#!/usr/bin/perl
use strict;
use warnings;
$|++;


Main();
exit;


## adapted from code found at: http://www.cs.tut.fi/~jkorpela/c/eng.html
sub eng
{
    my( $num, $digits )= @_;

    ## default to smallest number of digits allowing fixed width mantissa (4)
    $digits= defined $digits && 3 < $digits 
        ? $digits 
        : 4;
    
    my $neg;

    if( 0 > $num )
    {
        $neg= 'true';
        $num= -$num;
    }

    0 == $num and return sprintf '+%.*fe+%s' => $digits - 1, $num, 0;
    my $exp= 0 != $num 
        ## perl's log() is natural log, convert to common log
        ? int( log($num) / log(10) )
        ## short-circuit: can't do log(0)
        : 0;

    ## tricky integer casting ahead...
    $exp= 0 < $exp
        ? int( ( int( $exp / 3 ) ) * 3 )
        : int( int( ( -$exp + 3 ) / 3 ) * -3 );

    $num *= 10 ** -$exp;

    if( 1000 <= $num )
    { 
        $num /= 1000;
        $exp += 3;
    }
    elsif( 100 <= $num )
    {
        $digits -= 2;
    }
    elsif( 10 <= $num )
    {
        $digits -= 1;
    }

    0 <= $exp 
        and $exp= '+' . $exp;
    
    return ( $neg ? '-' : '+' ) 
        . sprintf '%.*fe%s' => $digits - 1, $num, $exp;
}


sub Main 
{
    my $digits= 2;

    for my $exp ( -101..-98, -11, -10..11, 98..101 )
    {
        for my $sign ( '', '-' ) 
        {
            my $num= 0 + ( $sign . "5.555555555e" . $exp );
            printf "%-20s (%s)\n", $num, eng( $num, $digits );
        }
    }

    for my $exp (  -10..11 ) 
    {
        for my $sign (  '', '-'  ) 
        {
            my $num= 0 + ( $sign . "1e" . $exp );
            printf "%-20s (%s)\n", $num, eng( $num, $digits );
            printf "%-20s (%s)\n", 0, eng( 0, $digits )
                if  1 == $num;
        }
    }
}