printer: add support for line number alignment

Closes #544

src/app.rs

@@ -102,6 +102,9 @@ pub fn app() -> App<'static, 'static> {
              .value_name("GLOB"))
         .arg(flag("ignore-case").short("i"))
         .arg(flag("line-number").short("n"))
+        .arg(flag("line-number-width")
+             .value_name("NUM").takes_value(true)
+             .validator(validate_line_number_width))
         .arg(flag("no-line-number").short("N").overrides_with("line-number"))
         .arg(flag("quiet").short("q"))
         .arg(flag("type").short("t")
@@ -318,6 +321,11 @@ lazy_static! {
              "Show line numbers.",
              "Show line numbers (1-based). This is enabled by default when \
               searching in a tty.");
+        doc!(h, "line-number-width",
+             "Left pad line numbers upto NUM width.",
+             "Left pad line numbers upto NUM width. Space is used as \
+              the default padding character. This has no effect if \
+              --no-line-number is enabled.");
         doc!(h, "no-line-number",
              "Suppress line numbers.",
              "Suppress line numbers. This is enabled by default when NOT \
@@ -575,6 +583,15 @@ lazy_static! {
     };
 }
 
+fn validate_line_number_width(s: String) -> Result<(), String> {
+    if s.starts_with("0") {
+        Err(String::from("Custom padding characters are currently not supported. \
+        Please enter only a numeric value."))
+    } else {
+        validate_number(s)
+    }
+}
+
 fn validate_number(s: String) -> Result<(), String> {
     s.parse::<usize>().map(|_|()).map_err(|err| err.to_string())
 }

src/args.rs

@@ -54,6 +54,7 @@ pub struct Args {
     invert_match: bool,
     line_number: bool,
     line_per_match: bool,
+    line_number_width: Option<usize>,
     max_columns: Option<usize>,
     max_count: Option<u64>,
     max_filesize: Option<u64>,
@@ -144,7 +145,8 @@ impl Args {
             .only_matching(self.only_matching)
             .path_separator(self.path_separator)
             .with_filename(self.with_filename)
-            .max_columns(self.max_columns);
+            .max_columns(self.max_columns)
+            .line_number_width(self.line_number_width);
         if let Some(ref rep) = self.replace {
             p = p.replace(rep.clone());
         }
@@ -336,6 +338,7 @@ impl<'a> ArgMatches<'a> {
             ignore_files: self.ignore_files(),
             invert_match: self.is_present("invert-match"),
             line_number: line_number,
+            line_number_width: try!(self.usize_of("line-number-width")),
             line_per_match: self.is_present("vimgrep"),
             max_columns: try!(self.usize_of("max-columns")),
             max_count: try!(self.usize_of("max-count")).map(|max| max as u64),

src/printer.rs

@@ -98,7 +98,11 @@ pub struct Printer<W> {
     /// The separator to use for file paths. If empty, this is ignored.
     path_separator: Option<u8>,
     /// Restrict lines to this many columns.
-    max_columns: Option<usize>
+    max_columns: Option<usize>,
+    /// Width of line number displayed. If the number of digits in the
+    /// line number is less than this, it is left padded with
+    /// spaces.
+    line_number_width: Option<usize>
 }
 
 impl<W: WriteColor> Printer<W> {
@@ -120,6 +124,7 @@ impl<W: WriteColor> Printer<W> {
             colors: ColorSpecs::default(),
             path_separator: None,
             max_columns: None,
+            line_number_width: None
         }
     }
 
@@ -208,6 +213,12 @@ impl<W: WriteColor> Printer<W> {
         self
     }
 
+    /// Configure the width of the displayed line number
+    pub fn line_number_width(mut self, line_number_width: Option<usize>) -> Printer<W> {
+        self.line_number_width = line_number_width;
+        self
+    }
+
     /// Returns true if and only if something has been printed.
     pub fn has_printed(&self) -> bool {
         self.has_printed
@@ -457,7 +468,11 @@ impl<W: WriteColor> Printer<W> {
     }
 
     fn line_number(&mut self, n: u64, sep: u8) {
-        self.write_colored(n.to_string().as_bytes(), |colors| colors.line());
+        let mut line_number = n.to_string();
+        if let Some(width) = self.line_number_width {
+            line_number = format!("{:>width$}", line_number, width = width);
+        }
+        self.write_colored(line_number.as_bytes(), |colors| colors.line());
         self.separator(&[sep]);
     }