(function (global, factory) {
    typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
    typeof define === 'function' && define.amd ? define(factory) :
    (global.OceanLayers = factory());
}(this, (function () { 'use strict';

  let cells, vertices, pointsN, used;

  const OceanLayers = function OceanLayers() {
    const outline = oceanLayers.attr("layers");
    if (outline === "none") return;
    TIME && console.time("drawOceanLayers");

    lineGen.curve(d3.curveBasisClosed);
    cells = grid.cells, pointsN = grid.cells.i.length, vertices = grid.vertices;
    const limits = outline === "random" ? randomizeOutline() : outline.split(",").map(s => +s);

    const chains = [];
    const opacity = rn(0.4 / limits.length, 2);
    used = new Uint8Array(pointsN); // to detect already passed cells

    for (const i of cells.i) {
      const t = cells.t[i];
      if (t > 0) continue;
      if (used[i] || !limits.includes(t)) continue;
      const start = findStart(i, t);
      if (!start) continue;
      used[i] = 1;
      const chain = connectVertices(start, t); // vertices chain to form a path
      if (chain.length < 4) continue;
      const relax = 1 + t * -2; // select only n-th point
      const relaxed = chain.filter((v, i) => !(i%relax) || vertices.c[v].some(c => c >= pointsN));
      if (relaxed.length < 4) continue;
      const points = clipPoly(relaxed.map(v => vertices.p[v]), 1);
      chains.push([t, points]); 
    }

    for (const t of limits) {
      const layer = chains.filter(c => c[0] === t);
      let path = layer.map(c => round(lineGen(c[1]))).join("");
      if (path) oceanLayers.append("path").attr("d", path).attr("fill", "#ecf2f9").style("opacity", opacity);
    }

    // find eligible cell vertex to start path detection
    function findStart(i, t) {
      if (cells.b[i]) return cells.v[i].find(v => vertices.c[v].some(c => c >= pointsN)); // map border cell
      return cells.v[i][cells.c[i].findIndex(c => cells.t[c] < t || !cells.t[c])];
    }

    TIME && console.timeEnd("drawOceanLayers");
  }

  function randomizeOutline() {
    const limits = [];
    let odd = .2
    for (let l = -9; l < 0; l++) {
      if (P(odd)) {odd = .2; limits.push(l);}
      else {odd *= 2;}
    }
    return limits;
  }

  // connect vertices to chain
  function connectVertices(start, t) {
    const chain = []; // vertices chain to form a path
    for (let i=0, current = start; i === 0 || current !== start && i < 10000; i++) {
      const prev = chain[chain.length - 1]; // previous vertex in chain
      chain.push(current); // add current vertex to sequence
      const c = vertices.c[current]; // cells adjacent to vertex
      c.filter(c => cells.t[c] === t).forEach(c => used[c] = 1);
      const v = vertices.v[current]; // neighboring vertices
      const c0 = !cells.t[c[0]] || cells.t[c[0]] === t-1;
      const c1 = !cells.t[c[1]] || cells.t[c[1]] === t-1;
      const c2 = !cells.t[c[2]] || cells.t[c[2]] === t-1;
      if (v[0] !== undefined && v[0] !== prev && c0 !== c1) current = v[0];
      else if (v[1] !== undefined && v[1] !== prev && c1 !== c2) current = v[1];
      else if (v[2] !== undefined && v[2] !== prev && c0 !== c2) current = v[2];
      if (current === chain[chain.length - 1]) {ERROR && console.error("Next vertex is not found"); break;}
    }
    chain.push(chain[0]); // push first vertex as the last one
    return chain;
  }

  return OceanLayers;

})));