log.js

import {ticks} from "d3-array";
import {format, formatSpecifier} from "d3-format";
import nice from "./nice.js";
import {copy, transformer} from "./continuous.js";
import {initRange} from "./init.js";

function transformLog(x) {
  return Math.log(x);
}

function transformExp(x) {
  return Math.exp(x);
}

function transformLogn(x) {
  return -Math.log(-x);
}

function transformExpn(x) {
  return -Math.exp(-x);
}

function pow10(x, k) {
  return isFinite(x) ? +(k + "e" + x) : x < 0 ? 0 : x;
}

function exp(x, k) {
  return Math.exp(x) * k;
}

function powp(base) {
  return base === 10 ? pow10
      : base === Math.E ? exp
      : (x, k) => Math.pow(base, x) * k;
}

function logp(base) {
  return base === Math.E ? Math.log
      : base === 10 && Math.log10
      || base === 2 && Math.log2
      || (base = Math.log(base), (x) => Math.log(x) / base);
}

function reflect(f) {
  return (x, k) => -f(-x, k);
}

export function loggish(transform) {
  const scale = transform(transformLog, transformExp);
  const domain = scale.domain;
  let base = 10;
  let logs;
  let pows;

  function rescale() {
    logs = logp(base), pows = powp(base);
    if (domain()[0] < 0) {
      logs = reflect(logs), pows = reflect(pows);
      transform(transformLogn, transformExpn);
    } else {
      transform(transformLog, transformExp);
    }
    return scale;
  }

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

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

  scale.ticks = count => {
    const d = domain();
    let u = d[0];
    let v = d[d.length - 1];
    const r = v < u;

    if (r) ([u, v] = [v, u]);

    let i = logs(u);
    let j = logs(v);
    let k;
    let t;
    const n = count == null ? 10 : +count;
    let z = [];

    if (!(base % 1) && j - i < n) {
      i = Math.floor(i), j = Math.ceil(j);
      if (u > 0) for (; i <= j; ++i) {
        for (k = 1; k < base; ++k) {
          t = pows(i, k);
          if (t < u) continue;
          if (t > v) break;
          z.push(t);
        }
      } else for (; i <= j; ++i) {
        for (k = base - 1; k >= 1; --k) {
          t = pows(i, k);
          if (t < u) continue;
          if (t > v) break;
          z.push(t);
        }
      }
      if (z.length * 2 < n) z = ticks(u, v, n);
    } else {
      z = ticks(i, j, Math.min(j - i, n)).map(i => pows(i, 1));
    }

    return r ? z.reverse() : z;
  };

  scale.tickFormat = (count, specifier) => {
    if (count == null) count = 10;
    if (specifier == null) specifier = base === 10 ? ".0e" : ",";
    if (typeof specifier !== "function") {
      if (!(base % 1) && (specifier = formatSpecifier(specifier)).precision == null) specifier.trim = true;
      specifier = format(specifier);
    }
    if (count === Infinity) return specifier;
    const k = Math.max(1, base * count / scale.ticks().length); // TODO fast estimate?
    return d => {
      let i = d / pows(Math.round(logs(d)), 1);
      if (i * base < base - 0.5) i *= base;
      return i <= k ? specifier(d) : "";
    };
  };

  scale.nice = () => {
    return domain(nice(domain(), {
      floor: x => pows(Math.floor(logs(x)), 1),
      ceil: x => pows(Math.ceil(logs(x)), 1)
    }));
  };

  return scale;
}

export default function log() {
  const scale = loggish(transformer()).domain([1, 10]);
  scale.copy = () => copy(scale, log()).base(scale.base());
  initRange.apply(scale, arguments);
  return scale;
}