Implementation of 'bat --diff'

This adds a new `--diff` option that can be used to only show lines
close to Git changes (added/removed/modified lines). The amount of
additional context can be controlled with `--diff-context=N`.

closes #23

7 files changed, 145 insertions, 36 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ced88213..973eba9a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 
 ## Features
+
+- Add a new `--diff` option that can be used to only show lines surrounding
+  Git changes, i.e. added, removed or modified lines. The amount of additional
+  context can be controlled with `--diff-context=N`. See #23 and #940
+
 ## Bugfixes
 ## Other
 ## `bat` as a library
diff --git a/src/bin/bat/app.rs b/src/bin/bat/app.rs
index e5221455..f0f519ea 100644
--- a/src/bin/bat/app.rs
+++ b/src/bin/bat/app.rs
@@ -15,7 +15,7 @@ use console::Term;
 
 use bat::{
     assets::HighlightingAssets,
-    config::Config,
+    config::{Config, VisibleLines},
     error::*,
     input::Input,
     line_range::{HighlightedLineRanges, LineRange, LineRanges},
@@ -196,13 +196,23 @@ impl App {
                     }
                 })
                 .unwrap_or_else(|| String::from(HighlightingAssets::default_theme())),
-            line_ranges: self
-                .matches
-                .values_of("line-range")
-                .map(|vs| vs.map(LineRange::from).collect())
-                .transpose()?
-                .map(LineRanges::from)
-                .unwrap_or_default(),
+            visible_lines: if self.matches.is_present("diff") {
+                VisibleLines::DiffContext(
+                    self.matches
+                        .value_of("diff-context")
+                        .and_then(|t| t.parse().ok())
+                        .unwrap_or(2),
+                )
+            } else {
+                VisibleLines::Ranges(
+                    self.matches
+                        .values_of("line-range")
+                        .map(|vs| vs.map(LineRange::from).collect())
+                        .transpose()?
+                        .map(LineRanges::from)
+                        .unwrap_or_default(),
+                )
+            },
             style_components,
             syntax_mapping,
             pager: self.matches.value_of("pager"),
diff --git a/src/bin/bat/clap_app.rs b/src/bin/bat/clap_app.rs
index c7344991..85edefde 100644
--- a/src/bin/bat/clap_app.rs
+++ b/src/bin/bat/clap_app.rs
@@ -106,6 +106,34 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                             the filename."),
         )
         .arg(
+            Arg::with_name("diff")
+                .long("diff")
+                .help("Only show lines that have been added/removed/modified.")
+                .long_help(
+                    "Only show lines that have been added/removed/modified with respect \
+                     to the Git index. Use --diff-context=N to control how much context you want to see.",
+                ),
+        )
+        .arg(
+            Arg::with_name("diff-context")
+                .long("diff-context")
+                .overrides_with("diff-context")
+                .takes_value(true)
+                .value_name("N")
+                .validator(
+                    |n| {
+                        n.parse::<usize>()
+                            .map_err(|_| "must be a number")
+                            .map(|_| ()) // Convert to Result<(), &str>
+                            .map_err(|e| e.to_string())
+                    }, // Convert to Result<(), String>
+                )
+                .hidden_short_help(true)
+                .long_help(
+                    "Include N lines of context around added/removed/modified lines when using '--diff'.",
+                ),
+        )
+        .arg(
             Arg::with_name("tabs")
                 .long("tabs")
                 .overrides_with("tabs")
@@ -339,6 +367,7 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                 .takes_value(true)
                 .number_of_values(1)
                 .value_name("N:M")
+                .conflicts_with("diff")
                 .help("Only print the lines from N to M.")
                 .long_help(
                     "Only print the specified range of lines for each file. \
diff --git a/src/config.rs b/src/config.rs
index d5df9910..3c24b77f 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -5,6 +5,32 @@ use crate::style::StyleComponents;
 use crate::syntax_mapping::SyntaxMapping;
 use crate::wrapping::WrappingMode;
 
+#[derive(Debug, Clone)]
+pub enum VisibleLines {
+    /// Show all lines which are included in the line ranges
+    Ranges(LineRanges),
+
+    #[cfg(feature = "git")]
+    /// Only show lines surrounding added/deleted/modified lines
+    DiffContext(usize),
+}
+
+impl VisibleLines {
+    pub fn diff_context(&self) -> bool {
+        match self {
+            Self::Ranges(_) => false,
+            #[cfg(feature = "git")]
+            Self::DiffContext(_) => true,
+        }
+    }
+}
+
+impl Default for VisibleLines {
+    fn default() -> Self {
+        VisibleLines::Ranges(LineRanges::default())
+    }
+}
+
 #[derive(Debug, Clone, Default)]
 pub struct Config<'a> {
     /// The explicitly configured language, if any
@@ -39,8 +65,8 @@ pub struct Config<'a> {
     #[cfg(feature = "paging")]
     pub paging_mode: PagingMode,
 
-    /// Specifies the lines that should be printed
-    pub line_ranges: LineRanges,
+    /// Specifies which lines should be printed
+    pub visible_lines: VisibleLines,
 
     /// The syntax highlighting theme
     pub theme: String,
@@ -62,10 +88,7 @@ pub struct Config<'a> {
 fn default_config_should_include_all_lines() {
     use crate::line_range::RangeCheckResult;
 
-    assert_eq!(
-        Config::default().line_ranges.check(17),
-        RangeCheckResult::InRange
-    );
+    assert_eq!(LineRanges::default().check(17), RangeCheckResult::InRange);
 }
 
 #[test]
diff --git a/src/controller.rs b/src/controller.rs
index 09c6ec2a..f636d5fd 100644
--- a/src/controller.rs
+++ b/src/controller.rs
@@ -1,9 +1,13 @@
 use std::io::{self, Write};
 
 use crate::assets::HighlightingAssets;
-use crate::config::Config;
+use crate::config::{Config, VisibleLines};
+#[cfg(feature = "git")]
+use crate::diff::{get_git_diff, LineChanges};
 use crate::error::*;
 use crate::input::{Input, InputReader, OpenedInput};
+#[cfg(feature = "git")]
+use crate::line_range::LineRange;
 use crate::line_range::{LineRanges, RangeCheckResult};
 use crate::output::OutputType;
 #[cfg(feature = "paging")]
@@ -68,6 +72,32 @@ impl<'b> Controller<'b> {
                     no_errors = false;
                 }
                 Ok(mut opened_input) => {
+                    #[cfg(feature = "git")]
+                    let line_changes = if self.config.visible_lines.diff_context()
+                        || (!self.config.loop_through && self.config.style_components.changes())
+                    {
+                        if let crate::input::OpenedInputKind::OrdinaryFile(ref path) =
+                            opened_input.kind
+                        {
+                            let diff = get_git_diff(path);
+
+                            if self.config.visible_lines.diff_context()
+                                && diff
+                                    .as_ref()
+                                    .map(|changes| changes.is_empty())
+                                    .unwrap_or(false)
+                            {
+                                continue;
+                            }
+
+                            diff
+                        } else {
+                            None
+                        }
+                    } else {
+                        None
+                    };
+
                     let mut printer: Box<dyn Printer> = if self.config.loop_through {
                         Box::new(SimplePrinter::new())
                     } else {
@@ -75,10 +105,18 @@ impl<'b> Controller<'b> {
                             &self.config,
                             &self.assets,
                             &mut opened_input,
+                            #[cfg(feature = "git")]
+                            &line_changes,
                         ))
                     };
 
-                    let result = self.print_file(&mut *printer, writer, &mut opened_input);
+                    let result = self.print_file(
+                        &mut *printer,
+                        writer,
+                        &mut opened_input,
+                        #[cfg(feature = "git")]
+                        &line_changes,
+                    );
 
                     if let Err(error) = result {
                         handle_error(&error);
@@ -96,13 +134,31 @@ impl<'b> Controller<'b> {
         printer: &mut dyn Printer,
         writer: &mut dyn Write,
         input: &mut OpenedInput,
+        #[cfg(feature = "git")] line_changes: &Option<LineChanges>,
     ) -> Result<()> {
         if !input.reader.first_line.is_empty() || self.config.style_components.header() {
             printer.print_header(writer, input)?;
         }
 
         if !input.reader.first_line.is_empty() {
-            self.print_file_ranges(printer, writer, &mut input.reader, &self.config.line_ranges)?;
+            let line_ranges = match self.config.visible_lines {
+                VisibleLines::Ranges(ref line_ranges) => line_ranges.clone(),
+                #[cfg(feature = "git")]
+                VisibleLines::DiffContext(context) => {
+                    let mut line_ranges: Vec<LineRange> = vec![];
+
+                    if let Some(line_changes) = line_changes {
+                        for line in line_changes.keys() {
+                            let line = *line as usize;
+                            line_ranges.push(LineRange::new(line - context, line + context));
+                        }
+                    }
+
+                    LineRanges::from(line_ranges)
+                }
+            };
+
+            self.print_file_ranges(printer, writer, &mut input.reader, &line_ranges)?;
         }
         printer.print_footer(writer, input)?;
 
diff --git a/src/pretty_printer.rs b/src/pretty_printer.rs
index 0c78ea90..13bd5dbc 100644
--- a/src/pretty_printer.rs
+++ b/src/pretty_printer.rs
@@ -6,7 +6,7 @@ use syntect::parsing::SyntaxReference;
 
 use crate::{
     assets::HighlightingAssets,
-    config::Config,
+    config::{Config, VisibleLines},
     controller::Controller,
     error::Result,
     input::Input,
@@ -205,7 +205,7 @@ impl<'a> PrettyPrinter<'a> {
 
     /// Specify the lines that should be printed (default: all)
     pub fn line_ranges(&mut self, ranges: LineRanges) -> &mut Self {
-        self.config.line_ranges = ranges;
+        self.config.visible_lines = VisibleLines::Ranges(ranges);
         self
     }
 
diff --git a/src/printer.rs b/src/printer.rs
index 5eed437e..2b245cfd 100644
--- a/src/printer.rs
+++ b/src/printer.rs
@@ -24,7 +24,7 @@ use crate::config::Config;
 use crate::decorations::LineChangesDecoration;
 use crate::decorations::{Decoration, GridBorderDecoration, LineNumberDecoration};
 #[cfg(feature = "git")]
-use crate::diff::{get_git_diff, LineChanges};
+use crate::diff::LineChanges;
 use crate::error::*;
 use crate::input::OpenedInput;
 use crate::line_range::RangeCheckResult;
@@ -90,7 +90,7 @@ pub(crate) struct InteractivePrinter<'a> {
     ansi_prefix_sgr: String,
     content_type: Option<ContentType>,
     #[cfg(feature = "git")]
-    pub line_changes: Option<LineChanges>,
+    pub line_changes: &'a Option<LineChanges>,
     highlighter: Option<HighlightLines<'a>>,
     syntax_set: &'a SyntaxSet,
     background_color_highlight: Option<Color>,
@@ -101,6 +101,7 @@ impl<'a> InteractivePrinter<'a> {
         config: &'a Config,
         assets: &'a HighlightingAssets,
         input: &mut OpenedInput,
+        #[cfg(feature = "git")] line_changes: &'a Option<LineChanges>,
     ) -> Self {
         let theme = assets.get_theme(&config.theme);
 
@@ -145,9 +146,6 @@ impl<'a> InteractivePrinter<'a> {
             panel_width = 0;
         }
 
-        #[cfg(feature = "git")]
-        let mut line_changes = None;
-
         let highlighter = if input
             .reader
             .content_type
@@ -155,18 +153,6 @@ impl<'a> InteractivePrinter<'a> {
         {
             None
         } else {
-            // Get the Git modifications
-            #[cfg(feature = "git")]
-            {
-                use crate::input::OpenedInputKind;
-
-                if config.style_components.changes() {
-                    if let OpenedInputKind::OrdinaryFile(ref path) = input.kind {
-                        line_changes = get_git_diff(path);
-                    }
-                }
-            }
-
             // Determine the type of syntax for highlighting
             let syntax = assets.get_syntax(config.language, input, &config.syntax_mapping);
             Some(HighlightLines::new(syntax, theme))