decode yields higher precision coordinates

[Fischerfredl]

Hi all,

we use a copied version of https://github.com/heremaps/flexible-polyline/blob/master/javascript/index.js in our codebase and I just noticed that the decode function returns higher precision values than expected (more decimal places). I guess this happens due to the assumption that adding two numbers of a certain order won't generate a lower order. But this is not true for IEEE 754 Floating-Point Arithmetic. Classic example:

16:39 $ node
Welcome to Node.js v16.14.0.
Type ".help" for more information.
> 0.1 + 0.2
0.30000000000000004
> 

I stumbled upon this as another library (polygon-clipping) was generating erroneous MultiPolygons when it was fed Polygons with the high precision. The following patch fixed this for us:

--- a/index.js
+++ b/index.js
@@ -33,6 +33,8 @@ function decode(encoded) {
 
   const factorDegree = 10 ** header.precision
   const factorZ = 10 ** header.thirdDimPrecision
+  const truncateDegree = num => Math.round(num * factorDegree) / factorDegree
+  const truncateZ = num => Math.round(num * factorZ) / factorZ
   const { thirdDim } = header
 
   let lastLat = 0
@@ -50,10 +52,10 @@ function decode(encoded) {
     if (thirdDim) {
       const deltaZ = toSigned(decoder[i + 2]) / factorZ
       lastZ += deltaZ
-      res.push([lastLat, lastLng, lastZ])
+      res.push([truncateDegree(lastLat), truncateDegree(lastLng), truncateZ(lastZ)])
       i += 3
     } else {
-      res.push([lastLat, lastLng])
+      res.push([truncateDegree(lastLat), truncateDegree(lastLng)])
       i += 2
     }
   }

This works for us as we only read Polylines from a third party API (HERE Isoline API) but I don't know if this is sufficient for a encoding/decoding setting. Maybe one should truncate the lastLat/lastLng/lastZ as you go for performance.

I just wanted to leave this here for a heads up if anyone stumbles upon a similar error.