Adds Get-FileHash for PowerShell < 4

Author: alexpilotti

GetFileHash.ps1

@@ -0,0 +1,37 @@
+# Get-FileHash has been introduced only in PowerShell 4.0
+# Here's a compatible implementation for previous versions (SHA1 only for now)
+function Get-FileHash {
+    [CmdletBinding()]
+    param
+    (
+        [parameter(Mandatory=$true)]
+        [string]$Path,
+
+        [string]$Algorithm = "SHA1"
+    )
+    process
+    {
+        if ($Algorithm -ne "SHA1") {
+            throw "Unsupported algorithm: $Algorithm"
+        }
+
+        $fullPath = Resolve-Path $Path
+        $f = [System.IO.File]::OpenRead($fullPath)
+        $sham = $null
+        try {
+            $sham = new-object System.Security.Cryptography.SHA1Managed
+            $hash = $sham.ComputeHash($f)
+
+            $hashSB = new-object System.Text.StringBuilder -ArgumentList ($hash.Length * 2)
+            foreach ($b in $hash) {
+                $sb = $hashSB.AppendFormat("{0:x2}", $b)
+            }
+
+            return [pscustomobject]@{Algorithm="SHA1"; Path=$fullPath; Hash=$hashSB.ToString()}
+        }
+        finally {
+            $f.Close()
+            if($sham) { $sham.Dispose() }
+        }
+    }
+}