PowerShell completion caching strategy

Adding a basic strategy which improves the performance of powershell completion.

Author: staxmanade

completion/powershell

@@ -11,7 +11,37 @@
 $gulp_completion_Process = {
     param($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameter)
 
-    (node -e "try { var gulp = require('gulp'); require('./gulpfile'); console.log(Object.keys(gulp.tasks).join(' ')); } catch (e) {}").split(' ') | 
+
+	# Load up an assembly to read the gulpfile's sha1
+	if(-not $global:GulpSHA1Managed) {
+		[Reflection.Assembly]::LoadWithPartialName("System.Security") | out-null
+		$global:GulpSHA1Managed = new-Object System.Security.Cryptography.SHA1Managed
+	}
+
+	# setup a global (in-memory) cache
+	if(-not $global:GulpfileShaCache) {
+		$global:GulpfileShaCache = @{};
+	}
+
+	$cache = $global:GulpfileShaCache;
+
+	# Get the gulpfile's sha1
+	$sha1gulpFile = (resolve-path gulpfile.js -ErrorAction Ignore | %{
+		$file = [System.IO.File]::Open($_.Path, "open", "read")
+		[string]::join('', ($global:GulpSHA1Managed.ComputeHash($file) | %{ $_.ToString("x2") }))
+		$file.Dispose()
+	})
+
+	# lookup the sha1 for previously cached task lists.
+	if($cache.ContainsKey($sha1gulpFile)){
+		$tasks = $cache[$sha1gulpFile];
+	} else {
+		$tasks = (node -e "try { var gulp = require('gulp'); require('./gulpfile'); console.log(Object.keys(gulp.tasks).join(' ')); } catch (e) {}").split(' ');
+		$cache[$sha1gulpFile] = $tasks;
+	}
+
+
+    $tasks | 
         where { $_.startswith($commandName) }
         Sort-Object | 
         foreach { New-Object System.Management.Automation.CompletionResult $_, $_, 'ParameterValue', ('{0}' -f $_) }