Store any string into level, and get a collision free hash of that value that you can use in an index or something.

For example if you put in the values hello, world, some very long string, hello, hello, and some very long string. What ends up being stored in level is this:

As you noticed, although some values were put in multiple times, they are only stored once. Each value is given a unique, collision free hash.

When making a query engine of some kind you often want to create indexes that stores the same values in several different arrangements. So instead of storing the full values multiple times in the indexes, you can instead just store the hashes, thus keeping the indexes from ballooning up in size.

var HashIndex = require('level-hash-index');
var h = HashIndex(db);// db is anything that exposes the levelUp API

h.putAndWrite("hello", function(err, hash){
  // hash === "23a4..."
  // ...
  // ...
  h.getHash(hash, function(err, val){
    // val === "hello"
  });
});

• db is your levelup db (i.e. level, sublevel, multilevel etc... )
• options.hashFn any hash function that takes in a string and returns a string
• options.index_prefix the level key = index_prefix + hash (default prefix is "hash!")

Given a value, get it's unique hash. The callback data is one of these two

• {hash: hash} - the hash of the value that's persisted to the db
• {hash: hash, is_new: true, key: db_key} - the hash of the value and it's raw db key that hasn't yet been persisted, but is the hash that will be used once it is persisted.

Given a value, get it's hash, and write to the db if it hasn't yet been persisted.

Given a value, get the hash for that value, if it has been persisted to the db. Uses level's NotFoundError if not found.

Given a hash get the value. Uses level's NotFoundError if not found.

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