Enhancement: Enhance NFunction evaluation and support advanced NFunctions (#2577)

# Description

Enhance NFunction sanitization and support advanced functions:
- Start page counting from 1 rather than 0 as PDFs are one based from
the user's perspective, thus functions results would be affected by
starting with "0" rather than "1".
- Ignore out of bound results rather than stopping iterations to work
with functions such as (n - 4) when page count is 10 as we would get
positive values when n > 4.
- Remove spaces to support expressions such as 2n + 1 rather just 2n+1.
- Support advanced functions as follows:
- Support expressions such as follows 5(n-1), n(n-1), expressions
followed by opening rounded without '*' operator.
- Support expressions such as follows (n-1)5, (n-1)n, expressions
preceded closing rounded without '*' operator.
- Support consecutive "n" expressions, examples: nnn, 2nn, nn*3, nnnn.

Closes #(issue_number)

## Checklist

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have performed a self-review of my own code
- [ ] I have attached images of the change if it is UI based
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] If my code has heavily changed functionality I have updated
relevant docs on [Stirling-PDFs doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
- [x] My changes generate no new warnings
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

src/test/java/stirling/software/SPDF/utils/GeneralUtilsTest.java

@@ -52,14 +52,68 @@ public class GeneralUtilsTest {
     @Test
     void nFuncAdvanced3() {
         List<Integer> result = GeneralUtils.parsePageList(new String[]{"4n+1"}, 9, true);
-        assertEquals(List.of(1, 5, 9), result, "'All' keyword should return all pages.");
+        assertEquals(List.of(5, 9), result, "'All' keyword should return all pages.");
+    }
+
+    @Test
+    void nFunc_spaces() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"n + 1"}, 9, true);
+        assertEquals(List.of(2, 3, 4, 5, 6, 7, 8, 9), result);
+    }
+
+    @Test
+    void nFunc_consecutive_Ns_nnn() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"nnn"}, 9, true);
+        assertEquals(List.of(1, 8), result);
+    }
+
+    @Test
+    void nFunc_consecutive_Ns_nn() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"nn"}, 9, true);
+        assertEquals(List.of(1, 4, 9), result);
+    }
+
+    @Test
+    void nFunc_opening_closing_round_brackets() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"(n-1)(n-2)"}, 9, true);
+        assertEquals(List.of(2, 6), result);
+    }
+
+    @Test
+    void nFunc_opening_round_brackets() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"2(n-1)"}, 9, true);
+        assertEquals(List.of(2, 4, 6, 8), result);
+    }
+
+    @Test
+    void nFunc_opening_round_brackets_n() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"n(n-1)"}, 9, true);
+        assertEquals(List.of(2, 6), result);
+    }
+
+    @Test
+    void nFunc_closing_round_brackets() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"(n-1)2"}, 9, true);
+        assertEquals(List.of(2, 4, 6, 8), result);
+    }
+
+    @Test
+    void nFunc_closing_round_brackets_n() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"(n-1)n"}, 9, true);
+        assertEquals(List.of(2, 6), result);
+    }
+
+    @Test
+    void nFunc_function_surrounded_with_brackets() {
+        List<Integer> result = GeneralUtils.parsePageList(new String[]{"(n-1)"}, 9, true);
+        assertEquals(List.of(1, 2, 3, 4, 5, 6, 7, 8), result);
     }
 
 
     @Test
     void nFuncAdvanced4() {
         List<Integer> result = GeneralUtils.parsePageList(new String[]{"3+2n"}, 9, true);
-        assertEquals(List.of(3, 5, 7, 9), result, "'All' keyword should return all pages.");
+        assertEquals(List.of(5, 7, 9), result, "'All' keyword should return all pages.");
     }
 
     @Test
@@ -80,7 +134,6 @@ public class GeneralUtilsTest {
         assertEquals(List.of(1, 2, 3), result, "Range should be parsed correctly.");
     }
 
-
     @Test
     void testParsePageListWithRangeZeroBaseOutput() {
         List<Integer> result = GeneralUtils.parsePageList(new String[]{"1-3"}, 5, false);