source: node_modules/d3-interpolate/src/string.js@ e4c61dd

import number from "./number.js";

var reA = /[-+]?(?:\d+\.?\d*|\.?\d+)(?:[eE][-+]?\d+)?/g,
    reB = new RegExp(reA.source, "g");

function zero(b) {
  return function() {
    return b;
  };
}

function one(b) {
  return function(t) {
    return b(t) + "";
  };
}

export default function(a, b) {
  var bi = reA.lastIndex = reB.lastIndex = 0, // scan index for next number in b
      am, // current match in a
      bm, // current match in b
      bs, // string preceding current number in b, if any
      i = -1, // index in s
      s = [], // string constants and placeholders
      q = []; // number interpolators

  // Coerce inputs to strings.
  a = a + "", b = b + "";

  // Interpolate pairs of numbers in a & b.
  while ((am = reA.exec(a))
      && (bm = reB.exec(b))) {
    if ((bs = bm.index) > bi) { // a string precedes the next number in b
      bs = b.slice(bi, bs);
      if (s[i]) s[i] += bs; // coalesce with previous string
      else s[++i] = bs;
    }
    if ((am = am[0]) === (bm = bm[0])) { // numbers in a & b match
      if (s[i]) s[i] += bm; // coalesce with previous string
      else s[++i] = bm;
    } else { // interpolate non-matching numbers
      s[++i] = null;
      q.push({i: i, x: number(am, bm)});
    }
    bi = reB.lastIndex;
  }

  // Add remains of b.
  if (bi < b.length) {
    bs = b.slice(bi);
    if (s[i]) s[i] += bs; // coalesce with previous string
    else s[++i] = bs;
  }

  // Special optimization for only a single match.
  // Otherwise, interpolate each of the numbers and rejoin the string.
  return s.length < 2 ? (q[0]
      ? one(q[0].x)
      : zero(b))
      : (b = q.length, function(t) {
          for (var i = 0, o; i < b; ++i) s[(o = q[i]).i] = o.x(t);
          return s.join("");
        });
}