func newReader()

in tsdb/index/index.go [1086:1192]
91 lines of code
21 McCabe index (conditional complexity)

func newReader(b ByteSlice, c io.Closer) (*Reader, error) {
	r := &Reader{
		b:        b,
		c:        c,
		postings: map[string][]postingOffset{},
	}

	// Verify header.
	if r.b.Len() < HeaderLen {
		return nil, errors.Wrap(encoding.ErrInvalidSize, "index header")
	}
	if m := binary.BigEndian.Uint32(r.b.Range(0, 4)); m != MagicIndex {
		return nil, errors.Errorf("invalid magic number %x", m)
	}
	r.version = int(r.b.Range(4, 5)[0])

	if r.version != FormatV1 && r.version != FormatV2 {
		return nil, errors.Errorf("unknown index file version %d", r.version)
	}

	var err error
	r.toc, err = NewTOCFromByteSlice(b)
	if err != nil {
		return nil, errors.Wrap(err, "read TOC")
	}

	r.symbols, err = NewSymbols(r.b, r.version, int(r.toc.Symbols))
	if err != nil {
		return nil, errors.Wrap(err, "read symbols")
	}

	if r.version == FormatV1 {
		// Earlier V1 formats don't have a sorted postings offset table, so
		// load the whole offset table into memory.
		r.postingsV1 = map[string]map[string]uint64{}
		if err := ReadOffsetTable(r.b, r.toc.PostingsTable, func(key []string, off uint64, _ int) error {
			if len(key) != 2 {
				return errors.Errorf("unexpected key length for posting table %d", len(key))
			}
			if _, ok := r.postingsV1[key[0]]; !ok {
				r.postingsV1[key[0]] = map[string]uint64{}
				r.postings[key[0]] = nil // Used to get a list of labelnames in places.
			}
			r.postingsV1[key[0]][key[1]] = off
			return nil
		}); err != nil {
			return nil, errors.Wrap(err, "read postings table")
		}
	} else {
		var lastKey []string
		lastOff := 0
		valueCount := 0
		// For the postings offset table we keep every label name but only every nth
		// label value (plus the first and last one), to save memory.
		if err := ReadOffsetTable(r.b, r.toc.PostingsTable, func(key []string, _ uint64, off int) error {
			if len(key) != 2 {
				return errors.Errorf("unexpected key length for posting table %d", len(key))
			}
			if _, ok := r.postings[key[0]]; !ok {
				// Next label name.
				r.postings[key[0]] = []postingOffset{}
				if lastKey != nil {
					// Always include last value for each label name.
					r.postings[lastKey[0]] = append(r.postings[lastKey[0]], postingOffset{value: lastKey[1], off: lastOff})
				}
				lastKey = nil
				valueCount = 0
			}
			if valueCount%32 == 0 {
				r.postings[key[0]] = append(r.postings[key[0]], postingOffset{value: key[1], off: off})
				lastKey = nil
			} else {
				lastKey = key
				lastOff = off
			}
			valueCount++
			return nil
		}); err != nil {
			return nil, errors.Wrap(err, "read postings table")
		}
		if lastKey != nil {
			r.postings[lastKey[0]] = append(r.postings[lastKey[0]], postingOffset{value: lastKey[1], off: lastOff})
		}
		// Trim any extra space in the slices.
		for k, v := range r.postings {
			l := make([]postingOffset, len(v))
			copy(l, v)
			r.postings[k] = l
		}
	}

	r.nameSymbols = make(map[uint32]string, len(r.postings))
	for k := range r.postings {
		if k == "" {
			continue
		}
		off, err := r.symbols.ReverseLookup(k)
		if err != nil {
			return nil, errors.Wrap(err, "reverse symbol lookup")
		}
		r.nameSymbols[off] = k
	}

	r.dec = &Decoder{LookupSymbol: r.lookupSymbol}

	return r, nil
}