code: func (idx *deltaIndex) findMatch(src, tgt []byte, tgtOffset int) (srcOffset, l int) {
  	if len(tgt) < tgtOffset+s {
  		return 0, len(tgt) - tgtOffset
  	}
  
  	if len(src) < blksz {
  		return 0, -1
  	}
  
  	if len(tgt) >= tgtOffset+s && len(src) >= blksz {
  		h := hashBlock(tgt, tgtOffset)
  		tIdx := h & idx.mask
  		eIdx := idx.table[tIdx]
  		if eIdx != 0 {
  			srcOffset = idx.entries[eIdx]
  		} else {
  			return
  		}
  
  		l = matchLength(src, tgt, tgtOffset, srcOffset)
  	}
  
  	return
  }
docs: // findMatch returns the offset of src where the block starting at tgtOffset
  // is and the length of the match. A length of 0 means there was no match. A
  // length of -1 means the src length is lower than the blksz and whatever
  // other positive length is the length of the match in bytes.