reproject-line/reproject-line.js

Re-project a line.

function reproject_line(coords, reproject, options) {
  const debug_level =
    typeof options === "object" && typeof options.debug_level === "number"
      ? options.debug_level
      : 0;
  let densify =
    typeof options === "object" && typeof options.densify === "number"
      ? options.densify
      : 0;
  const strategy =
    typeof options === "object" && typeof options.strategy === "string"
      ? options.strategy
      : "auto";

  // just in case densify isn't a round number
  densify = Math.round(densify);

  if (debug_level >= 1) {
    console.log("[reproject-line] debug_level:", debug_level);
    console.log("[reproject-line] densify:", densify);
    console.log("[reproject-line] strategy:", strategy);
  }

  // algorithm
  // drop point when the slope changes (and at the end)
  const out = [];

  let [xprev, yprev] = reproject(coords[0]);
  let mprev = null;
  let m = null;

  for (let i = 1; i < coords.length; i++) {
    const [x1, y1] = coords[i - 1];
    const [x2, y2] = coords[i];

    const xdist = x2 - x1;
    const ydist = y2 - y1;

    const xstep = xdist / (densify + 1);
    const ystep = ydist / (densify + 1);

    for (let ii = 1; ii <= densify; ii++) {
      const [rx, ry] = reproject([x1 + ii * xstep, y1 + ii * ystep]);
      m = (ry - yprev) / (rx - xprev);

      if (strategy === "always" || m !== mprev) {
        out.push([xprev, yprev]);
        mprev = m;
      }
      xprev = rx;
      yprev = ry;
    }

    // try with last coord in segment
    const [rx2, ry2] = reproject([x2, y2]);
    m = (ry2 - yprev) / (rx2 - xprev);

    // if slope changes, drop point
    if (strategy === "always" || m !== mprev) {
      out.push([xprev, yprev]);
      mprev = m;
    }

    xprev = rx2;
    yprev = ry2;
  }

  // drop last point
  out.push([xprev, yprev]);

  if (debug_level >= 3) {
    console.log("[reproject-line] returning", out);
  }

  return out;
}

if (typeof define === "function" && define.amd) {
  define(function () {
    return reproject_line;
  });
}

if (typeof module === "object") {
  module.exports = reproject_line;
  module.exports.default = reproject_line;
}