commit 32b79c1a635817f190f3125a813971b966726c51
Author: upstream <upstream@localhost>
Date:   Wed Sep 2 11:06:59 2026 +0200

    Upstream Wilfred/difftastic state of the touched files before 519f6a9c21

diff --git a/src/display/side_by_side.rs b/src/display/side_by_side.rs
new file mode 100644
index 0000000..e1ba2c2
--- /dev/null
+++ b/src/display/side_by_side.rs
@@ -0,0 +1,916 @@
+//! Side-by-side (two column) display of diffs.
+
+use std::cmp::{max, min};
+
+use line_numbers::{LineNumber, SingleLineSpan};
+use owo_colors::{OwoColorize, Style};
+
+use crate::constants::Side;
+use crate::display::context::all_matched_lines_filled;
+use crate::display::hunks::{matched_lines_indexes_for_hunk, Hunk};
+use crate::display::style::{
+    self, apply_colors, apply_line_number_color, color_positions, novel_style, replace_tabs,
+    split_and_apply, width_respecting_tabs, BackgroundColor,
+};
+use crate::hash::{DftHashMap, DftHashSet};
+use crate::lines::{format_line_num, format_line_num_padded, split_on_newlines};
+use crate::options::{DisplayMode, DisplayOptions};
+use crate::parse::syntax::{zip_pad_shorter, MatchedPos};
+use crate::summary::FileFormat;
+
+/// The space shown between LHS and RHS columns.
+const SPACER: &str = "  ";
+
+fn format_missing_line_num(
+    prev_num: LineNumber,
+    source_dims: &SourceDimensions,
+    side: Side,
+    is_continuation: bool,
+    use_color: bool,
+) -> String {
+    let column_width = match side {
+        Side::Left => source_dims.lhs_line_nums_width,
+        Side::Right => source_dims.rhs_line_nums_width,
+    };
+
+    let after_end = match side {
+        Side::Left => prev_num >= source_dims.lhs_max_line_in_file,
+        Side::Right => prev_num >= source_dims.rhs_max_line_in_file,
+    };
+
+    let mut style = Style::new();
+    if use_color {
+        style = style.dimmed();
+    }
+
+    let c = if is_continuation {
+        // Always show dots when this line is too long so we had to
+        // split it over several lines when displaying.
+        "."
+    } else if after_end {
+        // The file on this side has fewer lines, and we've already
+        // shown the last ones.
+        " "
+    } else {
+        // There are more lines in this file.
+        "."
+    };
+
+    let num_digits = prev_num.display().len();
+    format!(
+        "{:>width$} ",
+        c.repeat(num_digits),
+        width = column_width - 1
+    )
+    .style(style)
+    .to_string()
+}
+
+/// Display `src` in a single column (e.g. a file removal or addition).
+fn display_single_column(
+    display_path: &str,
+    old_path: Option<&String>,
+    file_format: &FileFormat,
+    src_lines: &[String],
+    side: Side,
+    display_options: &DisplayOptions,
+) -> Vec<String> {
+    let column_width = format_line_num((src_lines.len() as u32).into()).len();
+
+    let mut formatted_lines = Vec::with_capacity(src_lines.len());
+
+    let mut header_line = String::new();
+    header_line.push_str(&style::header(
+        display_path,
+        old_path,
+        1,
+        1,
+        file_format,
+        display_options,
+    ));
+    header_line.push('\n');
+    formatted_lines.push(header_line);
+
+    let mut style = Style::new();
+    if display_options.use_color {
+        style = novel_style(Style::new(), side, display_options.background_color);
+    }
+
+    for (i, line) in src_lines.iter().enumerate() {
+        let mut formatted_line = String::with_capacity(line.len());
+        formatted_line.push_str(
+            &format_line_num_padded((i as u32).into(), column_width)
+                .style(style)
+                .to_string(),
+        );
+        formatted_line.push_str(line);
+        formatted_lines.push(formatted_line);
+    }
+
+    formatted_lines
+}
+
+fn display_line_nums(
+    lhs_line_num: Option<LineNumber>,
+    rhs_line_num: Option<LineNumber>,
+    source_dims: &SourceDimensions,
+    display_options: &DisplayOptions,
+    lhs_has_novel: bool,
+    rhs_has_novel: bool,
+    prev_lhs_line_num: Option<LineNumber>,
+    prev_rhs_line_num: Option<LineNumber>,
+) -> (String, String) {
+    let display_lhs_line_num: String = match lhs_line_num {
+        Some(line_num) => {
+            let s = format_line_num_padded(line_num, source_dims.lhs_line_nums_width);
+            apply_line_number_color(&s, lhs_has_novel, Side::Left, display_options)
+        }
+        None => format_missing_line_num(
+            prev_lhs_line_num.unwrap_or_else(|| 1.into()),
+            source_dims,
+            Side::Left,
+            false,
+            display_options.use_color,
+        ),
+    };
+    let display_rhs_line_num: String = match rhs_line_num {
+        Some(line_num) => {
+            let s = format_line_num_padded(line_num, source_dims.rhs_line_nums_width);
+            apply_line_number_color(&s, rhs_has_novel, Side::Right, display_options)
+        }
+        None => format_missing_line_num(
+            prev_rhs_line_num.unwrap_or_else(|| 1.into()),
+            source_dims,
+            Side::Right,
+            false,
+            display_options.use_color,
+        ),
+    };
+
+    (display_lhs_line_num, display_rhs_line_num)
+}
+
+// Sizes used when displaying a hunk.
+#[derive(Debug)]
+struct SourceDimensions {
+    /// The number of characters used to display LHS source lines. Any
+    /// line that exceeds this length will be wrapped.
+    lhs_content_display_width: usize,
+    /// The number of characters used to display RHS source lines. Any
+    /// line that exceeds this length will be wrapped.
+    rhs_content_display_width: usize,
+    /// The number of characters required to display line numbers on
+    /// the LHS.
+    lhs_line_nums_width: usize,
+    /// The number of characters required to display line numbers on
+    /// the RHS.
+    rhs_line_nums_width: usize,
+    lhs_max_line_in_file: LineNumber,
+    rhs_max_line_in_file: LineNumber,
+}
+
+impl SourceDimensions {
+    fn new(
+        terminal_width: usize,
+        lhs_max_line_visible: LineNumber,
+        rhs_max_line_visible: LineNumber,
+        lhs_max_line_in_file: LineNumber,
+        rhs_max_line_in_file: LineNumber,
+        lhs_content_max_width: usize,
+        rhs_content_max_width: usize,
+    ) -> Self {
+        let lhs_line_nums_width = format_line_num(lhs_max_line_visible).len();
+        let rhs_line_nums_width = format_line_num(rhs_max_line_visible).len();
+
+        let content_max_width = max(lhs_content_max_width, rhs_content_max_width);
+
+        // If the file lines are extremely short, treat them as if
+        // they have a line of 25 characters.
+        let content_max_width = max(content_max_width, 25);
+
+        // If the terminal is very wide, we don't want to use the full
+        // 50% for the LHS column, we end up with too much space
+        // between LHS and RHS.
+        //
+        // Instead, cap the display width based on the maximum length
+        // of visible lines within the file.
+        //
+        // This naively assumes that byte length is the same as
+        // display length, which is generally OK because byte length
+        // will tend to be larger than the display length. Tab
+        // characters are an exception, where display width exceeds
+        // byte length, so we account for tab expansion explicitly.
+        let width_without_truncation = lhs_line_nums_width
+            + content_max_width
+            + SPACER.len()
+            + rhs_line_nums_width
+            + content_max_width;
+        let display_width = min(terminal_width, width_without_truncation);
+
+        assert!(
+            display_width > SPACER.len(),
+            "Terminal total width should not overflow"
+        );
+        let lhs_total_width = (display_width - SPACER.len()) / 2;
+
+        let lhs_content_width = if lhs_line_nums_width < lhs_total_width {
+            lhs_total_width - lhs_line_nums_width
+        } else {
+            // The terminal is so narrow that even the column numbers
+            // display doesn't fit. Ensure we show a non-zero number
+            // of content columns anyway.
+            1
+        };
+
+        let rhs_content_width = max(
+            1,
+            display_width as isize
+                - lhs_total_width as isize
+                - SPACER.len() as isize
+                - rhs_line_nums_width as isize,
+        ) as usize;
+
+        // We want the content width to be the same on both
+        // sides. This ensures that line wrapping splits lines at the
+        // same point on both sides.
+        let content_width = min(lhs_content_width, rhs_content_width);
+
+        Self {
+            lhs_content_display_width: content_width,
+            rhs_content_display_width: content_width,
+            lhs_line_nums_width,
+            rhs_line_nums_width,
+            lhs_max_line_in_file,
+            rhs_max_line_in_file,
+        }
+    }
+}
+
+pub(crate) fn lines_with_novel(
+    lhs_mps: &[MatchedPos],
+    rhs_mps: &[MatchedPos],
+) -> (DftHashSet<LineNumber>, DftHashSet<LineNumber>) {
+    let lhs_lines_with_novel: DftHashSet<LineNumber> = lhs_mps
+        .iter()
+        .filter(|mp| mp.kind.is_novel())
+        .map(|mp| mp.pos.line)
+        .collect();
+    let rhs_lines_with_novel: DftHashSet<LineNumber> = rhs_mps
+        .iter()
+        .filter(|mp| mp.kind.is_novel())
+        .map(|mp| mp.pos.line)
+        .collect();
+
+    (lhs_lines_with_novel, rhs_lines_with_novel)
+}
+
+/// Calculate positions of highlights on both sides. This includes
+/// both syntax highlighting and added/removed content highlighting.
+fn highlight_positions(
+    background: BackgroundColor,
+    syntax_highlight: bool,
+    file_format: &FileFormat,
+    lhs_mps: &[MatchedPos],
+    rhs_mps: &[MatchedPos],
+) -> (
+    DftHashMap<LineNumber, Vec<(SingleLineSpan, Style)>>,
+    DftHashMap<LineNumber, Vec<(SingleLineSpan, Style)>>,
+) {
+    let lhs_positions = color_positions(
+        Side::Left,
+        background,
+        syntax_highlight,
+        file_format,
+        lhs_mps,
+    );
+    let mut lhs_styles: DftHashMap<LineNumber, Vec<(SingleLineSpan, Style)>> =
+        DftHashMap::default();
+    for (span, style) in lhs_positions {
+        let styles = lhs_styles.entry(span.line).or_insert_with(Vec::new);
+        styles.push((span, style));
+    }
+
+    let rhs_positions = color_positions(
+        Side::Right,
+        background,
+        syntax_highlight,
+        file_format,
+        rhs_mps,
+    );
+    let mut rhs_styles: DftHashMap<LineNumber, Vec<(SingleLineSpan, Style)>> =
+        DftHashMap::default();
+    for (span, style) in rhs_positions {
+        let styles = rhs_styles.entry(span.line).or_insert_with(Vec::new);
+        styles.push((span, style));
+    }
+
+    (lhs_styles, rhs_styles)
+}
+
+fn highlight_as_novel(
+    line_num: Option<LineNumber>,
+    lines: &[&str],
+    opposite_line_num: Option<LineNumber>,
+    lines_with_novel: &DftHashSet<LineNumber>,
+) -> bool {
+    if let Some(line_num) = line_num {
+        // If this line contains any novel tokens, highlight it.
+        if lines_with_novel.contains(&line_num) {
+            return true;
+        }
+
+        let line_content = lines.get(line_num.as_usize()).map(|s| str::trim(s));
+        // If this is a blank line without a corresponding line on the
+        // other side, highlight it too. This helps highlight novel
+        // blank lines.
+        if line_content == Some("") && opposite_line_num.is_none() {
+            return true;
+        }
+    }
+
+    false
+}
+
+/// Find the longest line in `lhs_src` and `rhs_src` that will be
+/// displayed. Return the length of that line for both LHS and RHS.
+fn visible_content_max_display_width(
+    lhs_src: &str,
+    rhs_src: &str,
+    hunks: &[Hunk],
+    num_context_lines: u32,
+    tab_width: usize,
+) -> (usize, usize) {
+    let mut lhs_displayed_lines: DftHashSet<usize> = DftHashSet::default();
+    let mut rhs_displayed_lines: DftHashSet<usize> = DftHashSet::default();
+
+    for hunk in hunks {
+        let mut min_lhs_line: Option<LineNumber> = None;
+        let mut max_lhs_line: Option<LineNumber> = None;
+        let mut min_rhs_line: Option<LineNumber> = None;
+        let mut max_rhs_line: Option<LineNumber> = None;
+
+        for (lhs_line, rhs_line) in &hunk.lines {
+            if let Some(lhs_line) = lhs_line {
+                if let Some(current_min) = min_lhs_line {
+                    min_lhs_line = Some(min(current_min, *lhs_line));
+                } else {
+                    min_lhs_line = Some(*lhs_line);
+                }
+
+                if let Some(current_max) = max_lhs_line {
+                    max_lhs_line = Some(max(current_max, *lhs_line));
+                } else {
+                    max_lhs_line = Some(*lhs_line);
+                }
+            }
+
+            if let Some(rhs_line) = rhs_line {
+                if let Some(current_min) = min_rhs_line {
+                    min_rhs_line = Some(min(current_min, *rhs_line));
+                } else {
+                    min_rhs_line = Some(*rhs_line);
+                }
+
+                if let Some(current_max) = max_rhs_line {
+                    max_rhs_line = Some(max(current_max, *rhs_line));
+                } else {
+                    max_rhs_line = Some(*rhs_line);
+                }
+            }
+
+            if let (Some(min_lhs_line), Some(max_lhs_line)) = (min_lhs_line, max_lhs_line) {
+                let min_lhs_plus_padding =
+                    max(0, min_lhs_line.0 as isize - num_context_lines as isize) as usize;
+                let max_lhs_plus_padding = max_lhs_line.0 as usize + num_context_lines as usize;
+                for lhs_line_num in min_lhs_plus_padding..=max_lhs_plus_padding {
+                    lhs_displayed_lines.insert(lhs_line_num);
+                }
+            }
+
+            if let (Some(min_rhs_line), Some(max_rhs_line)) = (min_rhs_line, max_rhs_line) {
+                let min_rhs_plus_padding =
+                    max(0, min_rhs_line.0 as isize - num_context_lines as isize) as usize;
+                let max_rhs_plus_padding = max_rhs_line.0 as usize + num_context_lines as usize;
+                for rhs_line_num in min_rhs_plus_padding..=max_rhs_plus_padding {
+                    rhs_displayed_lines.insert(rhs_line_num);
+                }
+            }
+        }
+    }
+
+    let mut lhs_content_max_width: usize = 0;
+    let mut rhs_content_max_width: usize = 0;
+
+    for (lhs_i, lhs_line) in lhs_src.lines().enumerate() {
+        if lhs_displayed_lines.contains(&lhs_i) {
+            lhs_content_max_width = max(
+                lhs_content_max_width,
+                width_respecting_tabs(lhs_line, tab_width),
+            );
+        }
+    }
+    for (rhs_i, rhs_line) in rhs_src.lines().enumerate() {
+        if rhs_displayed_lines.contains(&rhs_i) {
+            rhs_content_max_width = max(
+                rhs_content_max_width,
+                width_respecting_tabs(rhs_line, tab_width),
+            );
+        }
+    }
+
+    (lhs_content_max_width, rhs_content_max_width)
+}
+
+pub(crate) fn print(
+    hunks: &[Hunk],
+    display_options: &DisplayOptions,
+    display_path: &str,
+    old_path: Option<&String>,
+    file_format: &FileFormat,
+    lhs_src: &str,
+    rhs_src: &str,
+    lhs_mps: &[MatchedPos],
+    rhs_mps: &[MatchedPos],
+) {
+    let (lhs_content_max_width, rhs_content_max_width) = visible_content_max_display_width(
+        lhs_src,
+        rhs_src,
+        hunks,
+        display_options.num_context_lines,
+        display_options.tab_width,
+    );
+
+    let (lhs_colored_lines, rhs_colored_lines) = if display_options.use_color {
+        (
+            apply_colors(
+                lhs_src,
+                Side::Left,
+                display_options.syntax_highlight,
+                file_format,
+                display_options.background_color,
+                lhs_mps,
+            ),
+            apply_colors(
+                rhs_src,
+                Side::Right,
+                display_options.syntax_highlight,
+                file_format,
+                display_options.background_color,
+                rhs_mps,
+            ),
+        )
+    } else {
+        (
+            split_on_newlines(lhs_src)
+                .map(|s| format!("{}\n", s))
+                .collect(),
+            split_on_newlines(rhs_src)
+                .map(|s| format!("{}\n", s))
+                .collect(),
+        )
+    };
+
+    // Style positions are relative to the source code offsets. Now
+    // that we've applied styling, we can replace tabs.
+    let lhs_colored_lines: Vec<_> = lhs_colored_lines
+        .iter()
+        .map(|l| replace_tabs(l, display_options.tab_width))
+        .collect();
+    let rhs_colored_lines: Vec<_> = rhs_colored_lines
+        .iter()
+        .map(|l| replace_tabs(l, display_options.tab_width))
+        .collect();
+
+    if lhs_src.is_empty()
+        && !matches!(
+            display_options.display_mode,
+            DisplayMode::SideBySideShowBoth
+        )
+    {
+        for line in display_single_column(
+            display_path,
+            old_path,
+            file_format,
+            &rhs_colored_lines,
+            Side::Right,
+            display_options,
+        ) {
+            print!("{}", line);
+        }
+        println!();
+        return;
+    }
+    if rhs_src.is_empty()
+        && !matches!(
+            display_options.display_mode,
+            DisplayMode::SideBySideShowBoth
+        )
+    {
+        for line in display_single_column(
+            display_path,
+            old_path,
+            file_format,
+            &lhs_colored_lines,
+            Side::Left,
+            display_options,
+        ) {
+            print!("{}", line);
+        }
+        println!();
+        return;
+    }
+
+    // TODO: this is largely duplicating the `apply_colors` logic.
+    let (lhs_highlights, rhs_highlights) = if display_options.use_color {
+        highlight_positions(
+            display_options.background_color,
+            display_options.syntax_highlight,
+            file_format,
+            lhs_mps,
+            rhs_mps,
+        )
+    } else {
+        (DftHashMap::default(), DftHashMap::default())
+    };
+
+    let (lhs_lines_with_novel, rhs_lines_with_novel) = lines_with_novel(lhs_mps, rhs_mps);
+
+    let mut prev_lhs_line_num = None;
+    let mut prev_rhs_line_num = None;
+
+    let mut lhs_lines = split_on_newlines(lhs_src).collect::<Vec<_>>();
+    let mut rhs_lines = split_on_newlines(rhs_src).collect::<Vec<_>>();
+
+    if lhs_lines.last() == Some(&"") && lhs_lines.len() > 1 {
+        lhs_lines.pop();
+    }
+    if rhs_lines.last() == Some(&"") && rhs_lines.len() > 1 {
+        rhs_lines.pop();
+    }
+
+    let matched_lines = all_matched_lines_filled(lhs_mps, rhs_mps, &lhs_lines, &rhs_lines);
+    let mut matched_lines_to_print = &matched_lines[..];
+
+    let mut lhs_max_visible_line = 1.into();
+    let mut rhs_max_visible_line = 1.into();
+
+    if let Some(hunk) = hunks.last() {
+        let (start_i, end_i) = matched_lines_indexes_for_hunk(
+            matched_lines_to_print,
+            hunk,
+            display_options.num_context_lines as usize,
+        );
+        let aligned_lines = &matched_lines_to_print[start_i..end_i];
+
+        for (lhs_line_num, rhs_line_num) in aligned_lines.iter().rev() {
+            if let Some(lhs_line_num) = *lhs_line_num {
+                lhs_max_visible_line = max(lhs_max_visible_line, lhs_line_num);
+            }
+            if let Some(rhs_line_num) = *rhs_line_num {
+                rhs_max_visible_line = max(rhs_max_visible_line, rhs_line_num);
+            }
+
+            if lhs_max_visible_line > 1.into() && rhs_max_visible_line > 1.into() {
+                break;
+            }
+        }
+    }
+
+    let lhs_max_line_in_file = LineNumber(lhs_lines.len().saturating_sub(1) as u32);
+    let rhs_max_line_in_file = LineNumber(rhs_lines.len().saturating_sub(1) as u32);
+
+    lhs_max_visible_line = LineNumber(min(
+        lhs_max_visible_line.0 + display_options.num_context_lines,
+        lhs_max_line_in_file.0,
+    ));
+    rhs_max_visible_line = LineNumber(min(
+        rhs_max_visible_line.0 + display_options.num_context_lines,
+        rhs_max_line_in_file.0,
+    ));
+
+    let source_dims = SourceDimensions::new(
+        display_options.terminal_width,
+        lhs_max_visible_line,
+        rhs_max_visible_line,
+        lhs_max_line_in_file,
+        rhs_max_line_in_file,
+        lhs_content_max_width,
+        rhs_content_max_width,
+    );
+
+    for (i, hunk) in hunks.iter().enumerate() {
+        println!(
+            "{}",
+            style::header(
+                display_path,
+                old_path,
+                i + 1,
+                hunks.len(),
+                file_format,
+                display_options
+            )
+        );
+
+        let (start_i, end_i) = matched_lines_indexes_for_hunk(
+            matched_lines_to_print,
+            hunk,
+            display_options.num_context_lines as usize,
+        );
+        let aligned_lines = &matched_lines_to_print[start_i..end_i];
+        // We iterate through hunks in order, so we know the next hunk
+        // must appear after start_i. This makes
+        // `matched_lines_indexes_for_hunk` faster on later
+        // iterations, and this function is hot on large textual
+        // diffs.
+        matched_lines_to_print = &matched_lines_to_print[start_i..];
+
+        let no_lhs_changes = hunk.novel_lhs.is_empty();
+        let no_rhs_changes = hunk.novel_rhs.is_empty();
+        let same_lines = aligned_lines.iter().all(|(l, r)| l == r);
+
+        for (lhs_line_num, rhs_line_num) in aligned_lines {
+            let lhs_line_novel = highlight_as_novel(
+                *lhs_line_num,
+                &lhs_lines,
+                *rhs_line_num,
+                &lhs_lines_with_novel,
+            );
+            let rhs_line_novel = highlight_as_novel(
+                *rhs_line_num,
+                &rhs_lines,
+                *lhs_line_num,
+                &rhs_lines_with_novel,
+            );
+
+            let (display_lhs_line_num, display_rhs_line_num) = display_line_nums(
+                *lhs_line_num,
+                *rhs_line_num,
+                &source_dims,
+                display_options,
+                lhs_line_novel,
+                rhs_line_novel,
+                prev_lhs_line_num,
+                prev_rhs_line_num,
+            );
+
+            let show_both = matches!(
+                display_options.display_mode,
+                DisplayMode::SideBySideShowBoth
+            );
+            if no_lhs_changes && !show_both {
+                match rhs_line_num {
+                    Some(rhs_line_num) => {
+                        let rhs_line = &rhs_colored_lines[rhs_line_num.as_usize()];
+                        if same_lines {
+                            print!("{}{}", display_rhs_line_num, rhs_line);
+                        } else {
+                            print!(
+                                "{}{}{}",
+                                display_lhs_line_num, display_rhs_line_num, rhs_line
+                            );
+                        }
+                    }
+                    None => {
+                        // We didn't have any changed RHS lines in the
+                        // hunk, but we had some contextual lines that
+                        // only occurred on the LHS (e.g. extra newlines).
+                        println!("{}{}", display_lhs_line_num, display_rhs_line_num);
+                    }
+                }
+            } else if no_rhs_changes && !show_both {
+                match lhs_line_num {
+                    Some(lhs_line_num) => {
+                        let lhs_line = &lhs_colored_lines[lhs_line_num.as_usize()];
+                        if same_lines {
+                            print!("{}{}", display_lhs_line_num, lhs_line);
+                        } else {
+                            print!(
+                                "{}{}{}",
+                                display_lhs_line_num, display_rhs_line_num, lhs_line
+                            );
+                        }
+                    }
+                    None => {
+                        println!("{}{}", display_lhs_line_num, display_rhs_line_num);
+                    }
+                }
+            } else {
+                let lhs_line = match lhs_line_num {
+                    Some(lhs_line_num) => split_and_apply(
+                        lhs_lines[lhs_line_num.as_usize()],
+                        source_dims.lhs_content_display_width,
+                        display_options.tab_width,
+                        lhs_highlights.get(lhs_line_num).unwrap_or(&vec![]),
+                        Side::Left,
+                    ),
+                    None => vec![" ".repeat(source_dims.lhs_content_display_width)],
+                };
+                let rhs_line = match rhs_line_num {
+                    Some(rhs_line_num) => split_and_apply(
+                        rhs_lines[rhs_line_num.as_usize()],
+                        source_dims.rhs_content_display_width,
+                        display_options.tab_width,
+                        rhs_highlights.get(rhs_line_num).unwrap_or(&vec![]),
+                        Side::Right,
+                    ),
+                    None => vec!["".into()],
+                };
+
+                for (i, (lhs_line, rhs_line)) in zip_pad_shorter(&lhs_line, &rhs_line)
+                    .into_iter()
+                    .enumerate()
+                {
+                    let lhs_line = lhs_line
+                        .unwrap_or_else(|| " ".repeat(source_dims.lhs_content_display_width));
+                    let rhs_line = rhs_line.unwrap_or_else(|| "".into());
+                    let lhs_num: String = if i == 0 {
+                        display_lhs_line_num.clone()
+                    } else {
+                        let mut s = format_missing_line_num(
+                            lhs_line_num
+                                .unwrap_or_else(|| prev_lhs_line_num.unwrap_or_else(|| 10.into())),
+                            &source_dims,
+                            Side::Left,
+                            true,
+                            display_options.use_color,
+                        );
+                        if let Some(line_num) = lhs_line_num {
+                            s = apply_line_number_color(
+                                &s,
+                                lhs_lines_with_novel.contains(line_num),
+                                Side::Left,
+                                display_options,
+                            );
+                        }
+                        s
+                    };
+                    let rhs_num: String = if i == 0 {
+                        display_rhs_line_num.clone()
+                    } else {
+                        let mut s = format_missing_line_num(
+                            rhs_line_num
+                                .unwrap_or_else(|| prev_rhs_line_num.unwrap_or_else(|| 10.into())),
+                            &source_dims,
+                            Side::Right,
+                            true,
+                            display_options.use_color,
+                        );
+                        if let Some(line_num) = rhs_line_num {
+                            s = apply_line_number_color(
+                                &s,
+                                rhs_lines_with_novel.contains(line_num),
+                                Side::Right,
+                                display_options,
+                            );
+                        }
+                        s
+                    };
+
+                    println!("{}{}{}{}{}", lhs_num, lhs_line, SPACER, rhs_num, rhs_line);
+                }
+            }
+
+            if lhs_line_num.is_some() {
+                prev_lhs_line_num = *lhs_line_num;
+            }
+            if rhs_line_num.is_some() {
+                prev_rhs_line_num = *rhs_line_num;
+            }
+        }
+        println!();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+
+    use super::*;
+    use crate::options::DEFAULT_TERMINAL_WIDTH;
+    use crate::parse::guess_language::Language;
+    use crate::syntax::{AtomKind, MatchKind, TokenKind};
+
+    #[test]
+    fn test_width_calculations() {
+        let source_dims = SourceDimensions::new(
+            DEFAULT_TERMINAL_WIDTH,
+            1.into(),
+            10.into(),
+            1.into(),
+            10.into(),
+            9999,
+            9999,
+        );
+
+        assert_eq!(source_dims.lhs_line_nums_width, 2);
+        assert_eq!(source_dims.rhs_line_nums_width, 3);
+    }
+
+    #[test]
+    fn test_format_missing_line_num() {
+        let source_dims = SourceDimensions::new(
+            DEFAULT_TERMINAL_WIDTH,
+            1.into(),
+            1.into(),
+            1.into(),
+            1.into(),
+            9999,
+            9999,
+        );
+
+        assert_eq!(
+            format_missing_line_num(0.into(), &source_dims, Side::Left, false, true),
+            ". ".dimmed().to_string()
+        );
+        assert_eq!(
+            format_missing_line_num(0.into(), &source_dims, Side::Left, false, false),
+            ". ".to_owned()
+        );
+    }
+
+    #[test]
+    fn test_format_missing_line_num_at_end() {
+        let source_dims =
+            SourceDimensions::new(80, 1.into(), 1.into(), 1.into(), 1.into(), 9999, 9999);
+
+        assert_eq!(
+            format_missing_line_num(1.into(), &source_dims, Side::Left, false, true),
+            "  ".dimmed().to_string()
+        );
+        assert_eq!(
+            format_missing_line_num(1.into(), &source_dims, Side::Left, false, false),
+            "  ".to_owned()
+        );
+    }
+
+    #[test]
+    fn test_display_single_column() {
+        // Basic smoke test.
+        let res_lines = display_single_column(
+            "foo.py",
+            None,
+            &FileFormat::SupportedLanguage(Language::Python),
+            &["print(123)\n".to_owned()],
+            Side::Right,
+            &DisplayOptions::default(),
+        );
+        let res = res_lines.join("");
+        assert!(res.len() > 10);
+    }
+
+    #[test]
+    fn test_display_hunks() {
+        // Simulate diffing:
+        //
+        // Old:
+        // foo
+        //
+        // New:
+        // bar
+        let lhs_mps = [MatchedPos {
+            kind: MatchKind::Novel {
+                highlight: TokenKind::Atom(AtomKind::Normal),
+            },
+            pos: SingleLineSpan {
+                line: 0.into(),
+                start_col: 0,
+                end_col: 3,
+            },
+        }];
+
+        let rhs_mps = [MatchedPos {
+            kind: MatchKind::Novel {
+                highlight: TokenKind::Atom(AtomKind::Normal),
+            },
+            pos: SingleLineSpan {
+                line: 0.into(),
+                start_col: 0,
+                end_col: 3,
+            },
+        }];
+
+        let mut novel_lhs = DftHashSet::default();
+        novel_lhs.insert(0.into());
+        let mut novel_rhs = DftHashSet::default();
+        novel_rhs.insert(0.into());
+
+        let hunks = [Hunk {
+            novel_lhs,
+            novel_rhs,
+            lines: vec![(Some(0.into()), Some(0.into()))],
+        }];
+
+        // Simple smoke test.
+        print(
+            &hunks,
+            &DisplayOptions::default(),
+            "foo-new.el",
+            None,
+            &FileFormat::SupportedLanguage(Language::EmacsLisp),
+            "foo",
+            "bar",
+            &lhs_mps,
+            &rhs_mps,
+        );
+    }
+}
diff --git a/tests/cli.rs b/tests/cli.rs
new file mode 100644
index 0000000..de8c88f
--- /dev/null
+++ b/tests/cli.rs
@@ -0,0 +1,293 @@
+use std::process::Command;
+
+use assert_cmd::prelude::*;
+use predicates::prelude::*;
+
+fn find_runner() -> Option<String> {
+    for (key, value) in std::env::vars() {
+        if key.starts_with("CARGO_TARGET_") && key.ends_with("_RUNNER") && !value.is_empty() {
+            return Some(value);
+        }
+    }
+    None
+}
+
+// Sample code from
+// https://github.com/assert-rs/assert_cmd/issues/139, supports
+// cross-compiled binaries.
+fn get_base_command() -> Command {
+    let mut cmd;
+    let path = assert_cmd::cargo_bin!("difft");
+    if let Some(runner) = find_runner() {
+        let mut runner = runner.split_whitespace();
+        cmd = Command::new(runner.next().unwrap());
+        for arg in runner {
+            cmd.arg(arg);
+        }
+        cmd.arg(path);
+    } else {
+        cmd = Command::new(path);
+    }
+    cmd
+}
+
+#[test]
+fn no_such_files() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("no_such_file").arg("no_such_file_either");
+    cmd.assert().failure().code(2);
+}
+
+#[test]
+fn inline() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--display=inline")
+        .arg("sample_files/simple_1.js")
+        .arg("sample_files/simple_2.js");
+    cmd.assert().success();
+}
+
+#[test]
+fn binary_changed() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--display=inline")
+        .arg("img/logo.png")
+        .arg("/dev/null");
+
+    let predicate_fn = predicate::str::contains("Binary file removed");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn binary_override() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--override-binary=*.js")
+        .arg("sample_files/simple_1.js")
+        .arg("sample_files/simple_2.js");
+    let predicate_fn = predicate::str::contains("Binary file modified");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn has_changes_default_exit_code() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/simple_1.js")
+        .arg("sample_files/simple_2.js");
+    cmd.assert().success();
+}
+
+#[test]
+fn has_changes_requested_exit_code() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--exit-code")
+        .arg("sample_files/simple_1.js")
+        .arg("sample_files/simple_2.js");
+    cmd.assert().failure().code(1);
+}
+
+#[test]
+fn ignore_comments() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--ignore-comments")
+        .arg("sample_files/comma_and_comment_1.js")
+        .arg("sample_files/comma_and_comment_2.js");
+
+    let predicate_fn = predicate::str::contains("No syntactic changes");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn check_only() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--check-only")
+        .arg("sample_files/simple_1.js")
+        .arg("sample_files/simple_2.js");
+
+    let predicate_fn = predicate::str::contains("Has syntactic changes");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn check_only_text_file() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--check-only")
+        .arg("sample_files/text_1.txt")
+        .arg("sample_files/text_2.txt");
+
+    let predicate_fn = predicate::str::contains("Has changes");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn text_changes_at_end_doesnt_crash() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/cli_tests/changes_at_end_1.txt")
+        .arg("sample_files/cli_tests/changes_at_end_2.txt");
+
+    cmd.assert().success();
+}
+
+#[test]
+fn makefile_text_as_atom() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/cli_tests/makefile_1.mk")
+        .arg("sample_files/cli_tests/makefile_2.mk");
+
+    let predicate_fn = predicate::str::contains("CCFLAGS");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn yaml_parse_errors() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/cli_tests/bad_yaml_1.yml")
+        .arg("sample_files/cli_tests/bad_yaml_2.yml");
+
+    let predicate_fn = predicate::str::contains("exceeded DFT_PARSE_ERROR_LIMIT");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn list_languages() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--list-languages");
+
+    let predicate_fn = predicate::str::contains("TOML");
+    cmd.assert().stdout(predicate_fn);
+
+    let predicate_fn = predicate::str::contains("*.toml");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn test_mime_type_false_positive() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/cli_tests/x_mod_false_positive.js")
+        .arg("sample_files/cli_tests/empty.js");
+
+    let predicate_fn = predicate::str::contains("JavaScript");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn slightly_invalid_utf8() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/cli_tests/MainWindowViewModel.cs")
+        .arg("/dev/null");
+
+    let predicate_fn = predicate::str::contains("using");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn directory_arguments() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/dir_1").arg("sample_files/dir_2");
+
+    let predicate_fn =
+        predicate::str::contains("only_in_1.c").and(predicate::str::contains("only_in_2.rs"));
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn git_style_arguments_rename() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("elisp_oldname.el")
+        .arg("sample_files/elisp_1.el")
+        .arg("lhs_hash_placeholder")
+        .arg("lhs_mode_placeholder")
+        .arg("sample_files/elisp_2.el")
+        .arg("rhs_hash_placeholder")
+        .arg("rhs_mode_placeholder")
+        .arg("elisp_newname.el")
+        .arg("similarity_placeholder");
+    let predicate_fn = predicate::str::contains("Renamed");
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn git_style_arguments_new_file() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("simple.txt")
+        .arg("/dev/null")
+        .arg(".")
+        .arg(".")
+        .arg("sample_files/simple_1.txt")
+        .arg("abcdef1234")
+        .arg("100644");
+    let predicate_fn = predicate::str::contains("File permissions changed").not();
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn drop_different_path_starts() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("sample_files/dir_1/clojure.clj")
+        .arg("sample_files/dir_2/clojure.clj");
+    let predicate_fn = predicate::str::contains("dir_2/clojure.clj").not();
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn dump_tree_sitter() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--dump-ts")
+        .arg("sample_files/simple_1.js")
+        .arg("sample_files/simple_2.js");
+    cmd.assert().success();
+}
+
+#[test]
+fn dump_syntax() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--dump-syntax")
+        .arg("sample_files/simple_1.js")
+        .arg("sample_files/simple_2.js");
+    cmd.assert().success();
+}
+
+#[test]
+fn walk_hidden_items() {
+    let mut cmd = get_base_command();
+
+    cmd.args(["sample_files/hidden_1", "sample_files/hidden_2"]);
+
+    let predicate_fn =
+        predicate::str::contains(format!(".hidden{}doc.txt", std::path::MAIN_SEPARATOR))
+            .and(predicate::str::contains(".hidden.txt"))
+            .and(predicate::str::contains("before"))
+            .and(predicate::str::contains("after"));
+    cmd.assert().stdout(predicate_fn);
+}
+
+#[test]
+fn git_unmerged_files() {
+    let mut cmd = get_base_command();
+
+    cmd.args(["sample_files/simple_1.js"]);
+    cmd.env("GIT_EXEC_PATH", "/usr/lib/git-core");
+
+    let predicate_fn = predicate::str::contains("Unmerged path");
+    cmd.assert().stdout(predicate_fn);
+}