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

  const generate = function Rivers() {
    console.time('generateRivers');
    Math.seedrandom(seed);
    const cells = pack.cells, p = cells.p, features = pack.features;
    features.forEach(f => {delete f.river; delete f.flux;});

    const riversData = []; // rivers data
    cells.fl = new Uint16Array(cells.i.length); // water flux array
    cells.r = new Uint16Array(cells.i.length); // rivers array
    cells.conf = new Uint8Array(cells.i.length); // confluences array
    let riverNext = 1; // first river id is 1, not 0

    void function drainWater() {
      const land = cells.i.filter(isLand).sort(highest);
    
      land.forEach(function(i) {
        cells.fl[i] += grid.cells.prec[cells.g[i]]; // flux from precipitation
        const x = p[i][0], y = p[i][1];

        // near-border cell: pour out of the screen
        if (cells.b[i]) {
          if (cells.r[i]) {
            const to = [];
            const min = Math.min(y, graphHeight - y, x, graphWidth - x);
            if (min === y) {to[0] = x; to[1] = 0;} else 
            if (min === graphHeight - y) {to[0] = x; to[1] = graphHeight;} else 
            if (min === x) {to[0] = 0; to[1] = y;} else 
            if (min === graphWidth - x) {to[0] = graphWidth; to[1] = y;}
            riversData.push({river: cells.r[i], cell: i, x: to[0], y: to[1]});
          }
          return;
        }

        const min = cells.c[i][d3.scan(cells.c[i], (a, b) => cells.h[a] - cells.h[b])]; // downhill cell
    
        // allow only one river can flow thought a lake
        const cf = features[cells.f[i]]; // current cell feature
        if (cf.river && cf.river !== cells.r[i]) {
          cells.fl[i] = 0; 
        }
    
        if (cells.fl[i] < 30) {
          if (cells.h[min] >= 20) cells.fl[min] += cells.fl[i];
          return; // flux is too small to operate as river
        }
    
        // Proclaim a new river
        if (!cells.r[i]) {
          cells.r[i] = riverNext;
          riversData.push({river: riverNext, cell: i, x, y});
          riverNext++;
        }
    
        if (cells.r[min]) { // downhill cell already has river assigned
          if (cells.fl[min] < cells.fl[i]) {
            cells.conf[min] = cells.fl[min];  // confluence
            cells.r[min] = cells.r[i]; // re-assign river if downhill part has less flux
          } else cells.conf[min] += cells.fl[i]; // confluence
        } else cells.r[min] = cells.r[i]; // assign the river to the downhill cell

        const nx = p[min][0], ny = p[min][1];
        if (cells.h[min] < 20) {
          // pour water to the sea haven
          riversData.push({river: cells.r[i], cell: cells.haven[i], x: nx, y: ny});
        } else {
          const mf = features[cells.f[min]]; // feature of min cell
          if (mf.type === "lake") {
            if (!mf.river || cells.fl[i] > mf.flux) {
              mf.river = cells.r[i]; // pour water to temporaly elevated lake
              mf.flux = cells.fl[i]; // entering flux
            }
          }
          cells.fl[min] += cells.fl[i]; // propagate flux
          riversData.push({river: cells.r[i], cell: min, x: nx, y: ny}); // add next River segment
        }

      });
    }()
  
    void function drawRivers() {
      const riverPaths = []; // to store data for all rivers

      for (let r = 1; r <= riverNext; r++) {
        const riverSegments = riversData.filter(d => d.river === r);
    
        if (riverSegments.length > 2) {
          const riverEnhanced = addMeandring(riverSegments);
          const width = rn(0.8 + Math.random() * 0.4, 1); // river width modifier
          const increment = rn(0.8 + Math.random() * 0.6, 1); // river bed widening modifier
          const path = getPath(riverEnhanced, width, increment);
          riverPaths.push([r, path, width, increment]);
        } else {
          // remove too short rivers
          riverSegments.filter(s => cells.r[s.cell] === r).forEach(s => cells.r[s.cell] = 0);
        }
      }

      rivers.selectAll("path").remove();
      rivers.selectAll("path").data(riverPaths).enter()
        .append("path").attr("d", d => d[1]).attr("id", d => "river"+d[0])
        .attr("data-width", d => d[2]).attr("data-increment", d => d[3]);
    }()
  
    console.timeEnd('generateRivers');
  }

  // add more river points on 1/3 and 2/3 of length
  const addMeandring = function(segments, rndFactor = 0.3) {
    const riverEnhanced = []; // to store enhanced segments
    let side = 1; // to control meandring direction
  
    for (let s = 0; s < segments.length; s++) {
      const sX = segments[s].x, sY = segments[s].y; // segment start coordinates
      const c = pack.cells.conf[segments[s].cell] || 0; // if segment is river confluence
      riverEnhanced.push([sX, sY, c]);
  
      if (s+1 === segments.length) break; // do not enhance last segment
  
      const eX = segments[s+1].x, eY = segments[s+1].y; // segment end coordinates
      const angle = Math.atan2(eY - sY, eX - sX);
      const sin = Math.sin(angle), cos = Math.cos(angle);
      const serpentine = 1 / (s + 1) + 0.3;
      const meandr = serpentine + Math.random() * rndFactor;
      if (Math.random() < 0.5) side *= -1; // change meandring direction in 50%
      const dist2 = (eX - sX) ** 2 + (eY - sY) ** 2;
      // if dist2 is big or river is small add extra points at 1/3 and 2/3 of segment
      if (dist2 > 64 || (dist2 > 16 && segments.length < 6)) {
        const p1x = (sX * 2 + eX) / 3 + side * -sin * meandr;
        const p1y = (sY * 2 + eY) / 3 + side * cos * meandr;
        if (Math.random() < 0.2) side *= -1; // change 2nd extra point meandring direction in 20%
        const p2x = (sX + eX * 2) / 3 + side * sin * meandr;
        const p2y = (sY + eY * 2) / 3 + side * cos * meandr;
        riverEnhanced.push([p1x, p1y], [p2x, p2y]);
      // if dist is medium or river is small add 1 extra middlepoint
      } else if (dist2 > 16 || segments.length < 6) {
        const p1x = (sX + eX) / 2 + side * -sin * meandr;
        const p1y = (sY + eY) / 2 + side * cos * meandr;
        riverEnhanced.push([p1x, p1y]);
      }
  
    }
    return riverEnhanced;
  }

  const getPath = function(points, width = 1, increment = 1) {
    let offset, extraOffset = .1; // starting river width (to make river source visible)  
    const riverLength = points.reduce((s, v, i, p) => s + (i ? Math.hypot(v[0] - p[i-1][0], v[1] - p[i-1][1]) : 0), 0); // summ of segments length
    const widening = rn((1000 + (riverLength * 30)) * increment);
    const riverPointsLeft = [], riverPointsRight = []; // store points on both sides to build a valid polygon
    const last = points.length - 1;
    const factor = riverLength / points.length;

    // first point
    let x = points[0][0], y = points[0][1], c;
    let angle = Math.atan2(y - points[1][1], x - points[1][0]);
    let sin = Math.sin(angle), cos = Math.cos(angle);
    let xLeft = x + -sin * extraOffset, yLeft = y + cos * extraOffset;
    riverPointsLeft.push([xLeft, yLeft]);
    let xRight = x + sin * extraOffset, yRight = y + -cos * extraOffset;
    riverPointsRight.unshift([xRight, yRight]);
  
    // middle points
    for (let p = 1; p < last; p++) {
      x = points[p][0], y = points[p][1], c = points[p][2] || 0;
      const xPrev = points[p-1][0], yPrev = points[p - 1][1];
      const xNext = points[p+1][0], yNext = points[p + 1][1];
      angle = Math.atan2(yPrev - yNext, xPrev - xNext);
      sin = Math.sin(angle), cos = Math.cos(angle);
      offset = (Math.atan(Math.pow(p * factor, 2) / widening) / 2 * width) + extraOffset;
      const confOffset = Math.atan(c * 5 / widening);
      extraOffset += confOffset;
      xLeft = x + -sin * offset, yLeft = y + cos * (offset + confOffset);
      riverPointsLeft.push([xLeft, yLeft]);
      xRight = x + sin * offset, yRight = y + -cos * offset;
      riverPointsRight.unshift([xRight, yRight]);
    }
  
    // end point
    x = points[last][0], y = points[last][1], c = points[last][2];
    if (c) extraOffset += Math.atan(c * 10 / widening); // add extra width on river confluence
    angle = Math.atan2(points[last-1][1] - y, points[last-1][0] - x);
    sin = Math.sin(angle), cos = Math.cos(angle);
    xLeft = x + -sin * offset, yLeft = y + cos * offset;
    riverPointsLeft.push([xLeft, yLeft]);
    xRight = x + sin * offset, yRight = y + -cos * offset;
    riverPointsRight.unshift([xRight, yRight]);
  
    // generate polygon path and return
    lineGen.curve(d3.curveCatmullRom.alpha(0.1));
    const right = lineGen(riverPointsRight);
    let left = lineGen(riverPointsLeft);
    left = left.substring(left.indexOf("C"));
    return round(right + left, 2);
  }

  return {generate, addMeandring, getPath};

})));