fix: optimize preview spawning

Don't show a preview when there are no lines selected, and don't respawn
a preview if the same line is selected between redraws.

src/Previewer.zig

@@ -1,5 +1,6 @@
 //! Manages child processes used for previewing information about the selected line
 
+const mem = std.mem;
 const process = std.process;
 const std = @import("std");
 
@@ -16,6 +17,8 @@ loop: *Loop,
 shell: []const u8,
 cmd_parts: [2][]const u8,
 
+current_arg: []const u8 = "",
+
 child: ?Child = null,
 stdout: ArrayList(u8),
 stderr: ArrayList(u8),
@@ -42,12 +45,21 @@ pub fn init(allocator: Allocator, loop: *Loop, cmd: []const u8, arg: []const u8)
     return previewer;
 }
 
-pub fn spawn(previewer: *Previewer, arg: []const u8) !void {
+pub fn reset(previewer: *Previewer) !void {
     previewer.stdout.clearRetainingCapacity();
     previewer.stderr.clearRetainingCapacity();
     if (previewer.child) |*child| {
         _ = try child.kill();
     }
+}
+
+pub fn spawn(previewer: *Previewer, arg: []const u8) !void {
+    // If the arg is already being previewed we don't need to do any work
+    if (mem.eql(u8, arg, previewer.current_arg)) {
+        return;
+    }
+
+    try previewer.reset();
 
     const command = try std.fmt.allocPrint(previewer.allocator, "{s}{s}{s} | expand -t4", .{ previewer.cmd_parts[0], arg, previewer.cmd_parts[1] });
 
@@ -58,8 +70,8 @@ pub fn spawn(previewer: *Previewer, arg: []const u8) !void {
     try child.spawn();
 
     previewer.loop.setChild(child.stdout.?.handle, child.stderr.?.handle);
-
     previewer.child = child;
+    previewer.current_arg = try previewer.allocator.dupe(u8, arg);
 }
 
 pub fn read(previewer: *Previewer, stream: enum { stdout, stderr }) !void {

src/ui.zig

@@ -424,12 +424,12 @@ pub fn run(
         }
 
         // The selection changed and the child process should be respawned
-        if (state.selection_changed and state.preview != null) {
+        if (state.selection_changed) if (state.preview) |*preview| {
             state.selection_changed = false;
             if (filtered.len > 0) {
-                try state.preview.?.spawn(filtered[state.selected + state.offset].str);
-            }
-        }
+                try preview.spawn(filtered[state.selected + state.offset].str);
+            } else try preview.reset();
+        };
 
         if (state.redraw) {
             state.redraw = false;