Illustrator: calculate rotation angle at which segment becomes 27mm width

rotateToFitWidth.jsx

/**
  * @file calculate rotation angle at which segment becomes 27mm width
  * @version 1.0.0
  * @author sttk3.com
  * @copyright © 2023 sttk3.com
*/

(function() {
  // config
  var dstWidth = 27 ; // as mm

  if(app.documents.length <= 0) {return ;}
  var doc = app.documents[0] ;
  var sel = doc.selection ;
  if(sel.length <= 0) {return ;}

  // mm to pt
  var adjacent = new UnitValue(dstWidth, 'mm').as('pt') ;
  
  // if main item is not path item, exit
  var mainItem = sel[0] ;
  if(mainItem.constructor.name != 'PathItem') {return ;}

  // get first and last anchor
  var pathPoints = mainItem.pathPoints ;
  var anchorLength = pathPoints.length ;
  if(anchorLength < 2) {return ;}
  var origin = pathPoints[0].anchor ;
  var anchorA = pathPoints[anchorLength - 1].anchor ;

  // get length of hypotenuse. exit if destination value is not expected to be reached
  var hypotenuse = Math.sqrt( Math.pow(anchorA[0] - origin[0], 2) + Math.pow(anchorA[1] - origin[1], 2) ) ;
  if(hypotenuse < adjacent) {
    alert('Line needs to increase longer.') ;
    return ;
  }

  // vector direction of destination segment
  // adjacent: bottom width，hypotenuse: radius
  var dstAngle0 = Math.acos(adjacent / hypotenuse) ;

  // vector direction of current segment
  var currentAngle = angle2(origin, anchorA) ;

  // rotaion angle
  var dstAngle1 = getClosestAngle(dstAngle0, currentAngle) ;
  var rotaionAngle = getAngleDiff(dstAngle1, currentAngle) ;

  prompt('Rotaion angle', degrees(rotaionAngle)) ;
})() ;

/**
  * get angle from two anchor points
  * @param {Array} p0 start point [x, y]
  * @param {Array} p1 end point [x, y]
  * @return {Number} angle (radians)
*/
function angle2(p0, p1) {
  return Math.atan2(p1[1] - p0[1], p1[0] - p0[0]) ;
}

/**
  * get angle of minimum rotation
  * @param {Number} dstAngle destination angle
  * @param {Number} currentAngle src angle
  * @return {Number} angle (radians)
*/
function getClosestAngle(dstAngle, currentAngle) {
  var diff1 = Math.abs(dstAngle - currentAngle) ;
  var diff2 = Math.abs(dstAngle + Math.PI - currentAngle) ;
  var diff3 = Math.abs(dstAngle - Math.PI - currentAngle) ;

  var minDiff = Math.min(diff1, diff2, diff3) ;

  if(minDiff === diff1) {
    return dstAngle ;
  } else if (minDiff === diff2) {
    return dstAngle + Math.PI ;
  } else {
    return dstAngle - Math.PI ;
  }
}

/**
  * get difference between dstAngle and currentAngle
  * @param {Number} dstAngle destination angle
  * @param {Number} currentAngle src angle
  * @return {Number} angle (radians)
*/
function getAngleDiff(dstAngle, currentAngle) {
  var difference = dstAngle - currentAngle ;

  if(difference > Math.PI) {
    difference -= 2 * Math.PI ;
  } else if(difference < -Math.PI) {
    difference += 2 * Math.PI ;
  }

  return difference ;
}

/**
  * convert radians to degrees
  * @param {Number} rad radians
  * @return {Number} degrees
*/
function degrees(rad) {
  return rad * 180 / Math.PI ;
}