Add --context flag for diffs.

Allows specifying the number of lines of context to show around diffs.
The logic was already in place, just some plumbing was needed.

CHANGELOG.md

@@ -80,6 +80,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj branch list` now supports a `--tracked/-t` option which can be used to
   show tracked branches only. Omits local Git-tracking branches by default.
 
+* Commands producing diffs now accept a `--context` flag for the number of
+  lines of context to show.
+
 ### Fixed bugs
 
 * On Windows, symlinks in the repo are now materialized as regular files in the

cli/src/diff_util.rs

@@ -44,6 +44,8 @@ use crate::merge_tools::{self, ExternalMergeTool};
 use crate::text_util;
 use crate::ui::Ui;
 
+const DEFAULT_CONTEXT_LINES: usize = 3;
+
 #[derive(clap::Args, Clone, Debug)]
 #[command(next_help_heading = "Diff Formatting Options")]
 #[command(group(clap::ArgGroup::new("short-format").args(&["summary", "stat", "types"])))]
@@ -73,15 +75,18 @@ pub struct DiffFormatArgs {
     /// Generate diff by external command
     #[arg(long)]
     pub tool: Option<String>,
+    /// Number of lines of context to show
+    #[arg(long)]
+    context: Option<usize>,
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum DiffFormat {
     Summary,
     Stat,
     Types,
-    Git,
-    ColorWords,
+    Git { context: usize },
+    ColorWords { context: usize },
     Tool(Box<ExternalMergeTool>),
 }
 
@@ -92,7 +97,7 @@ pub fn diff_formats_for(
 ) -> Result<Vec<DiffFormat>, config::ConfigError> {
     let formats = diff_formats_from_args(settings, args)?;
     if formats.is_empty() {
-        Ok(vec![default_diff_format(settings)?])
+        Ok(vec![default_diff_format(settings, args.context)?])
     } else {
         Ok(formats)
     }
@@ -108,7 +113,7 @@ pub fn diff_formats_for_log(
     let mut formats = diff_formats_from_args(settings, args)?;
     // --patch implies default if no format other than --summary is specified
     if patch && matches!(formats.as_slice(), [] | [DiffFormat::Summary]) {
-        formats.push(default_diff_format(settings)?);
+        formats.push(default_diff_format(settings, args.context)?);
         formats.dedup();
     }
     Ok(formats)
@@ -121,8 +126,18 @@ fn diff_formats_from_args(
     let mut formats = [
         (args.summary, DiffFormat::Summary),
         (args.types, DiffFormat::Types),
-        (args.git, DiffFormat::Git),
-        (args.color_words, DiffFormat::ColorWords),
+        (
+            args.git,
+            DiffFormat::Git {
+                context: args.context.unwrap_or(DEFAULT_CONTEXT_LINES),
+            },
+        ),
+        (
+            args.color_words,
+            DiffFormat::ColorWords {
+                context: args.context.unwrap_or(DEFAULT_CONTEXT_LINES),
+            },
+        ),
         (args.stat, DiffFormat::Stat),
     ]
     .into_iter()
@@ -136,7 +151,10 @@ fn diff_formats_from_args(
     Ok(formats)
 }
 
-fn default_diff_format(settings: &UserSettings) -> Result<DiffFormat, config::ConfigError> {
+fn default_diff_format(
+    settings: &UserSettings,
+    num_context_lines: Option<usize>,
+) -> Result<DiffFormat, config::ConfigError> {
     let config = settings.config();
     if let Some(args) = config.get("ui.diff.tool").optional()? {
         // External "tool" overrides the internal "format" option.
@@ -158,8 +176,12 @@ fn default_diff_format(settings: &UserSettings) -> Result<DiffFormat, config::Co
     match name.as_ref() {
         "summary" => Ok(DiffFormat::Summary),
         "types" => Ok(DiffFormat::Types),
-        "git" => Ok(DiffFormat::Git),
-        "color-words" => Ok(DiffFormat::ColorWords),
+        "git" => Ok(DiffFormat::Git {
+            context: num_context_lines.unwrap_or(DEFAULT_CONTEXT_LINES),
+        }),
+        "color-words" => Ok(DiffFormat::ColorWords {
+            context: num_context_lines.unwrap_or(DEFAULT_CONTEXT_LINES),
+        }),
         "stat" => Ok(DiffFormat::Stat),
         _ => Err(config::ConfigError::Message(format!(
             "invalid diff format: {name}"
@@ -190,13 +212,13 @@ pub fn show_diff(
                 let tree_diff = from_tree.diff_stream(to_tree, matcher);
                 show_types(formatter, workspace_command, tree_diff)?;
             }
-            DiffFormat::Git => {
+            DiffFormat::Git { context } => {
                 let tree_diff = from_tree.diff_stream(to_tree, matcher);
-                show_git_diff(formatter, workspace_command, tree_diff)?;
+                show_git_diff(formatter, workspace_command, *context, tree_diff)?;
             }
-            DiffFormat::ColorWords => {
+            DiffFormat::ColorWords { context } => {
                 let tree_diff = from_tree.diff_stream(to_tree, matcher);
-                show_color_words_diff(formatter, workspace_command, tree_diff)?;
+                show_color_words_diff(formatter, workspace_command, *context, tree_diff)?;
             }
             DiffFormat::Tool(tool) => {
                 merge_tools::generate_diff(ui, formatter.raw(), from_tree, to_tree, matcher, tool)?;
@@ -231,10 +253,10 @@ pub fn show_patch(
 fn show_color_words_diff_hunks(
     left: &[u8],
     right: &[u8],
+    num_context_lines: usize,
     formatter: &mut dyn Formatter,
 ) -> io::Result<()> {
     const SKIPPED_CONTEXT_LINE: &str = "    ...\n";
-    let num_context_lines = 3;
     let mut context = VecDeque::new();
     // Have we printed "..." for any skipped context?
     let mut skipped_context = false;
@@ -429,6 +451,7 @@ fn basic_diff_file_type(value: &MaterializedTreeValue) -> &'static str {
 pub fn show_color_words_diff(
     formatter: &mut dyn Formatter,
     workspace_command: &WorkspaceCommandHelper,
+    num_context_lines: usize,
     tree_diff: TreeDiffStream,
 ) -> Result<(), CommandError> {
     formatter.push_label("diff")?;
@@ -449,7 +472,12 @@ pub fn show_color_words_diff(
                 } else if right_content.is_binary {
                     writeln!(formatter.labeled("binary"), "    (binary)")?;
                 } else {
-                    show_color_words_diff_hunks(&[], &right_content.contents, formatter)?;
+                    show_color_words_diff_hunks(
+                        &[],
+                        &right_content.contents,
+                        num_context_lines,
+                        formatter,
+                    )?;
                 }
             } else if right_value.is_present() {
                 let description = match (&left_value, &right_value) {
@@ -508,6 +536,7 @@ pub fn show_color_words_diff(
                     show_color_words_diff_hunks(
                         &left_content.contents,
                         &right_content.contents,
+                        num_context_lines,
                         formatter,
                     )?;
                 }
@@ -523,7 +552,12 @@ pub fn show_color_words_diff(
                 } else if left_content.is_binary {
                     writeln!(formatter.labeled("binary"), "    (binary)")?;
                 } else {
-                    show_color_words_diff_hunks(&left_content.contents, &[], formatter)?;
+                    show_color_words_diff_hunks(
+                        &left_content.contents,
+                        &[],
+                        num_context_lines,
+                        formatter,
+                    )?;
                 }
             }
         }
@@ -694,8 +728,9 @@ fn show_unified_diff_hunks(
     formatter: &mut dyn Formatter,
     left_content: &[u8],
     right_content: &[u8],
+    num_context_lines: usize,
 ) -> Result<(), CommandError> {
-    for hunk in unified_diff_hunks(left_content, right_content, 3) {
+    for hunk in unified_diff_hunks(left_content, right_content, num_context_lines) {
         writeln!(
             formatter.labeled("hunk_header"),
             "@@ -{},{} +{},{} @@",
@@ -760,6 +795,7 @@ fn materialized_diff_stream<'a>(
 pub fn show_git_diff(
     formatter: &mut dyn Formatter,
     workspace_command: &WorkspaceCommandHelper,
+    num_context_lines: usize,
     tree_diff: TreeDiffStream,
 ) -> Result<(), CommandError> {
     formatter.push_label("diff")?;
@@ -778,7 +814,7 @@ pub fn show_git_diff(
                     writeln!(formatter, "--- /dev/null")?;
                     writeln!(formatter, "+++ b/{path_string}")
                 })?;
-                show_unified_diff_hunks(formatter, &[], &right_part.content)?;
+                show_unified_diff_hunks(formatter, &[], &right_part.content, num_context_lines)?;
             } else if right_value.is_present() {
                 let left_part = git_diff_part(&path, left_value)?;
                 let right_part = git_diff_part(&path, right_value)?;
@@ -803,7 +839,12 @@ pub fn show_git_diff(
                     }
                     Ok(())
                 })?;
-                show_unified_diff_hunks(formatter, &left_part.content, &right_part.content)?;
+                show_unified_diff_hunks(
+                    formatter,
+                    &left_part.content,
+                    &right_part.content,
+                    num_context_lines,
+                )?;
             } else {
                 let left_part = git_diff_part(&path, left_value)?;
                 formatter.with_label("file_header", |formatter| {
@@ -813,7 +854,7 @@ pub fn show_git_diff(
                     writeln!(formatter, "--- a/{path_string}")?;
                     writeln!(formatter, "+++ /dev/null")
                 })?;
-                show_unified_diff_hunks(formatter, &left_part.content, &[])?;
+                show_unified_diff_hunks(formatter, &left_part.content, &[], num_context_lines)?;
             }
         }
         Ok::<(), CommandError>(())

cli/tests/cli-reference@.md.snap

@@ -655,6 +655,7 @@ With the `--from` and/or `--to` options, shows the difference from/to the given
   Possible values: `true`, `false`
 
 * `--tool <TOOL>` — Generate diff by external command
+* `--context <CONTEXT>` — Number of lines of context to show
 
 
 
@@ -1000,6 +1001,7 @@ This excludes changes from other commits by temporarily rebasing `--from` onto `
   Possible values: `true`, `false`
 
 * `--tool <TOOL>` — Generate diff by external command
+* `--context <CONTEXT>` — Number of lines of context to show
 
 
 
@@ -1051,6 +1053,7 @@ Show commit history
   Possible values: `true`, `false`
 
 * `--tool <TOOL>` — Generate diff by external command
+* `--context <CONTEXT>` — Number of lines of context to show
 
 
 
@@ -1221,6 +1224,7 @@ Show how a change has evolved as it's been updated, rebased, etc.
   Possible values: `true`, `false`
 
 * `--tool <TOOL>` — Generate diff by external command
+* `--context <CONTEXT>` — Number of lines of context to show
 
 
 
@@ -1592,6 +1596,7 @@ Show commit description and changes in a revision
   Possible values: `true`, `false`
 
 * `--tool <TOOL>` — Generate diff by external command
+* `--context <CONTEXT>` — Number of lines of context to show
 
 
 

cli/tests/test_diff_command.rs

@@ -40,6 +40,17 @@ fn test_diff_basic() {
             1: foo
     "###);
 
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "--context=0"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Removed regular file file1:
+       1     : foo
+    Modified regular file file2:
+       1    1: foo
+            2: bar
+    Added regular file file3:
+            1: foo
+    "###);
+
     let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s"]);
     insta::assert_snapshot!(stdout, @r###"
     D file1
@@ -79,6 +90,30 @@ fn test_diff_basic() {
     +foo
     "###);
 
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "--git", "--context=0"]);
+    insta::assert_snapshot!(stdout, @r###"
+    diff --git a/file1 b/file1
+    deleted file mode 100644
+    index 257cc5642c..0000000000
+    --- a/file1
+    +++ /dev/null
+    @@ -1,1 +1,0 @@
+    -foo
+    diff --git a/file2 b/file2
+    index 257cc5642c...3bd1f0e297 100644
+    --- a/file2
+    +++ b/file2
+    @@ -2,0 +2,1 @@
+    +bar
+    diff --git a/file3 b/file3
+    new file mode 100644
+    index 0000000000..257cc5642c
+    --- /dev/null
+    +++ b/file3
+    @@ -1,0 +1,1 @@
+    +foo
+    "###);
+
     let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s", "--git"]);
     insta::assert_snapshot!(stdout, @r###"
     D file1
@@ -637,6 +672,74 @@ fn test_diff_skipped_context() {
     "###);
 }
 
+#[test]
+fn test_diff_skipped_context_nondefault() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    std::fs::write(repo_path.join("file1"), "a\nb\nc\nd").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["describe", "-m", "=== Left side of diffs"]);
+
+    test_env.jj_cmd_ok(&repo_path, &["new", "@", "-m", "=== Must skip 2 lines"]);
+    std::fs::write(repo_path.join("file1"), "A\nb\nc\nD").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new", "@-", "-m", "=== Don't skip 1 line"]);
+    std::fs::write(repo_path.join("file1"), "A\nb\nC\nd").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new", "@-", "-m", "=== No gap to skip"]);
+    std::fs::write(repo_path.join("file1"), "a\nB\nC\nd").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new", "@-", "-m", "=== 1 line at start"]);
+    std::fs::write(repo_path.join("file1"), "a\nB\nc\nd").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new", "@-", "-m", "=== 1 line at end"]);
+    std::fs::write(repo_path.join("file1"), "a\nb\nC\nd").unwrap();
+
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &[
+            "log",
+            "-Tdescription",
+            "-p",
+            "--no-graph",
+            "--reversed",
+            "--context=0",
+        ],
+    );
+    insta::assert_snapshot!(stdout, @r###"
+    === Left side of diffs
+    Added regular file file1:
+            1: a
+            2: b
+            3: c
+            4: d
+    === Must skip 2 lines
+    Modified regular file file1:
+       1    1: aA
+        ...
+       4    4: dD
+    === Don't skip 1 line
+    Modified regular file file1:
+       1    1: aA
+       2    2: b
+       3    3: cC
+       4    4: d
+    === No gap to skip
+    Modified regular file file1:
+       1    1: a
+       2    2: bB
+       3    3: cC
+       4    4: d
+    === 1 line at start
+    Modified regular file file1:
+       1    1: a
+       2    2: bB
+        ...
+    === 1 line at end
+    Modified regular file file1:
+        ...
+       3    3: cC
+       4    4: d
+    "###);
+}
+
 #[test]
 fn test_diff_external_tool() {
     let mut test_env = TestEnvironment::default();