2008 Scripting Games, Solution 3

# Advanced Event 3: Instant (Runoff) Winner
# http://www.microsoft.com/technet/scriptcenter/funzone/games/games08/aevent3.mspx

# Load all votes from the file
$allvotes=type c:\scripts\votes.txt

# While-true in lack of a real do-loop construct
while ($true) {
    # Pick up the first name from each line
    $votes=$allvotes | ? {$_} | % {$vote,$null=$_.split(","); $vote }
    # Save the count
    $count=$votes.count
    write-verbose "count=$count"
    # Use Group combined with select and a calculated property to get percentages
    $result=$votes | Group | Select Name,@{Name="Percentage";Expression={$_.count / $count * 100}}
    # Do we have a winner? If a percentage is larger than 50, we do
    if (($result | measure-object -Maximum Percentage).Maximum -gt 50) {
        # Get winner, only one can be returned
        $winner=$result | ? {$_.Percentage -gt 50}
        # Format output as requested
        "The winner is $($winner.name) with $($winner.percentage.tostring('##.#'))% of the vote."
        # Exit while
        break
    }
    write-verbose "Winner not found"
    # Find candidate that needs to be eliminated
    $fewestVotes=$result | sort Percentage | Select -First 1
    write-verbose "Fewest votes $fewestVotes"
    # Modify votes by removing candidate with fewest votes
    # Each line is turned into an array, the array is converted back into
    # a comma-separated line excluding the candidate
    # Note how -ne can be used to remove values from a collection/array
    $allvotes=$allvotes | % {
        $votes=$_.split(",")
        [string]::join(",",$votes -ne $fewestVotes.Name)
    }
}