feat: add srcset generation

README.md

@@ -81,3 +81,42 @@ echo $builder->createURL("bridge.png", $params);
 // Prints out:
 // http://demos.imgix.net/bridge.png?h=100&w=100&s=bb8f3a2ab832e35997456823272103a4
 ```
+
+## Srcset Generation
+
+The imgix library allows for generation of custom `srcset` attributes, which can be invoked through `createSrcSet()`. By default, the `srcset` generated will allow for responsive size switching by building a list of image-width mappings.
+
+```php
+$builder = new UrlBuilder("demos.imgix.net", true, "my-key", false);
+echo $builder->createSrcSet("bridge.png");
+```
+
+Will produce the following attribute value, which can then be served to the client:
+
+```html
+https://demos.imgix.net/bridge.png?w=100&s=ac331d314510e3039a33aa1a7ebc23ee 100w,
+https://demos.imgix.net/bridge.png?w=116&s=aac0667a00791c8c8801a2fef134e78a 116w,
+https://demos.imgix.net/bridge.png?w=134&s=2fcd42d984155efe26cc1e36e16b2897 134w,
+                                    ...
+https://demos.imgix.net/bridge.png?w=7400&s=6a6cbe01416dc4e0c65d1a2f87b868ac 7400w,
+https://demos.imgix.net/bridge.png?w=8192&s=9e6b0a94e81e929ad71829fcccf4d2d8 8192w
+```
+
+In cases where enough information is provided about an image's dimensions, `createSrcSet()` will instead build a `srcset` that will allow for an image to be served at different resolutions. The parameters taken into consideration when determining if an image is fixed-width are `w`, `h`, and `ar`. By invoking `createSrcSet()` with either a width **or** the height and aspect ratio (along with `fit=crop`, typically) provided, a different `srcset` will be generated for a fixed-size image instead.
+
+```php
+$builder = new UrlBuilder("demos.imgix.net", true, "my-key", false);
+echo $builder->createSrcSet("bridge.png", array("h"=>800, "ar"=>"3:2", "fit"=>"crop"));
+```
+
+Will produce the following attribute value:
+
+```html
+https://demos.imgix.net/bridge.png?ar=3%3A2&dpr=1&fit=crop&h=800&s=39eb37ad41acf7170343aa463424ae49 1x,
+https://demos.imgix.net/bridge.png?ar=3%3A2&dpr=2&fit=crop&h=800&s=a8ab13a2c7a17b91db42cb86e45f7c9d 2x,
+https://demos.imgix.net/bridge.png?ar=3%3A2&dpr=3&fit=crop&h=800&s=8fefe5daf312f04fb6912a101afbf704 3x,
+https://demos.imgix.net/bridge.png?ar=3%3A2&dpr=4&fit=crop&h=800&s=74a6167d6ef8ba410109feda814b9ac0 4x,
+https://demos.imgix.net/bridge.png?ar=3%3A2&dpr=5&fit=crop&h=800&s=4449b7f44ba7d6d0527a16d9a10b6e39 5x
+```
+
+For more information to better understand `srcset`, we highly recommend [Eric Portis' "Srcset and sizes" article](https://ericportis.com/posts/2014/srcset-sizes/) which goes into depth about the subject.

src/Imgix/UrlBuilder.php

@@ -9,6 +9,12 @@ class UrlBuilder {
     private $useHttps;
     private $signKey;
 
+    // define class constants
+    // should be private; but visibility modifiers are not supported php version <7.1
+    const TARGETRATIOS = array(1, 2, 3, 4, 5);
+    // constants cannot be dynamically assigned; keeping as a class variable instead
+    private $targetWidths;
+
     public function __construct($domain, $useHttps = false, $signKey = "", $includeLibraryParam = true) {
 
         if (!is_string($domain)) {
@@ -21,6 +27,7 @@ public function __construct($domain, $useHttps = false, $signKey = "", $includeL
         $this->useHttps = $useHttps;
         $this->signKey = $signKey;
         $this->includeLibraryParam = $includeLibraryParam;
+        $this->targetWidths = $this->targetWidths();
     }
 
     private function validateDomain($domain) {
@@ -57,4 +64,66 @@ public function createURL($path, $params=array()) {
 
         return $uh->getURL();
     }
+
+    public function createSrcSet($path, $params=array()) {
+        $width = array_key_exists('w', $params) ? $params['w'] : NULL;
+        $height = array_key_exists('h', $params) ? $params['h'] : NULL;
+        $aspectRatio = array_key_exists('ar', $params) ? $params['ar'] : NULL;
+
+        if (($width) || ($height && $aspectRatio)) {
+            return $this->createDPRSrcSet($path, $params);
+        }
+        else {
+            return $this->createSrcSetPairs($path, $params);
+        }
+    }
+
+    private function createDPRSrcSet($path, $params) {
+        $srcset = "";
+
+        $size = count(self::TARGETRATIOS);
+        for ($i = 0; $i < $size; $i++) {
+            $currentRatio = self::TARGETRATIOS[$i];
+            $currentParams = $params;
+            $currentParams['dpr'] = $i+1;
+            $srcset .= $this->createURL($path, $currentParams) . " " . $currentRatio . "x,\n";
+        }
+
+        return substr($srcset, 0, strlen($srcset) - 2);
+    }
+
+    private function createSrcSetPairs($path, $params) {
+        $srcset = "";
+        $currentWidth = NULL;
+        $currentParams = NULL;
+
+        $size = count($this->targetWidths);
+        for ($i = 0; $i < $size; $i++) {
+            $currentWidth = $this->targetWidths[$i];
+            $currentParams = $params;
+            $currentParams['w'] = $currentWidth;
+            $srcset .= $this->createURL($path, $currentParams) . " " . $currentWidth . "w,\n";
+        }
+
+        return substr($srcset, 0, strlen($srcset) - 2);
+    }
+
+    private function targetWidths() {
+        $resolutions = array();
+        $prev = 100;
+        $INCREMENT_PERCENTAGE = 8;
+        $MAX_SIZE = 8192;
+
+        $ensureEven = function($n) {
+            return 2 * round($n / 2);
+        };
+
+        while ($prev <= $MAX_SIZE) {
+            array_push($resolutions, $ensureEven($prev));
+            $prev *= 1 + ($INCREMENT_PERCENTAGE / 100) * 2;
+        }
+
+        array_push($resolutions, $MAX_SIZE);
+        return $resolutions;
+    }
 }