From 2b7e7b8dbd43a49c2f09a89036a860463fb7e14a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stephan=20B=C3=B6sch-Plepelits?= <skunk@xover.mud.at>
Date: Sat, 2 May 2020 18:35:44 +0200
Subject: [PATCH] Parameter 'lineOffset': offset line to the left or right.

---
 README.md                  |  1 +
 src/L.PolylineDecorator.js |  1 +
 src/patternUtils.js        | 18 ++++++++++++------
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index 09e3481..a2fc8c8 100644
--- a/README.md
+++ b/README.md
@@ -52,6 +52,7 @@ Property | Type | Required | Description
 `offset`| *see below* | No | Offset of the first pattern symbol, from the start point of the line. Default: 0.
 `endOffset`| *see below* | No | Minimum offset of the last pattern symbol, from the end point of the line. Default: 0.
 `repeat`| *see below* | Yes | Repetition interval of the pattern symbols. Defines the distance between each consecutive symbol's anchor point.
+`lineOffset` | number (pixels) | No | Offset line to the left (negative value) or the right (positive value). Default: 0.
 `symbol`| Symbol factory | Yes | Instance of a symbol factory class.
 
 `offset`, `endOffset` and `repeat` can be each defined as a number, in pixels, or in percentage of the line's length, as a string (ex: `'10%'`).
diff --git a/src/L.PolylineDecorator.js b/src/L.PolylineDecorator.js
index 307cef7..d4e9353 100644
--- a/src/L.PolylineDecorator.js
+++ b/src/L.PolylineDecorator.js
@@ -85,6 +85,7 @@ L.PolylineDecorator = L.FeatureGroup.extend({
             offset: parseRelativeOrAbsoluteValue(patternDef.offset),
             endOffset: parseRelativeOrAbsoluteValue(patternDef.endOffset),
             repeat: parseRelativeOrAbsoluteValue(patternDef.repeat),
+            lineOffset: patternDef.lineOffset
         };
     },
 
diff --git a/src/patternUtils.js b/src/patternUtils.js
index 7b0e44c..8a9c1d9 100644
--- a/src/patternUtils.js
+++ b/src/patternUtils.js
@@ -62,6 +62,7 @@ function projectPatternOnPointPath(pts, pattern) {
     const repeatIntervalPixels = totalPathLength * repeat;
     const startOffsetPixels = offset > 0 ? totalPathLength * offset : 0;
     const endOffsetPixels = endOffset > 0 ? totalPathLength * endOffset : 0;
+    const lineOffset = pattern.lineOffset || 0;
 
     // 2. generate the positions of the pattern as offsets from the path start
     const positionOffsets = [];
@@ -84,7 +85,7 @@ function projectPatternOnPointPath(pts, pattern) {
 
         const segmentRatio = (positionOffset - segment.distA) / (segment.distB - segment.distA);
         return {
-            pt: interpolateBetweenPoints(segment.a, segment.b, segmentRatio),
+            pt: interpolateBetweenPoints(segment.a, segment.b, segmentRatio, lineOffset, segment.distB - segment.distA),
             heading: segment.heading,
         };
     });
@@ -94,17 +95,22 @@ function projectPatternOnPointPath(pts, pattern) {
 * Finds the point which lies on the segment defined by points A and B,
 * at the given ratio of the distance from A to B, by linear interpolation.
 */
-function interpolateBetweenPoints(ptA, ptB, ratio) {
+function interpolateBetweenPoints(ptA, ptB, ratio, lineOffset, length) {
+    let n = {x: 0, y: 0}
+    if (lineOffset !== 0) {
+      n = {x: - (ptB.y - ptA.y) / length, y: (ptB.x - ptA.x) / length}
+    }
+
     if (ptB.x !== ptA.x) {
         return {
-            x: ptA.x + ratio * (ptB.x - ptA.x),
-            y: ptA.y + ratio * (ptB.y - ptA.y),
+            x: ptA.x + ratio * (ptB.x - ptA.x) + n.x * lineOffset,
+            y: ptA.y + ratio * (ptB.y - ptA.y) + n.y * lineOffset
         };
     }
     // special case where points lie on the same vertical axis
     return {
-        x: ptA.x,
-        y: ptA.y + (ptB.y - ptA.y) * ratio,
+        x: ptA.x + n.x * lineOffset,
+        y: ptA.y + (ptB.y - ptA.y) * ratio + n.y * lineOffset,
     };
 }