immutablejs源码中这个计算json对象哈希的的算法的怎么理解？

Question

immutablejs源码中这个计算json对象的的算法的怎么理解？为什么hashJSObj({a: 2})为1？它究竟有什么用，为了实现什么？，为什么实例里hash出来的是一个数字？

function hashJSObj(obj) {
  var hash;
  if (usingWeakMap) {
    hash = weakMap.get(obj);
    if (hash !== undefined) {
      return hash;
    }
  }
  hash = obj[UID_HASH_KEY];
  if (hash !== undefined) {
    return hash;
  }
  if (!canDefineProperty) {
    hash = obj.propertyIsEnumerable && obj.propertyIsEnumerable[UID_HASH_KEY];
    if (hash !== undefined) {
      return hash;
    }

    hash = getIENodeHash(obj);
    if (hash !== undefined) {
      return hash;
    }
  }

  hash = ++objHashUID;
  if (objHashUID & 0x40000000) {
    objHashUID = 0;
  }

  if (usingWeakMap) {
    weakMap.set(obj, hash);
  } else if (isExtensible !== undefined && isExtensible(obj) === false) {
    throw new Error('Non-extensible objects are not allowed as keys.');
  } else if (canDefineProperty) {
    Object.defineProperty(obj, UID_HASH_KEY, {
      'enumerable': false,
      'configurable': false,
      'writable': false,
      'value': hash
    });
  } else if (obj.propertyIsEnumerable !== undefined &&
             obj.propertyIsEnumerable === obj.constructor.prototype.propertyIsEnumerable) {
    // Since we can't define a non-enumerable property on the object
    // we'll hijack one of the less-used non-enumerable properties to
    // save our hash on it. Since this is a function it will not show up in
    // `JSON.stringify` which is what we want.
    obj.propertyIsEnumerable = function() {
      return this.constructor.prototype.propertyIsEnumerable.apply(this, arguments);
    };
    obj.propertyIsEnumerable[UID_HASH_KEY] = hash;
  } else if (obj.nodeType !== undefined) {
    // At this point we couldn't get the IE `uniqueID` to use as a hash
    // and we couldn't use a non-enumerable property to exploit the
    // dontEnum bug so we simply add the `UID_HASH_KEY` on the node
    // itself.
    obj[UID_HASH_KEY] = hash;
  } else {
    throw new Error('Unable to set a non-enumerable property on object.');
  }

  return hash;
}

// v8 has an optimization（优化组合） for storing 31-bit signed numbers（有正负符号数）.
// Values which have either 00 or 11 as the high order bits（高字节位） qualify（限定）.
// This function drops the highest order bit in a signed number（有正负符号数）, maintaining（坚持，保卫）
// the sign bit（符号位）.
function smi(i32) {
  return ((i32 >>> 1) & 0x40000000) | (i32 & 0xBFFFFFFF);
}