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

import {bisect} from "d3-array";
import {interpolate as interpolateValue, interpolateNumber, interpolateRound} from "d3-interpolate";
import constant from "./constant.js";
import number from "./number.js";

var unit = [0, 1];

export function identity(x) {
  return x;
}

function normalize(a, b) {
  return (b -= (a = +a))
      ? function(x) { return (x - a) / b; }
      : constant(isNaN(b) ? NaN : 0.5);
}

function clamper(a, b) {
  var t;
  if (a > b) t = a, a = b, b = t;
  return function(x) { return Math.max(a, Math.min(b, x)); };
}

// normalize(a, b)(x) takes a domain value x in [a,b] and returns the corresponding parameter t in [0,1].
// interpolate(a, b)(t) takes a parameter t in [0,1] and returns the corresponding range value x in [a,b].
function bimap(domain, range, interpolate) {
  var d0 = domain[0], d1 = domain[1], r0 = range[0], r1 = range[1];
  if (d1 < d0) d0 = normalize(d1, d0), r0 = interpolate(r1, r0);
  else d0 = normalize(d0, d1), r0 = interpolate(r0, r1);
  return function(x) { return r0(d0(x)); };
}

function polymap(domain, range, interpolate) {
  var j = Math.min(domain.length, range.length) - 1,
      d = new Array(j),
      r = new Array(j),
      i = -1;

  // Reverse descending domains.
  if (domain[j] < domain[0]) {
    domain = domain.slice().reverse();
    range = range.slice().reverse();
  }

  while (++i < j) {
    d[i] = normalize(domain[i], domain[i + 1]);
    r[i] = interpolate(range[i], range[i + 1]);
  }

  return function(x) {
    var i = bisect(domain, x, 1, j) - 1;
    return r[i](d[i](x));
  };
}

export function copy(source, target) {
  return target
      .domain(source.domain())
      .range(source.range())
      .interpolate(source.interpolate())
      .clamp(source.clamp())
      .unknown(source.unknown());
}

export function transformer() {
  var domain = unit,
      range = unit,
      interpolate = interpolateValue,
      transform,
      untransform,
      unknown,
      clamp = identity,
      piecewise,
      output,
      input;

  function rescale() {
    var n = Math.min(domain.length, range.length);
    if (clamp !== identity) clamp = clamper(domain[0], domain[n - 1]);
    piecewise = n > 2 ? polymap : bimap;
    output = input = null;
    return scale;
  }

  function scale(x) {
    return x == null || isNaN(x = +x) ? unknown : (output || (output = piecewise(domain.map(transform), range, interpolate)))(transform(clamp(x)));
  }

  scale.invert = function(y) {
    return clamp(untransform((input || (input = piecewise(range, domain.map(transform), interpolateNumber)))(y)));
  };

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

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

  scale.rangeRound = function(_) {
    return range = Array.from(_), interpolate = interpolateRound, rescale();
  };

  scale.clamp = function(_) {
    return arguments.length ? (clamp = _ ? true : identity, rescale()) : clamp !== identity;
  };

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

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

  return function(t, u) {
    transform = t, untransform = u;
    return rescale();
  };
}

export default function continuous() {
  return transformer()(identity, identity);
}