Don't color each path component separately

It can be expensive to color each path component separately, requiring a
stat() call on each component.  For deep hierarchies this can result in
quadratic overhead.  Instead, just color the path up to the basename as
a directory.

Fixes #720.

src/output.rs

@@ -1,8 +1,9 @@
+use std::borrow::Cow;
 use std::io::{self, StdoutLock, Write};
 use std::path::Path;
 use std::process;
 
-use lscolors::{LsColors, Style};
+use lscolors::{Indicator, LsColors, Style};
 
 use crate::error::print_error;
 use crate::exit_codes::ExitCode;
@@ -45,21 +46,40 @@ fn print_entry_colorized(
     config: &Options,
     ls_colors: &LsColors,
 ) -> io::Result<()> {
-    let default_style = ansi_term::Style::default();
+    // Split the path between the parent and the last component
+    let mut offset = 0;
+    let path_str = path.to_string_lossy();
 
-    // Traverse the path and colorize each component
-    for (component, style) in ls_colors.style_for_path_components(path) {
-        let style = style
-            .map(Style::to_ansi_term_style)
-            .unwrap_or(default_style);
+    if let Some(parent) = path.parent() {
+        offset = parent.to_string_lossy().len();
+        for c in path_str[offset..].chars() {
+            if std::path::is_separator(c) {
+                offset += c.len_utf8();
+            } else {
+                break;
+            }
+        }
+    }
 
-        let mut path_string = component.to_string_lossy();
+    if offset > 0 {
+        let mut parent_str = Cow::from(&path_str[..offset]);
         if let Some(ref separator) = config.path_separator {
-            *path_string.to_mut() = replace_path_separator(&path_string, separator);
+            *parent_str.to_mut() = replace_path_separator(&parent_str, separator);
         }
-        write!(stdout, "{}", style.paint(path_string))?;
+
+        let style = ls_colors
+            .style_for_indicator(Indicator::Directory)
+            .map(Style::to_ansi_term_style)
+            .unwrap_or_default();
+        write!(stdout, "{}", style.paint(parent_str))?;
     }
 
+    let style = ls_colors
+        .style_for_path(path)
+        .map(Style::to_ansi_term_style)
+        .unwrap_or_default();
+    write!(stdout, "{}", style.paint(&path_str[offset..]))?;
+
     if config.null_separator {
         write!(stdout, "\0")
     } else {