source: node_modules/d3-shape/src/stack.js

import array from "./array.js";
import constant from "./constant.js";
import offsetNone from "./offset/none.js";
import orderNone from "./order/none.js";

function stackValue(d, key) {
  return d[key];
}

function stackSeries(key) {
  const series = [];
  series.key = key;
  return series;
}

export default function() {
  var keys = constant([]),
      order = orderNone,
      offset = offsetNone,
      value = stackValue;

  function stack(data) {
    var sz = Array.from(keys.apply(this, arguments), stackSeries),
        i, n = sz.length, j = -1,
        oz;

    for (const d of data) {
      for (i = 0, ++j; i < n; ++i) {
        (sz[i][j] = [0, +value(d, sz[i].key, j, data)]).data = d;
      }
    }

    for (i = 0, oz = array(order(sz)); i < n; ++i) {
      sz[oz[i]].index = i;
    }

    offset(sz, oz);
    return sz;
  }

  stack.keys = function(_) {
    return arguments.length ? (keys = typeof _ === "function" ? _ : constant(Array.from(_)), stack) : keys;
  };

  stack.value = function(_) {
    return arguments.length ? (value = typeof _ === "function" ? _ : constant(+_), stack) : value;
  };

  stack.order = function(_) {
    return arguments.length ? (order = _ == null ? orderNone : typeof _ === "function" ? _ : constant(Array.from(_)), stack) : order;
  };

  stack.offset = function(_) {
    return arguments.length ? (offset = _ == null ? offsetNone : _, stack) : offset;
  };

  return stack;
}