Replace tabs with spaces

Fixes #16

Makefile

@@ -9,6 +9,7 @@ test:
 	cargo test
 	bash -c "diff -u <(git log -p | cut -c 2-) \
                      <(git log -p | delta --width variable \
+                                          --tabs 0 \
                                           --commit-style plain \
                                           --file-style plain \
                                           --hunk-style plain \

src/cli.rs

@@ -69,6 +69,13 @@ pub struct Opt {
     #[structopt(short = "w", long = "width")]
     pub width: Option<String>,
 
+    /// The number of spaces to replace tab characters with. Use --tabs=0 to pass tab characters
+    /// through directly, but note that in that case delta will calculate line widths assuming tabs
+    /// occupy one character's width on the screen: if your terminal renders tabs as more than than
+    /// one character wide then delta's output will look incorrect.
+    #[structopt(long = "tabs", default_value = "4")]
+    pub tab_width: usize,
+
     /// Show the command-line arguments for the current colors.
     #[structopt(long = "show-colors")]
     pub show_colors: bool,

src/config.rs

@@ -16,6 +16,7 @@ pub struct Config<'a> {
     pub syntax_set: &'a SyntaxSet,
     pub terminal_width: usize,
     pub width: Option<usize>,
+    pub tab_width: usize,
     pub pager: &'a str,
     pub opt: &'a cli::Opt,
     pub no_style: Style,
@@ -111,6 +112,7 @@ pub fn get_config<'a>(
         plus_emph_style_modifier,
         terminal_width,
         width,
+        tab_width: opt.tab_width,
         syntax_set,
         pager: "less",
         opt,

src/delta.rs

@@ -2,6 +2,8 @@ use std::io::Write;
 
 use ansi_term::Colour::{Blue, Yellow};
 use console::strip_ansi_codes;
+use itertools::Itertools;
+use unicode_segmentation::UnicodeSegmentation;
 
 use crate::bat::assets::HighlightingAssets;
 use crate::cli;
@@ -222,16 +224,16 @@ fn handle_hunk_line(painter: &mut Painter, line: &str, state: State, config: &Co
             if state == State::HunkPlus {
                 painter.paint_buffered_lines();
             }
-            painter.minus_lines.push(prepare(&line));
+            painter.minus_lines.push(prepare(&line, config.tab_width));
             State::HunkMinus
         }
         Some('+') => {
-            painter.plus_lines.push(prepare(&line));
+            painter.plus_lines.push(prepare(&line, config.tab_width));
             State::HunkPlus
         }
         _ => {
             painter.paint_buffered_lines();
-            let line = prepare(&line);
+            let line = prepare(&line, config.tab_width);
             let syntax_style_sections = Painter::get_line_syntax_style_sections(
                 &line,
                 &mut painter.highlighter,
@@ -251,13 +253,28 @@ fn handle_hunk_line(painter: &mut Painter, line: &str, state: State, config: &Co
     }
 }
 
-/// Replace initial -/+ character with ' ', and terminate with newline.
+/// Replace initial -/+ character with ' ', expand tabs as spaces, and terminate with newline.
 // Terminating with newline character is necessary for many of the sublime syntax definitions to
 // highlight correctly.
 // See https://docs.rs/syntect/3.2.0/syntect/parsing/struct.SyntaxSetBuilder.html#method.add_from_folder
-fn prepare(line: &str) -> String {
+fn prepare(line: &str, tab_width: usize) -> String {
     if !line.is_empty() {
-        format!(" {}\n", &line[1..])
+        let mut line = line.graphemes(true).peekable();
+        let mut left_fill = "".to_string();
+
+        // The first column contains a -/+/space character, added by git. Replace it with a space.
+        left_fill.push_str(" ");
+        line.next();
+
+        // Expand tabs as spaces
+        if tab_width > 0 {
+            // tab_width = 0 is documented to mean do not replace tabs.
+            let n_tabs = line.peeking_take_while(|c| *c == "\t").count();
+            if n_tabs > 0 {
+                left_fill.push_str(&" ".repeat(tab_width * n_tabs));
+            }
+        }
+        format!("{}{}\n", &left_fill, line.collect::<String>())
     } else {
         "\n".to_string()
     }