Add backwards fill

src/widgets/list.rs

@@ -1,3 +1,5 @@
+use std::collections::BTreeSet;
+
 use crate::{
     buffer::Buffer,
     layout::{Corner, Rect},
@@ -10,28 +12,38 @@ use unicode_width::UnicodeWidthStr;
 #[derive(Debug, Clone)]
 pub struct ListState {
     offset: usize,
-    selected: Option<usize>,
+    selected: BTreeSet<usize>,
 }
 
 impl Default for ListState {
     fn default() -> ListState {
         ListState {
             offset: 0,
-            selected: None,
+            selected: BTreeSet::new(),
         }
     }
 }
 
 impl ListState {
-    pub fn selected(&self) -> Option<usize> {
-        self.selected
+    pub fn offset(mut self, offset: usize) -> Self {
+        self.offset = offset;
+        self
+    }
+    
+    pub fn selected(&self) -> &BTreeSet<usize> {
+        &self.selected
     }
 
-    pub fn select(&mut self, index: Option<usize>) {
-        self.selected = index;
-        if index.is_none() {
-            self.offset = 0;
-        }
+    pub fn select(&mut self, index: usize) {
+        self.selected.insert(index);
+    }
+
+    pub fn deselect(&mut self, index: usize) {
+        self.selected.remove(&index);
+    }
+
+    pub fn deselect_all(&mut self) {
+        self.selected.clear();
     }
 }
 
@@ -131,7 +143,6 @@ impl<'a> List<'a> {
 
     fn get_items_bounds(
         &self,
-        selected: Option<usize>,
         offset: usize,
         max_height: usize,
     ) -> (usize, usize) {
@@ -139,6 +150,9 @@ impl<'a> List<'a> {
         let mut start = offset;
         let mut end = offset;
         let mut height = 0;
+
+        // Fills forwards until max_height is reached
+        // or no more items are left.
         for item in self.items.iter().skip(offset) {
             if height + item.height() > max_height {
                 break;
@@ -146,24 +160,17 @@ impl<'a> List<'a> {
             height += item.height();
             end += 1;
         }
-
-        let selected = selected.unwrap_or(0).min(self.items.len() - 1);
-        while selected >= end {
-            height = height.saturating_add(self.items[end].height());
-            end += 1;
-            while height > max_height {
-                height = height.saturating_sub(self.items[start].height());
-                start += 1;
+        
+        // Fills backwards, if offset is too large
+        // and we have insufficient items.
+        for item in self.items[..start].iter().rev() {
+            if height + item.height() > max_height {
+                break;
             }
-        }
-        while selected < start {
+            height += item.height();
             start -= 1;
-            height = height.saturating_add(self.items[start].height());
-            while height > max_height {
-                end -= 1;
-                height = height.saturating_sub(self.items[end].height());
-            }
         }
+
         (start, end)
     }
 }
@@ -191,19 +198,18 @@ impl<'a> StatefulWidget for List<'a> {
         }
         let list_height = list_area.height as usize;
 
-        let (start, end) = self.get_items_bounds(state.selected, state.offset, list_height);
-        state.offset = start;
+        let (start, end) = self.get_items_bounds(state.offset, list_height);
 
         let highlight_symbol = self.highlight_symbol.unwrap_or("");
         let blank_symbol = " ".repeat(highlight_symbol.width());
 
         let mut current_height = 0;
-        let has_selection = state.selected.is_some();
+        let has_selection = !state.selected.is_empty();
         for (i, item) in self
             .items
             .iter_mut()
             .enumerate()
-            .skip(state.offset)
+            .skip(start)
             .take(end - start)
         {
             let (x, y) = match self.start_corner {
@@ -226,7 +232,7 @@ impl<'a> StatefulWidget for List<'a> {
             let item_style = self.style.patch(item.style);
             buf.set_style(area, item_style);
 
-            let is_selected = state.selected.map(|s| s == i).unwrap_or(false);
+            let is_selected = state.selected.contains(&i);
             let elem_x = if has_selection {
                 let symbol = if is_selected {
                     highlight_symbol