Added checking for 4 spaces when pressing backspace/delete

Because this is overwriting the function of the keys, I have re-implemented
the standard functions of both to maintain usability.
Fix for uccser#377

codewof/static/js/question_types/function.js

@@ -22,10 +22,63 @@ $(document).ready(function () {
         autofocus: true,
         indentUnit: 4,
         viewportMargin: Infinity,
-        // Replace tabs with 4 spaces. Taken from https://stackoverflow.com/questions/15183494/codemirror-tabs-to-spaces
+        // Replace tabs with 4 spaces, and backspace all 4 when possible.
+        // Taken from https://stackoverflow.com/questions/32622128/codemirror-how-to-read-editor-text-before-or-after-cursor-position
         extraKeys: {
             "Tab": function(cm) {
                 cm.replaceSelection("    ", "end");
+            },
+            "Backspace": function(cm) {
+                doc = cm.getDoc();
+                line = doc.getCursor().line;   // Cursor line
+                ch = doc.getCursor().ch;       // Cursor character
+
+                if (doc.somethingSelected()) {  // Remove user-selected characters
+                    doc.replaceSelection("");
+                } else {    // Determine the ends of the selection to delete
+                    from = {line, ch};
+                    to = {line, ch};
+                    stringToTest = doc.getLine(line).substr(Math.max(ch - 4,0), Math.min(ch, 4));
+
+                    if (stringToTest === "    ") {  // Remove 4 spaces (dedent)
+                        from = {line, ch: ch - 4};
+                    } else if (ch == 0) {   // Remove last character of previous line
+                        if (line > 0) {
+                            from = {line: line - 1, ch: doc.getLine(line - 1).length};
+                        }
+                    } else {    // Remove preceding character
+                        from = {line, ch: ch - 1};
+                    }
+
+                    // Delete the selection
+                    doc.replaceRange("", from, to);
+                }
+            },
+            "Delete" : function(cm) {
+                doc = cm.getDoc();
+                line = doc.getCursor().line;   // Cursor line
+                ch = doc.getCursor().ch;       // Cursor character
+
+                if (doc.somethingSelected()) {  // Remove user-selected characters
+                    doc.replaceSelection("");
+                } else {    // Determine the ends of the selection to delete
+                    from = {line, ch};
+                    to = {line, ch};
+                    stringToTest = doc.getLine(line).substr(ch, 4);
+
+                    if (stringToTest === "    ") {  // Remove 4 spaces (dedent)
+                        to = {line, ch: ch + 4};
+                    } else if (ch == doc.getLine(line).length) {   // Remove first character of next line
+                        if (line < doc.size - 1) {
+                            to = {line: line + 1, ch: 0};
+                        }
+                    } else {    // Remove following character
+                        to = {line, ch: ch + 1};
+                    }
+
+                    // Delete the selection
+                    doc.replaceRange("", from, to);
+                }
             }
         }
     });