(maint) Refactor region folding detection

Previously the region comment detection used a little convoluted method to
detect regions in a document.  This commit simplifies the detection by
extracting the line from the document and using regex's similar to that used by
the PowerShell language configuration.  This also removes the need for the
emptyline and subsequentText method calls.  While the performance of the folder
is pretty quick, this should in theory make it faster on larger documents by
doing less calls to the VSCode Document API.

Author: glennsarti

src/features/Folding.ts

@@ -173,6 +173,14 @@ interface ILineNumberRangeList extends Array<LineNumberRange> { }
 export class FoldingProvider implements vscode.FoldingRangeProvider {
     private powershellGrammar: IGrammar;
 
+    /**
+     * These regular expressions are used to match lines which mark the start and end of region comment in a PowerShell
+     * script. They are based on the defaults in the VS Code Language Configuration at;
+     * https://github.com/Microsoft/vscode/blob/64186b0a26/extensions/powershell/language-configuration.json#L26-L31
+     */
+    private readonly startRegionText = /^\s*#region\b/i;
+    private readonly endRegionText = /^\s*#endregion\b/i;
+
     constructor(
         powershellGrammar: IGrammar,
     ) {
@@ -326,18 +334,16 @@ export class FoldingProvider implements vscode.FoldingRangeProvider {
     }
 
     /**
-     * Given a zero based offset, find the line text after it in the document
+     * Given a zero based offset, find the line in the document
      * @param offset   Zero based offset in the document
      * @param document The source text document
-     * @returns        The line text after the offset, not including the subsequent Line Feed
+     * @returns        The line at the offset
      */
-    private subsequentText(
+    private lineAtOffset(
         offset: number,
         document: vscode.TextDocument,
-    ): string {
-        const startPos: vscode.Position = document.positionAt(offset);
-        const endPos: vscode.Position = document.lineAt(document.positionAt(offset)).range.end;
-        return document.getText(new vscode.Range(startPos, endPos));
+    ): vscode.TextLine {
+        return document.lineAt(document.positionAt(offset));
     }
 
     /**
@@ -420,21 +426,14 @@ export class FoldingProvider implements vscode.FoldingRangeProvider {
         document: vscode.TextDocument,
     ): ITokenList {
         const result = [];
-
-        const emptyLine = /^\s*$/;
-        const startRegionText = /^#region\b/i;
-        const endRegionText = /^#endregion\b/i;
-
         tokens.forEach((token) => {
             if (token.scopes.indexOf("punctuation.definition.comment.powershell") !== -1) {
-                if (emptyLine.test(this.preceedingText(token.startIndex, document))) {
-                    const commentText = this.subsequentText(token.startIndex, document);
-                    if (startRegionText.test(commentText)) {
-                        result.push(this.addTokenScope(token, "custom.start.region"));
-                    }
-                    if (endRegionText.test(commentText)) {
-                        result.push(this.addTokenScope(token, "custom.end.region"));
-                    }
+                const line: string = this.lineAtOffset(token.startIndex, document).text;
+                if (this.startRegionText.test(line)) {
+                    result.push(this.addTokenScope(token, "custom.start.region"));
+                }
+                if (this.endRegionText.test(line)) {
+                    result.push(this.addTokenScope(token, "custom.end.region"));
                 }
             }
         });