ws page 31 - Dave's Blog

DIY Pepsi Challenge

Microsoft isn't completely shielded from our economies issues but I still have a job and still get free soda. While that's all still the case, I decided to test Sarah's claimed ability to differentiate between Pepsi, Coke, and their diet counterparts by taste alone. I poured the four sodas into marked cups and Sarah and I each took two runs through the cups with the following guesses.

As you can see from the results, Sarah's claimed ability to identify Coke and Pepsi by taste is confirmed. The first run through she got completely correct and on the second run only mistook Diet Pepsi for Diet Coke. Her excuse for the error on the second run was a tainted palate from the first run. I on the other hand was mostly incorrect. Surprisingly though my incorrect answers were mostly consistent between run one and two. For instance I thought Pepsi was Coke in both runs.

Tab Expansion in PowerShell

PowerShell gives us a real CLI for Windows based around .Net stuff. I don't like the creation of a new shell language but I suppose it makes sense given that they want something C# like but not C# exactly since that's much to verbose and strict for a CLI. One of the functions you can override is the TabExpansion function which is used when you tab complete commands. I really like this and so I've added on to the standard implementation to support replacing a variable name with its value, tab completion of available commands, previous command history, and drive names (there not restricted to just one letter in PS).

Learning the new language was a bit of a chore but MSDN helped. A couple of things to note, a statement that has a return value that you don't do anything with is implicitly the return value for the current function. That's why there's no explicit return's in my TabExpansion function. Also, if you're TabExpansion function fails or returns nothing then the builtin TabExpansion function runs which does just filenames. This is why you can see that the standard TabExpansion function doesn't handle normal filenames: it does extra stuff (like method and property completion on variables that represent .Net objects) but if there's no fancy extra stuff to be done it lets the builtin one take a crack.

Here's my TabExpansion function. Probably has bugs, so watch out!

function EscapePath([string] $path, [string] $original)
{
    if ($path.Contains(' ') -and !$original.Contains(' '))
    {
        '"'   $path   '"';
    }
    else
    {
        $path;
    }
}

function PathRelativeTo($pathDest, $pathCurrent)
{
    if ($pathDest.PSParentPath.ToString().EndsWith($pathCurrent.Path))
    {
        '.\'   $pathDest.name;
    }
    else
    {
        $pathDest.FullName;
    }
}

#  This is the default function to use for tab expansion. It handles simple
# member expansion on variables, variable name expansion and parameter completion
# on commands. It doesn't understand strings so strings containing ; | ( or { may
# cause expansion to fail.

function TabExpansion($line, $lastWord)
{
    switch -regex ($lastWord)
    {
         # Handle property and method expansion...
         '(^.*)(\$(\w|\.) )\.(\w*)$' {
             $method = [Management.Automation.PSMemberTypes] `
                 'Method,CodeMethod,ScriptMethod,ParameterizedProperty'
             $base = $matches[1]
             $expression = $matches[2]
             Invoke-Expression ('$val='   $expression)
             $pat = $matches[4]   '*'
             Get-Member -inputobject $val $pat | sort membertype,name |
                 where { $_.name -notmatch '^[gs]et_'} |
                 foreach {
                     if ($_.MemberType -band $method)
                     {
                         # Return a method...
                         $base   $expression   '.'   $_.name   '('
                     }
                     else {
                         # Return a property...
                         $base   $expression   '.'   $_.name
                     }
                 }
             break;
          }

         # Handle variable name expansion...
         '(^.*\$)([\w\:]*)$' {
             $prefix = $matches[1]
             $varName = $matches[2]
             foreach ($v in Get-Childitem ('variable:'   $varName   '*'))
             {
                 if ($v.name -eq $varName)
                 {
                     $v.value
                 }
                 else
                 {
                    $prefix   $v.name
                 }
             }
             break;
         }

         # Do completion on parameters...
         '^-([\w0-9]*)' {
             $pat = $matches[1]   '*'

             # extract the command name from the string
             # first split the string into statements and pipeline elements
             # This doesn't handle strings however.
             $cmdlet = [regex]::Split($line, '[|;]')[-1]

             #  Extract the trailing unclosed block e.g. ls | foreach { cp
             if ($cmdlet -match '\{([^\{\}]*)$')
             {
                 $cmdlet = $matches[1]
             }

             # Extract the longest unclosed parenthetical expression...
             if ($cmdlet -match '\(([^()]*)$')
             {
                 $cmdlet = $matches[1]
             }

             # take the first space separated token of the remaining string
             # as the command to look up. Trim any leading or trailing spaces
             # so you don't get leading empty elements.
             $cmdlet = $cmdlet.Trim().Split()[0]

             # now get the info object for it...
             $cmdlet = @(Get-Command -type 'cmdlet,alias' $cmdlet)[0]

             # loop resolving aliases...
             while ($cmdlet.CommandType -eq 'alias') {
                 $cmdlet = @(Get-Command -type 'cmdlet,alias' $cmdlet.Definition)[0]
             }

             # expand the parameter sets and emit the matching elements
             foreach ($n in $cmdlet.ParameterSets | Select-Object -expand parameters)
             {
                 $n = $n.name
                 if ($n -like $pat) { '-'   $n }
             }
             break;
         }

         default {
             $varNameStar = $lastWord   '*';

             foreach ($n in @(Get-Childitem $varNameStar))
             {
                 $name = PathRelativeTo ($n) ($PWD);

                 if ($n.PSIsContainer)
                 {
                     EscapePath ($name   '\') ($lastWord);
                 }
                 else
                 {
                     EscapePath ($name) ($lastWord);
                 }
             }

             if (!$varNameStar.Contains('\'))
             {
                foreach ($n in @(Get-Command $varNameStar))
                {
                    if ($n.CommandType.ToString().Equals('Application'))
                    {
                       foreach ($ext in @((cat Env:PathExt).Split(';')))
                       {
                          if ($n.Path.ToString().ToLower().EndsWith(($ext).ToString().ToLower()))
                          {
                              EscapePath($n.Path) ($lastWord);
                          }
                       }
                    }
                    else
                    {
                        EscapePath($n.Name) ($lastWord);
                    }
                }

                foreach ($n in @(Get-psdrive $varNameStar))
                {
                    EscapePath($n.name   ":") ($lastWord);
                }
             }

             foreach ($n in @(Get-History))
             {
                 if ($n.CommandLine.StartsWith($line) -and $n.CommandLine -ne $line)
                 {
                     $lastWord   $n.CommandLine.Substring($line.Length);
                 }
             }

             # Add the original string to the end of the expansion list.
             $lastWord;

             break;
         }
    }
}