feat: add srcset generation

README.md

@@ -68,5 +68,51 @@ using Imgix;
 var builder = new UrlBuilder("domain.imgix.net", includeLibraryParam: false);
 ```
 
+Srcset Generation
+-----------
+
+The imgix-csharp library allows for generation of custom `srcset` attributes, which can be invoked through `BuildSrcSet()`. By default, the `srcset` generated will allow for responsive size switching by building a list of image-width mappings.
+
+```csharp
+var builder = new UrlBuilder("domain.imgix.net", "my-token", false, true);
+String srcset = builder.BuildSrcSet("bridge.png");
+Debug.Print(srcset);
+```
+
+Will produce the following attribute value, which can then be served to the client:
+
+```html
+https://domain.imgix.net/bridge.png?w=100&s=494158d968e94ac8e83772ada9a83ad1 100w,
+https://domain.imgix.net/bridge.png?w=116&s=6a22236e189b6a9548b531330647ffa7 116w,
+https://domain.imgix.net/bridge.png?w=134&s=cbf91f556dd67c0b9e26cb9784a83794 134w,
+                                            ...
+https://domain.imgix.net/bridge.png?w=7400&s=503e3ba04588f1c301863c9a5d84fe91 7400w,
+https://domain.imgix.net/bridge.png?w=8192&s=152551ce4ec155f7a03f60f762a1ca33 8192w
+```
+
+In cases where enough information is provided about an image's dimensions, `BuildSrcSet()` 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` (width), `h` (height), and `ar` (aspect ratio). By invoking `BuildSrcSet()` 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.
+
+```csharp
+var builder = new UrlBuilder("domain.imgix.net", "my-token", false, true);
+var parameters = new Dictionary<String, String>();
+parameters["h"] = "200";
+parameters["ar"] = "3:2";
+parameters["fit"] = "crop";
+String srcset = builder.BuildSrcSet("bridge.png", parameters);
+Console.WriteLine(srcset);
+```
+
+Will produce the following attribute value:
+
+```html
+https://domain.imgix.net/bridge.png?h=200&ar=3%3A2&fit=crop&dpr=1&s=f39a78a6a2f245a70ba6aac910088435 1x,
+https://domain.imgix.net/bridge.png?h=200&ar=3%3A2&fit=crop&dpr=2&s=d5dfd75bd777283d82975ab18a3091ff 2x,
+https://domain.imgix.net/bridge.png?h=200&ar=3%3A2&fit=crop&dpr=3&s=8f25811130e3573530754c52f86a851d 3x,
+https://domain.imgix.net/bridge.png?h=200&ar=3%3A2&fit=crop&dpr=4&s=ec348479a843a688c2ef9be487ea9be8 4x,
+https://domain.imgix.net/bridge.png?h=200&ar=3%3A2&fit=crop&dpr=5&s=ce70bbfd682e683497f1afa6118ae2e3 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.
+
 ## Code of Conduct
 Users contributing to or participating in the development of this project are subject to the terms of imgix's [Code of Conduct](https://github.com/imgix/code-of-conduct).

src/Imgix/UrlBuilder.cs

@@ -14,9 +14,10 @@ public class UrlBuilder
 
         private String _signKey;
         public String SignKey { set { _signKey = value; } }
-
         private String Domain;
 
+        private static readonly List<int> SRCSET_TARGET_WIDTHS = GenerateTargetWidths();
+
         public UrlBuilder(String domain,
                           String signKey = null,
                           Boolean includeLibraryParam = true,
@@ -59,6 +60,29 @@ public String BuildUrl(String path, Dictionary<String, String> parameters)
             return GenerateUrl(Domain, path, parameters);
         }
 
+        public String BuildSrcSet(String path)
+        {
+            return BuildSrcSet(path, new Dictionary<string, string>());
+        }
+
+        public String BuildSrcSet(String path, Dictionary<String, String> parameters)
+        {
+            String srcset;
+            parameters.TryGetValue("w", out String width);
+            parameters.TryGetValue("h", out String height);
+            parameters.TryGetValue("ar", out String aspectRatio);
+
+            if ((width != null) || (height != null && aspectRatio != null))
+            {
+                srcset = GenerateSrcSetDPR(Domain, path, parameters);
+            }
+            else
+            {
+                srcset = GenerateSrcSetPairs(Domain, path, parameters);
+            }
+
+            return srcset;
+        }
 
         private String GenerateUrl(String domain, String path, Dictionary<String, String> parameters)
         {
@@ -77,6 +101,52 @@ private String GenerateUrl(String domain, String path, Dictionary<String, String
             return String.Format("{0}://{1}/{2}{3}", scheme, domain, path, localParams.Any() ? "?" + GenerateUrlStringFromDict(localParams) : String.Empty);
         }
 
+        private String GenerateSrcSetDPR(String domain, String path, Dictionary<String, String> parameters)
+        {
+            String srcset = "";
+            int[] targetRatios = { 1, 2, 3, 4, 5 };
+
+            foreach(int ratio in targetRatios)
+            {
+                parameters["dpr"] = ratio.ToString();
+                parameters.Remove("ixlib");
+                srcset += BuildUrl(path, parameters) + " " + ratio.ToString()+ "x,\n";
+            }
+
+            return srcset.Substring(0, srcset.Length - 2);
+        }
+
+        private String GenerateSrcSetPairs(String domain, String path, Dictionary<String, String> parameters)
+        {
+            String srcset = "";
+
+            foreach(int width in SRCSET_TARGET_WIDTHS)
+            {
+                parameters["w"] = width.ToString();
+                parameters.Remove("ixlib");
+                srcset += BuildUrl(path, parameters) + " " + width + "w,\n";
+            }
+
+            return srcset.Substring(0, srcset.Length - 2);
+        }
+
+        private static List<int> GenerateTargetWidths()
+        {
+            List<int> resolutions = new List<int>();
+            int MAX_SIZE = 8192, roundedPrev;
+            double SRCSET_PERCENT_INCREMENT = 8;
+            double prev = 100;
+
+            while (prev < MAX_SIZE)
+            {
+                roundedPrev = (int)(2 * Math.Round(prev / 2));
+                resolutions.Add(roundedPrev);
+                prev *= 1 + (SRCSET_PERCENT_INCREMENT / 100) * 2;
+            }
+            resolutions.Add(MAX_SIZE);
+
+            return resolutions;
+        }
 
         private String HashString(String input)
         {