 # The speedup uses global storage space to simulate pass-and-return by
  10 # reference so that you can capture the side effects of a function call without
  11 # writing to stdout and wrapping the call in a subshell. How to use:
  12 #
  13 #    Turn on  "__shellmath_isOptimized"  as shown below.
  14 #    Then instead of invoking  "mySum = $(_shellmath_add  $x  $y)",
  15 #    call  "_shellmath_add  $x  $y;  _shellmath_getReturnValue  mySum".
  16 ###############################################################################
  17
  18 source shellmath.sh
  19
  20 # Setting the '-t' flag will cause the script to time the algorithm
  21 if [[ "$1" == '-t' ]]; then
  22     do_timing=${__shellmath_true}
  23     shift
  24 fi
  25
  26 if [[ $# -ne 1 ]]; then
  27     echo "USAGE: ${BASH_SOURCE##*/}  [-t]  *N*"
  28     echo "       Approximates 'e' using the N-th order Maclaurin polynomial"
  29     echo "       (i.e. the Taylor polynomial centered at 0)."
  30     echo "       Specify the '-t' flag to time the main algorithm."
  31     exit 0
  32 elif [[ ! "$1" =~ ^[0-9]+$ ]]; then
  33     echo "Illegal argument. Whole numbers only, please."
  34     exit 1
  35 fi
  36
  37 __shellmath_isOptimized=${__shellmath_true}
  38
  39
  40 function run_algorithm()
  41 {
  42     # Initialize
  43     n=0;  N=$1;  zero_factorial=1
  44
  45     # Initialize "e" to its zeroth-order term
  46     _shellmath_divide  1  $zero_factorial
  47     _shellmath_getReturnValue term
  48     e=$term
  49
  50     # Compute successive terms T(n) := T(n-1)/n and accumulate into e
  51     for ((n=1; n<=N; n++)); do
  52         _shellmath_divide  "$term"  "$n"
  53         _shellmath_getReturnValue term
  54         _shellmath_add  "$e"  "$term"
  55         _shellmath_getReturnValue e
  56     done
  57
  58     echo "e = $e"
  59 }
  60
  61 if (( do_timing == __shellmath_true )); then
  62     time run_algorithm "$1"
  63 else
  64     run_algorithm "$1"
  65 fi
  66
  67 exit 0
  68