Fix parsing of non-extended file headers and lines starting with --

[mrnugget]

This fixes #54 by making the detection of non-extended file headers
(which start with --- directly) that was introduced in #53 more
robust.

Instead of simply aborting when the current line starts with ---
(which is a valid hunk line, if you, say, remove a line starting with
--), we confirm that the next line also starts with +++ by peeking a
bit ahead.

That's also what git does: https://sourcegraph.com/github.com/git/git/-/blob/apply.c#L1574-1576

[codecov]

Codecov Report

Merging #55 into master will decrease coverage by 1.88%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
- Coverage   75.99%   74.10%   -1.89%     
==========================================
  Files           4        4              
  Lines         454      421      -33     
==========================================
- Hits          345      312      -33     
+ Misses         63       62       -1     
- Partials       46       47       +1     

Impacted Files	Coverage Δ
diff/parse.go	81.16% <71.42%> (-1.04%)	⬇️
diff/print.go	35.29% <0.00%> (-6.82%)	⬇️
diff/diff.go	84.37% <0.00%> (-2.12%)	⬇️
diff/reader_util.go	84.61% <0.00%> (-2.06%)	⬇️

---

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[eseliger]

Cool, 2 inline questions

[eseliger]

Suggested change

	@@ -1,4 +1,3 @@
	select 1;
	--- this is my query
	select 2;
	select 3;
	@@ -1,4 +1,3 @@
	select 1;
	--- this is my query
	+++ this could break?
	select 2;
	select 3;

[eseliger]

Is that a case that works?

[mrnugget]

You mean, with the fix here or in general?

[eseliger]

With the logic above that checks the next hunk line for +++ and fails if it finds it.
Like when I change a line prefix from -- to ++, the diff would look like

line
---line
+++line
line

I guess. Would this be parseable?

[mrnugget]

If I use this diff, it crashes:

diff --git a/foobar.sql b/foobar.sql
index 9537d7b..f73c856 100644
--- a/foobar.sql
+++ b/foobar.sql
@@ -1,5 +1,5 @@
 select 1;
--- this is my query
+++ this is my query
 select 2;
 select 3;
--- this is the last line
+++ this is the last line

I'll try to see if I can make that work.

[mrnugget]

@keegancsmith @eseliger what do you think of this? #56

It adds a custom type, lineReader, so we can do two-line lookahead and check whether the --- and +++ are followed by a @@ , which is what git does. It's built on top of this PR and all tests pass, including the case raised here by @eseliger.

[keegancsmith]

some inline comments, not approving just yet :)

[mrnugget]

I'm merging this and will take another stab at fixing the --- foo\n+++ bar problem in a separate PR, since that behaviour was never supported and this PR here is to intended to fix and restore old behaviour.