iD.Entity = function(a, b, c) {
    if (!(this instanceof iD.Entity)) return new iD.Entity(a, b, c);

    this.tags = {};
    this.transients = {};

    var sources = [a, b, c], source;
    for (var i = 0; i < sources.length; ++i) {
        source = sources[i];
        for (var prop in source) {
            if (prop !== 'transients' && Object.prototype.hasOwnProperty.call(source, prop)) {
                this[prop] = source[prop];
            }
        }
    }

    if (!this.id && this.type) {
        this.id = iD.Entity.id(this.type);
        this._updated = true;
    }

    if (iD.debug) {
        Object.freeze(this);
        Object.freeze(this.tags);

        if (this.nodes) Object.freeze(this.nodes);
        if (this.members) Object.freeze(this.members);
    }
};

iD.Entity.id = function (type) {
    return iD.Entity.id.fromOSM(type, iD.Entity.id.next[type]--);
};

iD.Entity.id.next = {node: -1, way: -1, relation: -1};

iD.Entity.id.fromOSM = function (type, id) {
    return type[0] + id;
};

iD.Entity.id.toOSM = function (id) {
    return +id.slice(1);
};

iD.Entity.prototype = {
    osmId: function() {
        return iD.Entity.id.toOSM(this.id);
    },

    update: function(attrs) {
        return iD.Entity(this, attrs, {_updated: true});
    },

    created: function() {
        return this._updated && this.osmId() < 0;
    },

    modified: function() {
        return this._updated && this.osmId() > 0;
    },

    intersects: function(extent, resolver) {
        if (this.type === 'node') {
            return this.loc[0] > extent[0][0] &&
                   this.loc[0] < extent[1][0] &&
                   this.loc[1] < extent[0][1] &&
                   this.loc[1] > extent[1][1];
        } else if (this.type === 'way') {
            var _extent = this.transients.extent;

            if (!_extent) {
                _extent = this.transients.extent = [[-Infinity, Infinity], [Infinity, -Infinity]];
                for (var i = 0, l = this.nodes.length; i < l; i++) {
                    var node = resolver.entity(this.nodes[i]);
                    if (node.loc[0] > _extent[0][0]) _extent[0][0] = node.loc[0];
                    if (node.loc[0] < _extent[1][0]) _extent[1][0] = node.loc[0];
                    if (node.loc[1] < _extent[0][1]) _extent[0][1] = node.loc[1];
                    if (node.loc[1] > _extent[1][1]) _extent[1][1] = node.loc[1];
                }
            }

            return _extent[0][0] > extent[0][0] &&
                   _extent[1][0] < extent[1][0] &&
                   _extent[0][1] < extent[0][1] &&
                   _extent[1][1] > extent[1][1];
        } else {
            return false;
        }
    },

    hasInterestingTags: function() {
        return _.keys(this.tags).some(function (key) {
            return key != "attribution" &&
                key != "created_by" &&
                key != "source" &&
                key != 'odbl' &&
                key.indexOf('tiger:') !== 0;
        });
    }
};

iD.Node = function(attrs) {
    return iD.Entity(attrs || {}, {type: 'node'});
};

iD.Way = function(attrs) {
    return iD.Entity({nodes: []}, attrs || {}, {type: 'way'});
};

iD.Way.isOneWay = function(d) {
    return !!(d.tags.oneway && d.tags.oneway === 'yes');
};

iD.Way.isClosed = function(d) {
    return (!d.nodes.length) || d.nodes[d.nodes.length - 1].id === d.nodes[0].id;
};

iD.Way.isArea = function(d) {
    return iD.Way.isClosed(d) || (d.tags.area && d.tags.area === 'yes');
};

iD.Relation = function(attrs) {
    return iD.Entity({members: []}, attrs || {}, {type: 'relation'});
};