Off-by-one in unified diff with zero context and no deleted lines

[aray-openai]

It looks like the leading line number is off by one in at least this case, where there are no deleted lines, and there is zero context.

Example: (note the hunk header is -1,0 +2,3

$> cat /tmp/a
hello
alice
$> cat /tmp/b
hello
my
name
is
alice
$> diff -U 0 /tmp/a /tmp/b
--- /tmp/a	2020-08-27 18:06:40.000000000 -0700
+++ /tmp/b	2020-08-27 18:06:47.000000000 -0700
@@ -1,0 +2,3 @@
+my
+name
+is

And the same comparison with JsDiff -- notice the hunk header is -2,0 +2,3

$> node
> const JsDiff = require('diff')
> const fs = require('fs')
> var a = fs.readFileSync('/tmp/a', 'utf8')
> var b = fs.readFileSync('/tmp/b', 'utf8')
> JsDiff.createTwoFilesPatch('a', 'b', a, b, '', '', {'context':0})
'===================================================================\n' +
  '--- a\t\n' +
  '+++ b\t\n' +
  '@@ -2,0 +2,3 @@\n' +
  '+my\n' +
  '+name\n' +
  '+is\n'

[aray-openai]

If JsDiff applies the results from diff then it produces incorrect results:

$> diff -U 0 /tmp/a /tmp/b > /tmp/diff

$> cat /tmp/diff
--- /tmp/a	2020-08-27 18:06:40.000000000 -0700
+++ /tmp/b	2020-08-27 18:06:47.000000000 -0700
@@ -1,0 +2,3 @@
+my
+name
+is

$> node
> const JsDiff = require('diff')
> const fs = require('fs')
> var a = fs.readFileSync('/tmp/a', 'utf8')
> var b = fs.readFileSync('/tmp/b', 'utf8')
> var d = fs.readFileSync('/tmp/diff', 'utf8')
> var jd = JsDiff.createTwoFilesPatch('a', 'b', a, b, '', '', {'context':0})
> jd
'===================================================================\n' +
  '--- a\t\n' +
  '+++ b\t\n' +
  '@@ -2,0 +2,3 @@\n' +
  '+my\n' +
  '+name\n' +
  '+is\n'
> d
'--- /tmp/a\t2020-08-27 18:06:40.000000000 -0700\n' +
  '+++ /tmp/b\t2020-08-27 18:06:47.000000000 -0700\n' +
  '@@ -1,0 +2,3 @@\n' +
  '+my\n' +
  '+name\n' +
  '+is\n'
> JsDiff.applyPatch
[Function: applyPatch]
> JsDiff.applyPatch(a, d)
'my\nname\nis\nhello\nalice\n'
> JsDiff.applyPatch(a, b)
'hello\nalice\n'
> JsDiff.applyPatch(a, jd)
'hello\nmy\nname\nis\nalice\n'

JsDiff does produce correct results when applying diffs made with createTwoFilesPatch.

Is the expectation that JsDiff is consistent with diff/patch here?

[aray-openai]

More context

If the chunk size is 0, the first number is one lower than one would expect (it is the line number after which the chunk should be inserted or deleted; in all other cases it gives the first line number or the replaced range of lines).

From https://www.artima.com/weblogs/viewpost.jsp?thread=164293

Which is the best description I've found so far explaining this behavior from patch/diff