Separate the scroll state

src/list/scroll_state.rs

@@ -0,0 +1,104 @@
+pub struct ScrollState {
+    n_rows: usize,
+    max_n_rows_to_display: usize,
+    selected: Option<usize>,
+    offset: usize,
+    scroll_padding: usize,
+    max_scroll_padding: usize,
+}
+
+impl ScrollState {
+    pub fn new(n_rows: usize, selected: Option<usize>, max_scroll_padding: usize) -> Self {
+        Self {
+            n_rows,
+            max_n_rows_to_display: 0,
+            selected,
+            offset: selected.map_or(0, |selected| selected.saturating_sub(max_scroll_padding)),
+            scroll_padding: 0,
+            max_scroll_padding,
+        }
+    }
+
+    #[inline]
+    pub fn offset(&self) -> usize {
+        self.offset
+    }
+
+    fn update_offset(&mut self) {
+        let Some(selected) = self.selected else {
+            return;
+        };
+
+        let min_offset = (selected + self.scroll_padding)
+            .saturating_sub(self.max_n_rows_to_display.saturating_sub(1));
+        let max_offset = selected.saturating_sub(self.scroll_padding);
+        let global_max_offset = self.n_rows.saturating_sub(self.max_n_rows_to_display);
+
+        self.offset = self
+            .offset
+            .max(min_offset)
+            .min(max_offset)
+            .min(global_max_offset);
+    }
+
+    #[inline]
+    pub fn selected(&self) -> Option<usize> {
+        self.selected
+    }
+
+    fn set_selected(&mut self, selected: usize) {
+        self.selected = Some(selected);
+        self.update_offset();
+    }
+
+    pub fn select_next(&mut self) {
+        if let Some(selected) = self.selected {
+            self.set_selected((selected + 1).min(self.n_rows - 1));
+        }
+    }
+
+    pub fn select_previous(&mut self) {
+        if let Some(selected) = self.selected {
+            self.set_selected(selected.saturating_sub(1));
+        }
+    }
+
+    pub fn select_first(&mut self) {
+        if self.n_rows > 0 {
+            self.set_selected(0);
+        }
+    }
+
+    pub fn select_last(&mut self) {
+        if self.n_rows > 0 {
+            self.set_selected(self.n_rows - 1);
+        }
+    }
+
+    pub fn set_n_rows(&mut self, n_rows: usize) {
+        self.n_rows = n_rows;
+
+        if self.n_rows == 0 {
+            self.selected = None;
+            return;
+        }
+
+        self.set_selected(self.selected.map_or(0, |selected| selected.min(n_rows - 1)));
+    }
+
+    #[inline]
+    fn update_scroll_padding(&mut self) {
+        self.scroll_padding = (self.max_n_rows_to_display / 4).min(self.max_scroll_padding);
+    }
+
+    #[inline]
+    pub fn max_n_rows_to_display(&self) -> usize {
+        self.max_n_rows_to_display
+    }
+
+    pub fn set_max_n_rows_to_display(&mut self, max_n_rows_to_display: usize) {
+        self.max_n_rows_to_display = max_n_rows_to_display;
+        self.update_scroll_padding();
+        self.update_offset();
+    }
+}