1 files changed, 33 insertions, 0 deletions
diff --git a/packages/gitbook/src/utils/reducedObject.js b/packages/gitbook/src/utils/reducedObject.js
new file mode 100644
index 0000000..196a72c
--- /dev/null
+++ b/packages/gitbook/src/utils/reducedObject.js
@@ -0,0 +1,33 @@
+const Immutable = require('immutable');
+
+/**
+ *  Reduce the difference between a map and its default version
+ *  @param {Map} defaultVersion
+ *  @param {Map} currentVersion
+ *  @return {Map} The properties of currentVersion that differs from defaultVersion
+ */
+function reducedObject(defaultVersion, currentVersion) {
+    if (defaultVersion === undefined) {
+        return currentVersion;
+    }
+
+    return currentVersion.reduce(function(result, value, key) {
+        const defaultValue = defaultVersion.get(key);
+
+        if (Immutable.Map.isMap(value)) {
+            const diffs = reducedObject(defaultValue, value);
+
+            if (diffs.size > 0) {
+                return result.set(key, diffs);
+            }
+        }
+
+        if (Immutable.is(defaultValue, value)) {
+            return result;
+        }
+
+        return result.set(key, value);
+    }, Immutable.Map());
+}
+
+module.exports = reducedObject;