/**
* Makes a diff copy of two {@link State} maps.
*
* <p>The returned diff state contains only the key/values that changed. {@code null} values are
* equivalent to absent values.
*
* <p>For values set to null or removed, the value is null.
*
* <p>When setting a delta, the old value is checked to know if the delta should be kept or if a
* full value should be set instead.
*
* <p>For sub-documents, a recursive diff is returned.
*
* @return a {@link StateDiff} which, when applied to a, gives b.
*/
public static StateDiff diff(State a, State b) {
StateDiff diff = new StateDiff();
for (Entry<String, Serializable> en : a.entrySet()) {
Serializable va = en.getValue();
if (va == null) {
// checked by loop on b
continue;
}
String key = en.getKey();
Serializable vb = b.get(key);
if (vb == null) {
// value must be cleared
diff.put(key, null);
} else {
// compare values
Serializable elemDiff = diff(va, vb);
if (elemDiff != NOP) {
if (elemDiff instanceof Delta) {
Delta delta = (Delta) elemDiff;
Serializable deltaBase = delta.getBase();
if (!Objects.equals(va, deltaBase)) {
// delta's base is not the old value
// -> set a new value, don't use a delta update
elemDiff = delta.getFullValue();
}
// else delta's base is the in-database value
// because base is consistent with old value, assume the delta is already properly
// computed
}
diff.put(key, elemDiff);
}
}
}
for (Entry<String, Serializable> en : b.entrySet()) {
String key = en.getKey();
Serializable va = a.get(key);
if (va != null) {
// already checked by loop on a
continue;
}
Serializable vb = en.getValue();
if (!equalsLoose(null, vb)) {
// value must be added
diff.put(key, vb);
}
}
return diff;
}
private static final StateDiff stateDiff(Serializable... values) {
assertTrue(values.length % 2 == 0);
StateDiff diff = new StateDiff();
for (int i = 0; i < values.length; i += 2) {
diff.put((String) values[i], values[i + 1]);
}
return diff;
}