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copied!<p>As the question says that <code>(A,1) (A,2) (B,1) (B,2) (C,1) (B,3) (B,4)</code> is a valid output, we'll find that we need to store the waiting number for each letter when you looking at the <code>(C,1)</code> between <code>(B,2)</code> and <code>(B,3)</code>.</p> <p>What's more, as the question says:</p> <blockquote> <p>Other than as dictated by those rules, the expectation is that things will arrive on the output in the order they are submitted to the input.</p> </blockquote> <p>So after forwarding some values, we may still have some queued values. Take the question's output as an example, we must still have <code>(B,4)</code> queued for the coming <code>(B,3)</code> after forwarding <code>(C,1)</code>.</p> <p>Here is a code example that RAC parts are almost the same as Justin's while I highlight the differences in comments:</p> <p>Also, I post a full and runnable code example: <a href="http://d.pr/X59S/9p9bT58U" rel="nofollow">http://d.pr/X59S/9p9bT58U</a>.</p> <pre class="lang-c prettyprint-override"><code>RACSubject *input = [RACSubject subject]; RACSignal *output = [[[[input map:^(NSString *combo) { NSArray *components = [combo componentsSeparatedByString:@","]; NSInteger number = [components[1] integerValue]; return RACTuplePack(components[0], @(number)); }] // !!!: DIFF // We need there state parameters: // 1. The letters and numbers we're waiting for. // 2. Values received that cannot be forwarded until a certain // letter/number. // 3. The values to forward at each step. scanWithStart:RACTuplePack(@{ @"A": @1 }, @[], @[]) reduce:^id(RACTuple *state, RACTuple *letterAndNumber) { __block NSDictionary *waitingLettersAndNumbers = state[0]; __block NSArray *queuedValues = state[1]; // Enqueue this value until we're ready to send it (which may or may not // occur on this step of the scan). queuedValues = [queuedValues arrayByAddingObject:letterAndNumber]; char letterChar = [letterAndNumber.first characterAtIndex:0]; // !!!: DIFF // (letter + 1, number) may be a valid output after forwarding (letter, number + 1) if ( [[waitingLettersAndNumbers objectForKey:letterAndNumber.first] isEqualToNumber:letterAndNumber.second] && (letterChar == 'A' || [[waitingLettersAndNumbers objectForKey:[NSString stringWithFormat:@"%c", letterChar - 1]] integerValue] > [letterAndNumber.second integerValue]) ) { NSMutableDictionary *mutableWaitingLettersAndNumbers = [NSMutableDictionary dictionaryWithDictionary:waitingLettersAndNumbers]; // Sort queuedValues lexically and numerically. queuedValues = ... // Determine the next letter and number. NSMutableArray *forwardValues = [NSMutableArray array]; NSMutableArray *remindValues = [NSMutableArray array]; [queuedValues enumerateObjectsUsingBlock:^(RACTuple *tuple, NSUInteger idx, BOOL *stop) { ... }]; queuedValues = [remindValues copy]; waitingLettersAndNumbers = [mutableWaitingLettersAndNumbers copy]; // !!!: DIFF // After forwarding some values, we may still have some queued values. return RACTuplePack(waitingLettersAndNumbers, queuedValues, forwardValues); } else { // No values should escape the scan yet. Just pass on our queued // values. return RACTuplePack(waitingLettersAndNumbers, queuedValues, @[]); } }] map:^(RACTuple *state) { // Convert the array of values into a signal. NSArray *forwardValues = state.last; return [forwardValues.rac_sequence signalWithScheduler:[RACScheduler immediateScheduler]]; }] // Forward values from each inner signal in the correct, sorted order. concat]; </code></pre>

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