// Function that appends a path to selection that has sankey path data attached
// The path is formatted as dash array, and triangle paths to create arrows along the path
function appendArrows (selection, arrowLength, gapLength, arrowHeadSize) {
    let totalDashArrayLength = arrowLength + gapLength
  
    arrows = selection
      .append('path')
      .attr('d', function(d){
      	return d.path;
    	})
      .style('stroke-width', 1)
      .style('stroke', 'black')
      .style('stroke-dasharray', arrowLength + ',' + gapLength)
  
    arrows.each(function (arrow) {
      let thisPath = d3.select(this).node()
      let parentG =  d3.select(this.parentNode)
      let pathLength = thisPath.getTotalLength()
      let numberOfArrows = Math.ceil(pathLength / totalDashArrayLength)
  
      // remove the last arrow head if it will overlap the target node
      if (
        (numberOfArrows - 1) * totalDashArrayLength +
          (arrowLength + (arrowHeadSize + 1)) >
        pathLength
      ) {
        numberOfArrows = numberOfArrows - 1
      }
  
      let arrowHeadData = d3.range(numberOfArrows).map(function (d, i) {
        let length = i * totalDashArrayLength + arrowLength
  
        let point = thisPath.getPointAtLength(length)
        let previousPoint = thisPath.getPointAtLength(length - 2)
  
        let rotation = 0
  
        if (point.y == previousPoint.y) {
          rotation = point.x < previousPoint.x ? 180 : 0
        } else if (point.x == previousPoint.x) {
          rotation = point.y < previousPoint.y ? -90 : 90
        } else {
          let adj = Math.abs(point.x - previousPoint.x)
          let opp = Math.abs(point.y - previousPoint.y)
          let angle = Math.atan(opp / adj) * (180 / Math.PI)
          if (point.x < previousPoint.x) {
            angle = angle + (90 - angle) * 2
          }
          if (point.y < previousPoint.y) {
            rotation = -angle
          } else {
            rotation = angle
          }
        }
  
        return { x: point.x, y: point.y, rotation: rotation }
      })
  
      let arrowHeads = parentG
        .selectAll('.arrow-heads')
        .data(arrowHeadData)
        .enter()
        .append('path')
        .attr('d', function (d) {
          return (
            'M' +
            d.x +
            ',' +
            (d.y - arrowHeadSize / 2) +
            ' ' +
            'L' +
            (d.x + arrowHeadSize) +
            ',' +
            d.y +
            ' ' +
            'L' +
            d.x +
            ',' +
            (d.y + arrowHeadSize / 2)
          )
        })
        .attr('class', 'arrow-head')
        .attr('transform', function (d) {
          return 'rotate(' + d.rotation + ',' + d.x + ',' + d.y + ')'
        })
        .style('fill', 'black')
    })
  }