PARSER.md

Parser Deep Dive

The parser in TrailerCodecService.decode() walks backwards from the bottom of the commit message to efficiently locate the trailer block. This is faster than scanning each line from the top because trailers are guaranteed to live at the end of the message.

Steps

1. Normalize line endings to \n and split the message.
2. Consume the title (the first line) and drop the optional blank line that separates it from the body.
3. Walk backward with _findTrailerStartIndex until either a non-matching line or an empty line appears; contiguous Key: Value patterns form the trailer block.
4. Validate the separator: _validateTrailerSeparation ensures there is a blank line before the trailers. Messages that omit the blank line now throw TrailerNoSeparatorError.
5. Trim the body without double allocations: _trimBody trims leading/trailing blank lines via index arithmetic and one join.
6. Parse trailers using the schema bundle’s keyPattern, instantiating trailers via the injected trailerFactory.

The backward walk plus the blank-line guard ensure attackers cannot append arbitrary trailers to the body without the required separator.