Storage format is simple: Zstandard-compressed list of records:

* 16-bit BE size of the following name
* entity (file, directory, symbolic link, etc) name itself.
  Directory has trailing "/"
* single byte indicating current file's depth
* 64-bit BE mtime seconds
* 64-bit BE file or directory (sum of all files and directories) size

Index algorithm:

* traverse over all filesystem hierarchy in a *sorted* order. All
  records are written to temporary file, without directory sizes,
  because they are not known in advance during the walk
* during the walk, remember in memory each directory's total size
* read all records from that temporary file, writing to another one,
  replacing directory sizes with ones remembered

Search is trivial:

* searching is performed on each record streamed from the database
* if -root is specified, then search will stop after that hierarchy
  part is over
* by default all elements are printed, unless you provide a single
  argument that becomes "*X*" pattern matched on case-lowered path
  elements

Update algorithm:

* read all [-+MR] actions from "zfs diff -FH", validating the whole
  format
* each "R" for the file becomes "-" and "+" actions
* if there are "R"s for directories, then stream current database and
  get each file entity for those directories, making "-" and "+"
  actions correspondingly
* each "+" also adds an entry to the list of "M"s
* sort all "-", "+" and "M" filenames in ascending order
* get entity's information for each "M" (remembering its size and mtime)
* stream current database records, writing them to temporary file,
  taking into account, that:
  * if record exists in "-"-list, then skip it
  * if any "+" exists in the *sorted* list, that has precedence over
    the record from database, then insert it into the stream, taking
    size and mtime information from "M"-list
  * if any "M" exists for the read record, then use it to alter it
* all that time, directory size calculating algorithm, same used during
  the index procedure, also works in parallel
* create another temporary file to copy the records with actualized
  directory sizes