Do not use "..." between diff chunks when it only replaces 1 line of the diff

The number of lines in the diff output is unchanged.

This makes diffs a little more readable when the "..." would otherwise hide a
single line of code that helps in understanding the surrounding context lines.

This change mostly rearranges the loop that consumes the diff lines, so it can
buffer up to num_context_lines*2+1 lines instead of just num_context_lines.
There's a bit of extra code to handle times when a "..." replaces the last line
of a diff.

Note that `jj diff --git` is unchanged, and will still output `@@` lines that
replace a single line of context.

CHANGELOG.md

@@ -21,7 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 Thanks to the people who made this release happen!
 
  * Martin von Zweigbergk (@martinvonz)
-
+ * Danny Hooper (hooper@google.com)
 
 ## [0.6.1] - 2022-12-05
 

src/commands.rs

@@ -1296,6 +1296,7 @@ fn show_color_words_diff_hunks(
     right: &[u8],
     formatter: &mut dyn Formatter,
 ) -> io::Result<()> {
+    const SKIPPED_CONTEXT_LINE: &[u8] = b"    ...\n";
     let num_context_lines = 3;
     let mut context = VecDeque::new();
     // Have we printed "..." for any skipped context?
@@ -1305,23 +1306,24 @@ fn show_color_words_diff_hunks(
     for diff_line in files::diff(left, right) {
         if diff_line.is_unmodified() {
             context.push_back(diff_line.clone());
-            if context.len() > num_context_lines {
+            let mut start_skipping_context = false;
-                if context_before {
+            if context_before {
+                if skipped_context && context.len() > num_context_lines {
                     context.pop_front();
-                } else {
+                } else if !skipped_context && context.len() > num_context_lines + 1 {
-                    context.pop_back();
+                    start_skipping_context = true;
                 }
-                if !context_before {
+            } else if context.len() > num_context_lines * 2 + 1 {
-                    for line in &context {
+                for line in context.drain(..num_context_lines) {
-                        show_color_words_diff_line(formatter, line)?;
+                    show_color_words_diff_line(formatter, &line)?;
-                    }
-                    context.clear();
-                    context_before = true;
-                }
-                if !skipped_context {
-                    formatter.write_bytes(b"    ...\n")?;
-                    skipped_context = true;
                 }
+                start_skipping_context = true;
+            }
+            if start_skipping_context {
+                context.drain(..2);
+                formatter.write_bytes(SKIPPED_CONTEXT_LINE)?;
+                skipped_context = true;
+                context_before = true;
             }
         } else {
             for line in &context {
@@ -1334,9 +1336,17 @@ fn show_color_words_diff_hunks(
         }
     }
     if !context_before {
+        if context.len() > num_context_lines + 1 {
+            context.truncate(num_context_lines);
+            skipped_context = true;
+            context_before = true;
+        }
         for line in &context {
             show_color_words_diff_line(formatter, line)?;
         }
+        if context_before {
+            formatter.write_bytes(SKIPPED_CONTEXT_LINE)?;
+        }
     }
 
     // If the last diff line doesn't end with newline, add it.

tests/test_diff_command.rs

@@ -270,7 +270,7 @@ fn test_color_words_diff_missing_newline() {
         ...
     === Modify middle line
     Modified regular file file1:
-        ...
+       1    1: A
        2    2: b
        3    3: c
        4    4: d
@@ -278,7 +278,7 @@ fn test_color_words_diff_missing_newline() {
        6    6: f
        7    7: g
        8    8: h
-        ...
+       9    9: i
     === Modify last line
     Modified regular file file1:
         ...
@@ -313,3 +313,115 @@ fn test_color_words_diff_missing_newline() {
        9     : I
     "###);
 }
+
+#[test]
+fn test_diff_skipped_context() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(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\ne\nf\ng\nh\ni\nj").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m", "=== Left side of diffs"]);
+
+    test_env.jj_cmd_success(&repo_path, &["new", "@", "-m", "=== Must skip 2 lines"]);
+    std::fs::write(repo_path.join("file1"), "A\nb\nc\nd\ne\nf\ng\nh\ni\nJ").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new", "@-", "-m", "=== Don't skip 1 line"]);
+    std::fs::write(repo_path.join("file1"), "A\nb\nc\nd\ne\nf\ng\nh\nI\nj").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new", "@-", "-m", "=== No gap to skip"]);
+    std::fs::write(repo_path.join("file1"), "a\nB\nc\nd\ne\nf\ng\nh\nI\nj").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new", "@-", "-m", "=== No gap to skip"]);
+    std::fs::write(repo_path.join("file1"), "a\nb\nC\nd\ne\nf\ng\nh\nI\nj").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new", "@-", "-m", "=== 1 line at start"]);
+    std::fs::write(repo_path.join("file1"), "a\nb\nc\nd\nE\nf\ng\nh\ni\nj").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new", "@-", "-m", "=== 1 line at end"]);
+    std::fs::write(repo_path.join("file1"), "a\nb\nc\nd\ne\nF\ng\nh\ni\nj").unwrap();
+
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &["log", "-Tdescription", "-p", "--no-graph", "--reversed"],
+    );
+    insta::assert_snapshot!(stdout, @r###"
+    (no description set)
+    === Left side of diffs
+    Added regular file file1:
+            1: a
+            2: b
+            3: c
+            4: d
+            5: e
+            6: f
+            7: g
+            8: h
+            9: i
+           10: j
+    === Must skip 2 lines
+    Modified regular file file1:
+       1    1: aA
+       2    2: b
+       3    3: c
+       4    4: d
+        ...
+       7    7: g
+       8    8: h
+       9    9: i
+      10   10: jJ
+    === Don't skip 1 line
+    Modified regular file file1:
+       1    1: aA
+       2    2: b
+       3    3: c
+       4    4: d
+       5    5: e
+       6    6: f
+       7    7: g
+       8    8: h
+       9    9: iI
+      10   10: j
+    === No gap to skip
+    Modified regular file file1:
+       1    1: a
+       2    2: bB
+       3    3: c
+       4    4: d
+       5    5: e
+       6    6: f
+       7    7: g
+       8    8: h
+       9    9: iI
+      10   10: j
+    === No gap to skip
+    Modified regular file file1:
+       1    1: a
+       2    2: b
+       3    3: cC
+       4    4: d
+       5    5: e
+       6    6: f
+       7    7: g
+       8    8: h
+       9    9: iI
+      10   10: j
+    === 1 line at start
+    Modified regular file file1:
+       1    1: a
+       2    2: b
+       3    3: c
+       4    4: d
+       5    5: eE
+       6    6: f
+       7    7: g
+       8    8: h
+        ...
+    === 1 line at end
+    Modified regular file file1:
+        ...
+       3    3: c
+       4    4: d
+       5    5: e
+       6    6: fF
+       7    7: g
+       8    8: h
+       9    9: i
+      10   10: j
+    "###);
+}