fix(pagination): rewrite calculate_pages to track lines instead of chars, newlines consume vertical height not horizontal

src/reader.rs

@@ -17,10 +17,9 @@ pub fn recalculate_pages(book: &mut Book, font_size: f32, line_height: f32, pane
     } else {
         1
     };
-    let chars_per_page = chars_per_line * lines_per_page;
     for section in &mut book.sections {
         let styled = style.apply_to_text(&section.content);
-        section.pages = calculate_pages(&styled, chars_per_page);
+        section.pages = calculate_pages(&styled, chars_per_line, lines_per_page);
     }
 }
 
@@ -391,60 +390,66 @@ fn render_bookmarks(
     }
 }
 
-pub fn calculate_pages(text: &str, chars_per_page: usize) -> Vec<usize> {
+pub fn calculate_pages(text: &str, chars_per_line: usize, lines_per_page: usize) -> Vec<usize> {
     let mut pages = Vec::new();
     pages.push(0);
 
-    if text.is_empty() || chars_per_page == 0 {
+    if text.is_empty() || chars_per_line == 0 || lines_per_page == 0 {
         return pages;
     }
 
     let chars: Vec<char> = text.chars().collect();
-    let total_chars = chars.len();
-    if total_chars <= chars_per_page {
-        pages.push(total_chars);
+    let total = chars.len();
+    if total == 0 {
         return pages;
     }
 
-    let mut pos: usize = 0;
-    while pos < total_chars {
-        let next = pos + chars_per_page;
-        if next >= total_chars {
-            pages.push(total_chars);
-            break;
+    let mut page_start: usize = 0;
+    let mut line_count: usize = 0;
+    let mut line_pos: usize = 0;
+    let mut i: usize = 0;
+
+    while i < total {
+        let ch = chars[i];
+
+        if ch == '\n' {
+            line_count += 1;
+            line_pos = 0;
+            i += 1;
+        } else if line_pos >= chars_per_line {
+            line_count += 1;
+            line_pos = 0;
+        } else {
+            line_pos += 1;
+            i += 1;
         }
 
-        // 在页面后半段查找段落边界(\n\n)或行边界(\n)，不回超理想页长
-        let search_start = pos + chars_per_page / 2;
-        let search_end = next.min(total_chars);
-        let mut split = next;
-
-        // Prefer double newline (paragraph), then single newline
-        let mut found = false;
-        for i in (search_start..search_end).rev() {
-            if chars[i] == '\n' && i > 0 && chars[i - 1] == '\n' {
-                split = i - 1;
-                found = true;
-                break;
-            }
-        }
-        if !found {
-            for i in (search_start..search_end).rev() {
-                if chars[i] == '\n' {
-                    split = i + 1;
+        if line_count >= lines_per_page && i > page_start {
+            let window_back = chars_per_line * lines_per_page / 3;
+            let search_start = page_start.max(i.saturating_sub(window_back));
+            let mut split = i;
+            for j in (search_start..i).rev() {
+                if j > 0 && chars[j] == '\n' && chars[j - 1] == '\n' {
+                    split = j + 1;
                     break;
                 }
             }
+            if split <= page_start {
+                split = i;
+            }
+            if split < total {
+                pages.push(split);
+                page_start = split;
+                line_count = 0;
+                line_pos = 0;
+                i = split;
+            }
         }
-
-        if split <= pos {
-            split = next;
-        }
-
-        pos = split.min(total_chars);
-        pages.push(pos);
     }
 
+    if *pages.last().unwrap() < total {
+        pages.push(total);
+    }
     pages
 }
 
@@ -454,38 +459,38 @@ mod tests {
 
     #[test]
     fn test_pagination_empty() {
-        let pages = calculate_pages("", 100);
+        let pages = calculate_pages("", 10, 5);
         assert_eq!(pages, vec![0]);
     }
 
     #[test]
     fn test_pagination_shorter_than_page() {
-        let pages = calculate_pages("Hello World", 100);
+        let pages = calculate_pages("Hello World", 10, 10);
         assert_eq!(pages, vec![0, 11]);
     }
 
     #[test]
     fn test_pagination_exact_fit() {
-        let pages = calculate_pages("ABCD", 4);
+        let pages = calculate_pages("ABCD", 2, 2);
         assert_eq!(pages, vec![0, 4]);
     }
 
     #[test]
     fn test_pagination_multiple_pages() {
         let text = "A".repeat(100);
-        let pages = calculate_pages(&text, 30);
+        let pages = calculate_pages(&text, 10, 3);
         assert_eq!(pages, vec![0, 30, 60, 90, 100]);
     }
 
     #[test]
     fn test_pagination_single_char() {
-        let pages = calculate_pages("A", 1);
+        let pages = calculate_pages("A", 10, 5);
         assert_eq!(pages, vec![0, 1]);
     }
 
     #[test]
     fn test_pagination_zero_chars_per_page() {
-        let pages = calculate_pages("test", 0);
+        let pages = calculate_pages("test", 0, 5);
         assert_eq!(pages, vec![0]);
     }
 }