Bug: PGNs without headers can not be parsed

[Epigene]

Hi!

The gem (v0.2.0) seems unable to parse PGNs without headers, which seem optional.

Does not parse:

1. e4 a6 2. Bc4 a5 3. Ne2 a4 4. O-O Ra7
5. e5 f5 6. exf6 a5 0-1

Error:

Whittle::ParseError: Parse error: expected one of :$end, "[" but got :move_number_indication on line 1.

1. e4 a6 2. Bc4 a5 3. Ne2 a4 4. O-O Ra7
^ ... occurred here

Parses:

[Event "Live Chess"]

1. e4 a6 2. Bc4 a5 3. Ne2 a4 4. O-O Ra7
5. e5 f5 6. exf6 a5 0-1

[tobiasvl]

Well, the Seven Tag Roster tags aren't actually optional, although parsers should in general choose to be lax about stuff like that. See Wikipedia.

However, I agree that this gem seems to take a weird middle ground. Either it should require all seven STR tags, or not have any tag requirements at all. It seems strange that it needs the "Event" tag, of all tags (although I guess it just needs any arbitrary tag to be present).

[tobiasvl]

Scratch that. From section 8.1 of the PGN specification

The tag pair section is composed of a series of zero or more tag pairs.

So for the "import format" it's explicitly noted that the tags are optional, so this seems like a bug.