Apply numeric sort to object keys when rendering

Fixes #228 reported by @derrickburns; since we can interpret
numeric keys as lists, we sort the keys numerically to make
it easy to read the list value in order.

config/src/main/java/com/typesafe/config/impl/SimpleConfigObject.java

@@ -418,6 +418,45 @@ final class SimpleConfigObject extends AbstractConfigObject implements Serializa
         });
     }
 
+    static final private class RenderComparator implements java.util.Comparator<String> {
+        private static boolean isAllDigits(String s) {
+            int length = s.length();
+
+            // empty string doesn't count as a number
+            if (length == 0)
+                return false;
+
+            for (int i = 0; i < length; ++i) {
+                char c = s.charAt(i);
+
+                if (Character.isDigit(c))
+                    continue;
+                else
+                    return false;
+            }
+            return true;
+        }
+
+        // This is supposed to sort numbers before strings,
+        // and sort the numbers numerically. The point is
+        // to make objects which are really list-like
+        // (numeric indices) appear in order.
+        @Override
+        public int compare(String a, String b) {
+            boolean aDigits = isAllDigits(a);
+            boolean bDigits = isAllDigits(b);
+            if (aDigits && bDigits) {
+                return Integer.compare(Integer.parseInt(a), Integer.parseInt(b));
+            } else if (aDigits) {
+                return -1;
+            } else if (bDigits) {
+                return 1;
+            } else {
+                return a.compareTo(b);
+            }
+        }
+    }
+
     @Override
     protected void render(StringBuilder sb, int indent, boolean atRoot, ConfigRenderOptions options) {
         if (isEmpty()) {
@@ -438,7 +477,7 @@ final class SimpleConfigObject extends AbstractConfigObject implements Serializa
 
             int separatorCount = 0;
             String[] keys = keySet().toArray(new String[size()]);
-            Arrays.sort(keys);
+            Arrays.sort(keys, new RenderComparator());
             for (String k : keys) {
                 AbstractConfigValue v;
                 v = value.get(k);

config/src/test/scala/com/typesafe/config/impl/ConfigValueTest.scala

@@ -14,6 +14,7 @@ import com.typesafe.config.ConfigList
 import com.typesafe.config.ConfigException
 import com.typesafe.config.ConfigValueType
 import com.typesafe.config.ConfigOrigin
+import com.typesafe.config.ConfigRenderOptions
 import com.typesafe.config.ConfigValueFactory
 import com.typesafe.config.ConfigFactory
 
@@ -959,4 +960,11 @@ class ConfigValueTest extends TestUtils {
 
         assertEquals(conf, parsed)
     }
+
+    @Test
+    def renderSorting(): Unit = {
+        val config = parseConfig("""0=a,1=b,2=c,3=d,10=e,20=f,30=g""")
+        val rendered = config.root.render(ConfigRenderOptions.concise())
+        assertEquals("""{"0":"a","1":"b","2":"c","3":"d","10":"e","20":"f","30":"g"}""", rendered)
+    }
 }