Hasher

hashing~ Hasher

Source:

The Hasher in­ter­face is used to im­ple­ment cus­tom hash func­tions and con­struct hash­ers with spe­cial be­ha­viours.

It is a reg­u­lar old pro­tocol spe­cify­ing which meth­ods/​prop­er­ties must be im­ple­men­ted. It is not a trait.

Im­ple­ment­a­tions

  • De­faultHasher is the stand­ard hash func­tion used by fer­rum
  • Un­ordered­Hasher is a Hasher wrap­per that makes sure .up­date() calls are pro­cessed in such a way so that the sub­mis­sion or­der does not mat­ter.

Mem­bers

Ad­apter that treats un­defined as null

Be­cause Hash­ers are called for every ele­ment in the hash tree like a vis­itor, they can be used to im­ple­ment spe­cial be­ha­vi­ors.

In this ex­ample we im­ple­ment a gen­eric hasher that ap­plies a func­tion to each ele­ment be­ing hashed, be­fore hash­ing. This al­lows re­pla­cing val­ues and more com­plex trans­form­a­tions.

const as­sert = re­quire('assert');
const {
  de­fault­Build­Hasher, curry, hash­With, bytes2hex, builder,
  ran­dom­Build­Hasher, type, Hash­able,
} = re­quire('fer­rum');

class Un­defi­nedIs­Null­Hasher {
  con­structor(build­Hasher = de­fault­Build­Hasher()) {
    Ob­ject.as­sign(this, {
      // The build­Hasher prop­erty must be provided; this is
      // used in some spe­cial cases, like un­ordered hash­ing
      build­Hasher: () => Un­defi­nedIs­Null­Hasher.new(build­Hasher),
      _in­ner: build­Hasher(),
    })
  }

  up­date(v) {
    // Uint8Ar­ray is our base type for hash­ing; all other types
    // re­duce to Uint8Ar­ray
    if(type(v) === Uint8Ar­ray) {
      this._in­ner.up­date(v);
    } else {
      // Re­place each un­defined with null
      Hash­able.in­voke(v === un­defined ? null : v, this);
    }
    re­turn this;
  }

  di­gest() {
    re­turn this._in­ner.di­gest();
  }
};

Un­defi­nedIs­Null­Hasher.new = curry('Un­defi­nedIs­Null­Hasher.new',
  builder(Un­defi­nedIs­Null­Hasher));

const r = ran­dom­Build­Hasher();
const h = hash­With(() => Un­defi­nedIs­Null­Hasher.new());
as­sert.stric­tEqual(
  bytes2hex(h([[[null]]])),
  bytes2hex(h([[[un­defined]]])));

Soph­ist­ic­ated hasher in­teg­rat­ing ob­ject hash

Fer­rum re­quires you to im­ple­ment Hash­able for each type; you could get around this by im­ple­ment­ing a hasher that in­teg­rates the ob­ject-hash lib­rary which can ar­bit­rary types. You could also in­teg­rate the lib­rary by us­ing Hash­able.im­pl­Wild, but this Ver­sion is less dan­ger­ous as it does­n't change the be­ha­viour glob­ally.

const liboh = re­quire('ob­ject-hash');
const as­sert = re­quire('assert');
const {
  de­fault­Build­Hasher, curry, hash­With, bytes2hex, Hash­Set,
  as­ser­tEquals, Hash­able, val­ue­Sup­ports, builder, Equals,
  ran­dom­Build­Hasher, type,
} = re­quire('fer­rum');

class LibO­H­Hasher {
  con­structor(build­Hasher) {
    Ob­ject.as­sign(this, {
      build­Hasher: () => LibO­H­Hasher.new(build­Hasher),
      _in­ner: build­Hasher(),
    });
  }

  up­date(v) {
    // Uint8Ar­ray is our base type for hash­ing; all other types
    // re­duce to Uint8Ar­ray
    if(type(v) === Uint8Ar­ray) {
      this._in­ner.up­date(v);
      re­turn this;
    }

    // Check if a fer­rum base im­ple­ment­a­tion ex­ists
    const impl = Hash­able.look­up­Value(v);
    if (impl) {
      impl(v, this);
      re­turn this;
    }

    // No fer­rum im­ple­ment­a­tion, fall­back to ob­ject hash;
    // provid­ing a re­pla­cer makes sure we try to use fer­rum
    // hash­ing for in­ner val­ues be­fore fall­ing back to OH again
    this._in­ner.up­date(new Uint8Ar­ray(liboh(v, {
      en­cod­ing: 'buf­fer',
      re­pla­cer: (w) =>
        val­ue­Sup­ports(w, Hash­able)
          ? bytes2hex(hash­With(w, this.build­Hasher))
          : w})));
    re­turn this;
  }

  di­gest() {
    re­turn this._in­ner.di­gest();
  }
};

LibO­H­Hasher.new = curry('LibO­H­Hasher.new', builder(LibO­H­Hasher));

const libo­hRan­dom­Build­Hasher = () => {
  const r = ran­dom­Build­Hasher();
  re­turn () => LibO­H­Hasher.new(r);
};

class Un­sup­por­ted­Type {
  con­structor(fields) {
    Ob­ject.as­sign(this, fields);
  }
};
Un­sup­por­ted­Type.new = curry('Un­sup­por­ted­Type.new', builder(Un­sup­por­ted­Type));

class Sup­por­ted­Type {
  [Hash­able.sym](hasher) {
    hasher.up­date(42); // So we can test this
  }
  // eq(a, b) <=> hash(a) = hash(B)
  [Equals.sym](otr) {
    re­turn type(otr) === Sup­por­ted­Type;
  }
};
Sup­por­ted­Type.new = curry('Sup­por­ted­Type.new', builder(Sup­por­ted­Type));

// A nor­mal Hash­Set will not be able to store our cus­tom type
as­sert.throws(() =>
  Hash­Set.from­Seq([Un­sup­por­ted­Type.new({})]));

// But we can sub­sti­tute our cus­tom hasher which will be able to use ob­ject-hash
// as a fall­back (us­ing cur­ry­ing here)
const libO­h­Hash­Set­From­Seq = Hash­Set.from­SeqWith­Opts({
  gen­Build­Hasher: libo­hRan­dom­Build­Hasher
});

const hm = libO­h­Hash­Set­From­Seq([
  Un­sup­por­ted­Type.new({}),
  Un­sup­por­ted­Type.new({}), // Same as pre­vi­ous
  Un­sup­por­ted­Type.new({ bar: Sup­por­ted­Type.new() }),
  Un­sup­por­ted­Type.new({ bar: 42 }), // Same as pre­vi­ous
]);
as­sert.stric­tEqual(hm.size, 2);

Ver­sion his­tory

  • 1.9.0 Ini­tial im­ple­ment­a­tion