GeezerVison

In context format (-c, -C), when a block of lines is replaced by a block of a different length, only as many lines as pair up get !; the rest are shown as plain deletions or insertions. GNU diff marks every line of the block with !:

$ printf 'a\nb\nc\nd\n' > foo; printf 'a\nx\nd\n' > bar
$ diff -c foo bar
GNU diff 3.12         uutils
*** 1,4 ****          *** 1,4 ****
  a                     a
! b                   - b
! c                   ! c
  d                     d
--- 1,3 ----          --- 1,3 ----
  a                     a
! x                   ! x
  d                     d

Both versions apply with patch, but the output differs from GNU's for any change that isn't line-for-line, and tools that read context diffs see a deletion plus a change instead of one change.

make_diff now tracks each run of changed lines with no common line in between. When the run both deletes and inserts lines, all of its lines on both sides become Change. Runs that only delete or only insert keep - and +. This replaces the one-to-one pairing through expected_lines_change_idx.

New test test_change_block_lines_all_changed (2 lines replaced by 1, 1 replaced by 3) fails without the change. cargo test: 287 passed, clippy unchanged, fmt clean. On 6000 random file pairs with 0, 1 and 3 context lines, wherever uutils -U output equals GNU's (the same alignment), -C output now also equals GNU's in all 14682 cases; before, 3381 of 7373 differed.