Re: Can I convince the diff algorithm to behave better?

[Thomas Braun <thomas.braun@virtuell-zuhause.de>]

On 3/3/2021 3:03 AM, Tom Ritter wrote:

Hi Tom,

> (For a specific, nuanced, and personal definition of better...)
> 
> I have a frequent behavior that arises when I am copy/pasting chunks
> of code, typically in tests.  Here is an example:
> 
> My Original code:
> 
> def function():
>    line 1
>    line 2
>    line 3
>    line 4
>    line 5
>    line 6
> 
> --------------------------------
> I add, after it:
> 
> def function2():
>    line 1
>    line 2
>    line 3
>    line 4
>    line 5
>    line 6
> 
> --------------------------------
> My diff is:
> 
> +   line 3
> +   line 4
> +   line 5
> +   line 6
> +
> +def function2():
> +   line 1
> +   line 2
> 
> --------------------------------
> I'd like my diff to be
> 
> +
> +def function2():
> +   line 1
> +   line 2
> +   line 3
> +   line 4
> +   line 5
> +   line 6

I tried to reproduce and got exactly the diff you wanted to have. I need
to add a newline after the first "line 4" to get the not-sought-for diff.

Commit:

+++ b/test.py
@@ -0,0 +1,7 @@
+def function():
+    line 1
+    line 2
+    line 3
+    line 4
+    line 5
+    line 6

and then the following change:

--- a/test.py
+++ b/test.py
@@ -3,5 +3,14 @@ def function():
     line 2
     line 3
     line 4
+
+    line 5
+    line 6
+
+def function2():
+    line 1
+    line 2
+    line 3
+    line 4
     line 5
     line 6

I usually play around with --anchored when I want to solve an issue like
that.

The documentation of anchored says

If a line exists in both the source and destination, exists only once,
and starts with this text, this algorithm attempts to prevent it from
appearing as a deletion or addition in the output. It uses the "patience
diff" algorithm internally.

But I can't get it working here as the "exists only once" premise is broken.

Stepping back: It might also make sense to rethink the code as repeating
the same 6 lines in every function might not be the best possible design.

Thomas

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