Binary searcher

Find-String.ps1

function Find-String {
    <#
    .SYNOPSIS
        Find a matching string from an alphabetically sorted file.
    .DESCRIPTION
        Find-String is a specialised binary (half interval) searcher designed to find matches in sorted ASCII encoded text files.
    .NOTES
        Change log:
            11/08/2014 - Chris Dent - First release.
    #>

    [CmdletBinding()]
    [OutputType([String])]
    param (
        # The string to find. The string is treated as a regular expression and must match the beginning of the line.
        [Parameter(Mandatory = $true)]
        [String]$String,

        # The name of the file to search.
        [Parameter(Mandatory = $true)]
        [ValidateScript( { Test-Path $_ } )]
        [String]$FileName
    )

    $FileName = $pscmdlet.GetUnresolvedProviderPathFromPSPath($FileName)
    $FileStream = New-Object IO.FileStream($FileName, [IO.FileMode]::Open)
    $BinaryReader = New-Object IO.BinaryReader($FileStream)

    $Length = $BinaryReader.BaseStream.Length
    $Position = $Length / 2

    [Int64]$HalfInterval = $Length / 2
    $Position = $Length - $HalfInterval

    while ($Position -gt 1 -and $Position -lt $Length -and $Position -ne $LastPosition) {
        $LastPosition = $Position
        $HalfInterval = $HalfInterval / 2

        $BinaryReader.BaseStream.Seek($Position, [IO.SeekOrigin]::Begin) | Out-Null

        # Track back to the start of the line
        while ($true) {
            $Character = $BinaryReader.ReadByte()
            if ($BinaryReader.BaseStream.Position -eq 1) {
                $BinaryReader.BaseStream.Seek(-1, [IO.SeekOrigin]::Current) | Out-Null
                break
            } elseif ($Character -eq [Byte][Char]"`n") {
                break
            } else {
                $BinaryReader.BaseStream.Seek(-2, [IO.SeekOrigin]::Current) | Out-Null
            }
        }

        # Read the line
        $Characters = @()
        if ($BinaryReader.BaseStream.Position -lt $BinaryReader.BaseStream.Length) {
            do {
                $Characters += [Char][Int]$BinaryReader.ReadByte()
            } until ($Characters[-1] -eq [Char]"`n" -or $BinaryReader.BaseStream.Position -eq $BinaryReader.BaseStream.Length)
            $Line = (New-Object String (,[Char[]]$Characters)).Trim()
        } else {
            # End of file
            $FileStream.Close()
            return $null
        }

        if ($Line -match "^$String") {
            # Close the file stream and return the match immediately
            $FileStream.Close()
            return $Line
        } elseif ($Line -lt $String) {
            $Position = $Position + $HalfInterval
        } elseif ($Line -gt $String) {
            $Position = $Position - $HalfInterval
        }
    }

    # Close the file stream if no matches are found.
    $FileStream.Close()
}