Bulkify the Logic

public void afterUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) {
    Map<Id, Decimal> amountAdjustmentByAccountId = new Map<Id, Decimal>();

    for (Opportunity opp : newOpportunities) {
        Opportunity oldOpp = oldMap.get(opp.Id);
        Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won';
        Boolean justUnwon = opp.StageName != 'Closed Won' && oldOpp.StageName == 'Closed Won';
        Decimal amount = opp.Amount == null ? 0 : opp.Amount;

        if (justWon) {
            addAdjustment(amountAdjustmentByAccountId, opp.AccountId, amount);
        } else if (justUnwon) {
            addAdjustment(amountAdjustmentByAccountId, opp.AccountId, -amount);
        }
    }
    // ... apply the adjustments (next section)
}

private void addAdjustment(Map<Id, Decimal> adjustments, Id accountId, Decimal amount) {
    if (! adjustments.containsKey(accountId)) {
        adjustments.put(accountId, 0);
    }
    adjustments.put(accountId, adjustments.get(accountId) + amount);
}

1. public void afterUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) { — the method.
2. Map<Id, Decimal> amountAdjustmentByAccountId = new Map<Id, Decimal>(); — this is Module 16's "Maps of sObjects" pattern — one entry per affected Account.
3. (blank line)
4. for (Opportunity opp : newOpportunities) { — loops over the updated records — pure in-memory work so far.
5. Opportunity oldOpp = oldMap.get(opp.Id); — the prior version.
6. Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won'; — the same transition check as before.
7. Boolean justUnwon = opp.StageName != 'Closed Won' && oldOpp.StageName == 'Closed Won'; — the reverse.
8. Decimal amount = opp.Amount == null ? 0 : opp.Amount; — Module 6's ternary operator, defaulting a null Amount to 0.
9. (blank line)
10. if (justWon) { — the won case.
11. addAdjustment(amountAdjustmentByAccountId, opp.AccountId, amount); — adds the amount to this Account's running total.
12. } else if (justUnwon) { — the unwon case.
13. addAdjustment(amountAdjustmentByAccountId, opp.AccountId, -amount); — subtracts it back out.
14. } — closes the if/else if.
15. } — closes the loop.
16. // ... apply the adjustments (next section) — the next step, covered separately.
17. } — closes the method.
18. (blank line)
19. private void addAdjustment(Map<Id, Decimal> adjustments, Id accountId, Decimal amount) { — a small helper.
20. if (! adjustments.containsKey(accountId)) { — the first time this Account is seen...
21. adjustments.put(accountId, 0); — ...its running total starts at 0.
22. } — closes the if.
23. adjustments.put(accountId, adjustments.get(accountId) + amount); — adds this adjustment, so several Opportunities for the same Account in one transaction combine into a single running total.
24. } — closes the helper.

private void applyAdjustments(Map<Id, Decimal> amountAdjustmentByAccountId) {
    if (amountAdjustmentByAccountId.isEmpty()) {
        return;
    }

    List<Account> accounts = [
        SELECT Id, Total_Won_Revenue__c
        FROM Account
        WHERE Id IN :amountAdjustmentByAccountId.keySet()
    ];

    for (Account acc : accounts) {
        Decimal current = acc.Total_Won_Revenue__c == null ? 0 : acc.Total_Won_Revenue__c;
        acc.Total_Won_Revenue__c = current + amountAdjustmentByAccountId.get(acc.Id);
    }

    update accounts;
}

1. private void applyAdjustments(Map<Id, Decimal> amountAdjustmentByAccountId) { — the method.
2. if (amountAdjustmentByAccountId.isEmpty()) { — if nothing needs adjusting...
3. return; — ...exits early, avoiding a wasted query and update.
4. } — closes the if.
5. (blank line)
6. List<Account> accounts = [ — this is Module 25's core pattern applied directly — one query.
7. SELECT Id, Total_Won_Revenue__c — the fields needed.
8. FROM Account — the object.
9. WHERE Id IN :amountAdjustmentByAccountId.keySet() — Module 16's keySet(), fetching exactly the affected Accounts.
10. ]; — closes the query.
11. (blank line)
12. for (Account acc : accounts) { — loops over the affected accounts, in memory.
13. Decimal current = acc.Total_Won_Revenue__c == null ? 0 : acc.Total_Won_Revenue__c; — defaults a null rollup to 0.
14. acc.Total_Won_Revenue__c = current + amountAdjustmentByAccountId.get(acc.Id); — applies this Account's adjustment.
15. } — closes the loop.
16. (blank line)
17. update accounts; — one update, regardless of whether 1 or 200 Opportunities changed in this transaction — exactly what makes this feature bulk-safe.
18. } — closes the method.