context hunk merging in diff

[WayneD]

In playing with diffutils in the uutils playground web page, I noticed that -U1 on the shopping files was outputting a second hunk that has no prior context line. The official diff utility joins hunks together when the context overlaps or even just abuts.

My test:

uutils $ diff shopping-old.txt shopping-new.txt -U1
--- shopping-old.txt    2026-08-09 03:43:13.212000000 +0000
+++ shopping-new.txt    2026-08-09 03:43:13.212000000 +0000
@@ -2,2 +2,3 @@
 eggs
+yogurt
 bread
@@ -4,2 +5,2 @@
-butter
+honey
 apples

As another test, I added another line of context between the chunks:

uutils $ sed shopping-old.txt >s1 -e 's/bread/bread\nrolls/'
uutils $ sed shopping-new.txt >s2 -e 's/bread/bread\nrolls/'

Now the 1-line context is present on hunk 2, but the normal diff util would join them together since they abut:

uutils $ diff s1 s2 -u1
--- s1  1970-01-01 00:00:00.000000000 +0000
+++ s2  1970-01-01 00:00:00.000000000 +0000
@@ -2,2 +2,3 @@
 eggs
+yogurt
 bread
@@ -4,3 +5,3 @@
 rolls
-butter
+honey
 apples

At this point -U2 has the missing context in the same manner as in the first example.