From 76eac04bf3670b3131d96558221df663f4f085e8 Mon Sep 17 00:00:00 2001
From: Bart Nagel <bart@tremby.net>
Date: Wed, 4 Nov 2020 07:03:05 -0800
Subject: [PATCH] fix(buildSrcSet): ensure largest size can't be repeated

The largest size was being repeated in certain conditions, since the
value stored in `prev` was exact (possibly fractional) while the number
saved in the list of resolutions is rounded to an even number, but the
value stored in `prev` was the one being compared to `maxWidth`.

In this fix we remove the code which pushes `maxWidth` to the list
*after* the loop finishes and compare the last value in the list to
`maxWidth` rather than the running exact fractional width value. These
together eliminate the areas where the bug can subtly appear. It is then
necessary to refactor the loop, by initializing the list to already
contain the `minWidth` and then exiting early if `maxWidth` is equal
(which has the side benefit that the cache won't be polluted with
trivial cases) and then in the loop body push the *new* rather than
previous value to the list, after rounding and clamping.
---
 src/imgix-core-js.js | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/src/imgix-core-js.js b/src/imgix-core-js.js
index 56398af9b..2f9bf01f5 100644
--- a/src/imgix-core-js.js
+++ b/src/imgix-core-js.js
@@ -224,10 +224,15 @@
 
     // returns an array of width values used during scrset generation
     ImgixClient.prototype._generateTargetWidths = function(widthTolerance, minWidth, maxWidth) {
-      var resolutions = [];
+      var resolutions = [minWidth];
       var INCREMENT_PERCENTAGE = widthTolerance;
       var minWidth = Math.floor(minWidth);
       var maxWidth = Math.floor(maxWidth);
+
+      if (minWidth === maxWidth) {
+        return resolutions;
+      }
+
       var cacheKey = INCREMENT_PERCENTAGE + '/' + minWidth + '/' + maxWidth;
 
       if (cacheKey in this.targetWidthsCache) {
@@ -238,14 +243,12 @@
         return 2 * Math.round(n / 2);
       };
 
-      var prev = minWidth;
-      while (prev < maxWidth) {
-        resolutions.push(ensureEven(prev));
-        prev *= 1 + (INCREMENT_PERCENTAGE * 2);
+      var exact = minWidth;
+      while (resolutions[resolutions.length - 1] < maxWidth) {
+        exact *= 1 + (INCREMENT_PERCENTAGE * 2);
+        resolutions.push(Math.min(ensureEven(exact), maxWidth));
       }
 
-      resolutions.push(maxWidth);
-
       this.targetWidthsCache[cacheKey] = resolutions;
 
       return resolutions;