source: node_modules/d3-scale/src/ordinal.js@ e4c61dd

import {InternMap} from "d3-array";
import {initRange} from "./init.js";

export const implicit = Symbol("implicit");

export default function ordinal() {
  var index = new InternMap(),
      domain = [],
      range = [],
      unknown = implicit;

  function scale(d) {
    let i = index.get(d);
    if (i === undefined) {
      if (unknown !== implicit) return unknown;
      index.set(d, i = domain.push(d) - 1);
    }
    return range[i % range.length];
  }

  scale.domain = function(_) {
    if (!arguments.length) return domain.slice();
    domain = [], index = new InternMap();
    for (const value of _) {
      if (index.has(value)) continue;
      index.set(value, domain.push(value) - 1);
    }
    return scale;
  };

  scale.range = function(_) {
    return arguments.length ? (range = Array.from(_), scale) : range.slice();
  };

  scale.unknown = function(_) {
    return arguments.length ? (unknown = _, scale) : unknown;
  };

  scale.copy = function() {
    return ordinal(domain, range).unknown(unknown);
  };

  initRange.apply(scale, arguments);

  return scale;
}