You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: difflib.go
    path of file: ./repos/darktile/vendor/github.com/pmezard/go-difflib/difflib
    
    the code of the file until where you have to start completion: // Package difflib is a partial port of Python difflib module.
//
// It provides tools to compare sequences of strings and generate textual diffs.
//
// The following class and functions have been ported:
//
// - SequenceMatcher
//
// - unified_diff
//
// - context_diff
//
// 
    
func (m *SequenceMatcher) chainB() {
	// Populate line -> index mapping
	b2j := map[string][]int{}
	for i, s := range m.b {
		indices := b2j[s]
		indices = append(indices, i)
		b2j[s] = indices
	}

	// Purge junk elements
	m.bJunk = map[string]struct{}{}
	if m.IsJunk != nil {
		junk := m.bJunk
		for s, _ := range b2j {
			if m.IsJunk(s) {
				junk[s] = struct{}{}
			}
		}
		for s, _ := range junk {
			delete(b2j, s)
		}
	}

	// Purge remaining popular elements
	popular := map[string]struct{}{}
	n := len(m.b)
	if m.autoJunk && n >= 200 {
		ntest := n/100 + 1
		for s, indices := range b2j {
			if len(indices) > ntest {
				popular[s] = struct{}{}
			}
		}
		for s, _ := range popular {
			delete(b2j, s)
		}
	}
	m.bPopular = popular
	m.b2j = b2j
}