Add URL decode filter

src/main/java/com/hubspot/jinjava/lib/filter/FilterLibrary.java

@@ -89,6 +89,7 @@ protected void registerDefaults() {
       ForceEscapeFilter.class,
       StripTagsFilter.class,
       UrlEncodeFilter.class,
+      UrlDecodeFilter.class,
       XmlAttrFilter.class,
       EscapeJsonFilter.class,
       EscapeJinjavaFilter.class,

src/main/java/com/hubspot/jinjava/lib/filter/UrlDecodeFilter.java

@@ -0,0 +1,75 @@
+package com.hubspot.jinjava.lib.filter;
+
+import com.hubspot.jinjava.doc.annotations.JinjavaDoc;
+import com.hubspot.jinjava.doc.annotations.JinjavaParam;
+import com.hubspot.jinjava.doc.annotations.JinjavaSnippet;
+import com.hubspot.jinjava.interpret.JinjavaInterpreter;
+import java.io.UnsupportedEncodingException;
+import java.net.URLDecoder;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import org.apache.commons.lang3.StringUtils;
+
+@JinjavaDoc(
+  value = "Decodes encoded URL strings back to the original URL. It accepts both dictionaries and regular strings as well as pairwise iterables.",
+  input = @JinjavaParam(
+    value = "url",
+    type = "string",
+    desc = "the url to decode",
+    required = true
+  ),
+  snippets = {
+    @JinjavaSnippet(
+      code = "{{ \"http%3A%2F%2Ffoo.com%3Fbar%26food\"|urldecode }}",
+      output = "http://foo.com?bar&food"
+    )
+  }
+)
+public class UrlDecodeFilter implements Filter {
+
+  @Override
+  public String getName() {
+    return "urldecode";
+  }
+
+  @Override
+  public Object filter(Object var, JinjavaInterpreter interpreter, String... args) {
+    if (var == null && args.length == 0) {
+      return "";
+    }
+
+    if (var != null) {
+      if (Map.class.isAssignableFrom(var.getClass())) {
+        @SuppressWarnings("unchecked")
+        Map<Object, Object> dict = (Map<Object, Object>) var;
+
+        List<String> paramPairs = new ArrayList<>();
+
+        for (Map.Entry<Object, Object> param : dict.entrySet()) {
+          StringBuilder paramPair = new StringBuilder();
+          paramPair.append(urlDecode(Objects.toString(param.getKey())));
+          paramPair.append("=");
+          paramPair.append(urlDecode(Objects.toString(param.getValue())));
+
+          paramPairs.add(paramPair.toString());
+        }
+
+        return StringUtils.join(paramPairs, "&");
+      }
+
+      return urlDecode(var.toString());
+    }
+
+    return urlDecode(args[0]);
+  }
+
+  private String urlDecode(String s) {
+    try {
+      return URLDecoder.decode(s, "UTF-8");
+    } catch (UnsupportedEncodingException e) {
+      throw new RuntimeException(e);
+    }
+  }
+}

src/test/java/com/hubspot/jinjava/lib/filter/UrlDecodeFilterTest.java

@@ -0,0 +1,38 @@
+package com.hubspot.jinjava.lib.filter;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.hubspot.jinjava.BaseInterpretingTest;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import org.junit.Before;
+import org.junit.Test;
+
+public class UrlDecodeFilterTest extends BaseInterpretingTest {
+  private UrlDecodeFilter filter;
+
+  @Before
+  public void setup() {
+    filter = new UrlDecodeFilter();
+  }
+
+  @Test
+  public void itDecodesDictAsParamPairs() {
+    Map<String, String> dict = new LinkedHashMap<>();
+    dict.put("foo", "bar%3Dset");
+    dict.put("other", "val");
+
+    assertThat(filter.filter(dict, interpreter)).isEqualTo("foo=bar=set&other=val");
+  }
+
+  @Test
+  public void itDecodesVarString() {
+    assertThat(filter.filter("http%3A%2F%2Ffoo.com%3Fbar%26food", interpreter))
+      .isEqualTo("http://foo.com?bar&food");
+  }
+
+  @Test
+  public void itDecodesArgWhenNoVar() {
+    assertThat(filter.filter(null, interpreter, "foo%26you")).isEqualTo("foo&you");
+  }
+}