grammar KeepAChangelog
  rule changelog
    changelog_header unreleased_section releases_section diff_section?
  end

  rule changelog_header
    '# Changelog' new_line
    'All notable changes to this project will be documented in this file.' new_line
    new_line
    keep_a_changelog_format '.'? new_line
    (semver_convention new_line)?
    new_line
  end

  rule keep_a_changelog_format
    'The format is based on [Keep a Changelog](http' 's'? '://keepachangelog.com/en/1.0.0/)'
  end

  rule semver_convention
    'and this project adheres to [Semantic Versioning](http' 's'? '://semver.org/spec/v2.0.0.html).'
  end

  rule unreleased_section
    unreleased_header change_section? (new_line change_section)*
  end

  rule unreleased_header
    '## ' '['? 'Unreleased' ']'? new_line
  end

  rule change_section
    change_header change+
  end

  rule change_header
    '### ' ('Added' / 'Changed' / 'Deprecated' / 'Removed' / 'Fixed' / 'Security') new_line
  end

  rule change
    ('- ' / '  ') (!new_line .)+ new_line?
  end

  rule releases_section
    release*
  end

  rule release
    new_line release_header change_section? (new_line change_section)*
  end

  rule release_header
    '## ' '['? release_version ']'? ' - ' release_date new_line
  end

  rule release_version
    [0-9]+ '.' [0-9]+ '.' [0-9]+ pre_release_version metadata
  end

  rule pre_release_version
    ( '-' [0-9A-Za-z-]+ dot_separated_group* ) / ''
  end

  rule metadata
    ( '+' [0-9A-Za-z-]+ dot_separated_group*  ) / ''
  end

  rule dot_separated_group
    '.' [0-9A-Za-z-]+
  end

  rule release_date
    [0-9] [0-9] [0-9] [0-9] '-' [0-9] [0-9] '-' [0-9] [0-9]
  end

  rule diff_section
    new_line unreleased_diff release_diff*  initial_release?
  end

  rule unreleased_diff
    '[Unreleased]: ' release_url new_line
  end

  rule initial_release
    '[' release_version ']: ' 'https://github.com/' (!'/' .)+ '/' (!'/' .)+ '/releases/tag/v' release_version new_line
  end

  rule release_diff
    '[' release_version ']: ' release_url new_line
  end

  rule release_url
    'https://github.com/' (!'/' .)+ '/' (!'/' .)+ '/compare/' diff_version '...' diff_version
  end

  rule diff_version
    'HEAD' / ( 'v' release_version )
  end
        
  rule new_line
    "\n"
  end
end