Files · Items: one icon map, file icon, file row and file grid

The crate had two maps from a file's name to its icon, in chat and in the
file tree, and they disagreed. One map now lives in primitives, over the
Lucide file-* icons, and chat, agent, the file tree and the directory
listing use it. FileIcon draws it for a file or a folder. FileItem is a
file's row, a ListItem with its size and when it changed. FileList is the
directory listing. FileGrid lays tiles in the order given, with a cursor
seeded from the owner's pick: arrows by tile and by row, Home, End, Enter
and a double press to open. The count of what fits a width has one home,
layout's columns_for, and fit builds on it.

src/layout/grid.rs

@@ -5,11 +5,18 @@ use gpui::{
 use smallvec::SmallVec;
 
 /// Columns for `width`, each at least `min`, `gap` apart.
-fn columns_for(width: Pixels, min: Pixels, gap: Pixels) -> u16 {
+pub(crate) fn columns_for(width: Pixels, min: Pixels, gap: Pixels) -> u16 {
     let fit = ((width + gap) / (min + gap)).floor();
     fit.clamp(1.0, f32::from(u16::MAX)) as u16
 }
 
+/// How many items `item` wide with `gap` between fit across `width`, at least one, and the inset that centers them.
+pub(crate) fn fit(width: Pixels, item: Pixels, gap: Pixels) -> (usize, Pixels) {
+    let count = columns_for(width, item, gap) as usize;
+    let used = (item + gap) * count as f32 - gap;
+    (count, ((width - used) / 2.0).max(Pixels::ZERO))
+}
+
 /// Index of the shortest column; ties go left.
 fn shortest(heights: &[Pixels]) -> usize {
     heights
@@ -174,7 +181,14 @@ impl RenderOnce for Masonry {
 mod tests {
     use gpui::px;
 
-    use super::{columns_for, shortest};
+    use super::{columns_for, fit, shortest};
+
+    #[test]
+    fn what_fits_sits_centered() {
+        assert_eq!(fit(px(100.0), px(3.0), px(2.0)), (20, px(1.0)));
+        assert_eq!(fit(px(98.0), px(3.0), px(2.0)), (20, px(0.0)));
+        assert_eq!(fit(px(2.0), px(3.0), px(2.0)), (1, px(0.0)));
+    }
 
     #[test]
     fn columns_fit_the_width() {

src/layout/mod.rs

@@ -19,6 +19,7 @@ pub use collapse::{Accordion, AccordionItem, Collapsible};
 pub use dock::{Dock, DockLayout, DockPanel, DockSide, Spot};
 pub use floating::FloatingPanel;
 pub use grid::{Masonry, SimpleGrid};
+pub(crate) use grid::{columns_for, fit};
 pub use page::{AppShell, MasterDetail, Page};
 pub use panes::{PaneGroup, PaneId, PaneLayout};
 pub(crate) use scroll::bring_into_view;