Paste number 27626: interview problem

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Paste number 27626: interview problem
Pasted by: psykotic
When:7 years, 6 months ago
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The problem is to write a set of functions to manage a variable number of byte queues, each with variable length, in a small, fixed amount of memory. You should provide implementations of the following four functions:

Q * createQ();               //Creates a FIFO byte queue, returning a handle to it.
void destroyQ(Q * q);        //Destroy an earlier created byte queue.
void enQ(Q * q, unsigned char b);    //Adds a new byte to a queue.
unsigned char deQ(Q * q);    //Pops the next byte off the FIFO queue.

So, the output from the following set of calls:

Q * q0 = createQ();
enQ(q0, 0);
enQ(q0, 1);
Q * q1 = createQ();
enQ(q1, 3);
enQ(q0, 2);
enQ(q1, 4);
printf("%d", deQ(q0));
printf("%d\n", deQ(q0));
enQ(q0, 5);
enQ(q1, 6);
printf("%d", deQ(q0));
printf("%d\n", deQ(q0));
printf("%d", deQ(q1));
printf("%d", deQ(q1));
printf("%d\n", deQ(q1));

should be:

0 1
2 5
3 4 6

You can define the type Q to be whatever you want.

Your code is not allowed to call malloc() or other heap management routines. Instead, all storage (other than local variables in your functions) should be done in a provided array:

unsigned char data[2048];

Memory efficiency is important. On average while your system is running, there will be a dozen or so queues with an average of 80 or so bytes in each queue. Your functions may be asked to create a larger number of queues with less bytes in each. Your functions may be asked to create a smaller number of queues with more bytes in each.

If you are unable to satisfy a request due to lack of memory, your code should call a provided failure function, which will not return:

void onOutOfMemory();

If the caller makes an illegal request, like attempting to deQ a byte from an empty queue, your code should call a provided failure function, which will not return:

void onIllegalOperation();

Worst-case performance when adding and removing bytes is more important than average-case performance.

There may be spikes in the number of queues allocated, or in the size of an individual queue. Your code should not assume a maximum number of bytes in a queue (other than that imposed by the total amount of memory available, of course!) You can assume that no more than 64 queues will be created at once.

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Annotation number 1: solution
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When:7 years, 6 months ago
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It is late and I didn't think this through as well as I should, but this
should work:

 - using the data area as a buffer containing all queues, commingled
 - each entry is two bytes: a queue number and the data byte
 - creation is simply incrementing a static max queue number 
 - destruction zeroes out all pairs whose queue number is being destructed
   followed by a pass that backfills the created holes
 - adding sticks (queue,value) pair in a zeroed field
    - if we reach the end of buffer before finding empty, call out of memory
 - removing searches forward for first pair from this queue, returns/zeroes it
   and moves up the rest of the table.
    - call error if can't find the requested queue number

A typical case, 12 queues of 80 bytes is 960 bytes. Using pairs means that
the data requirement is double the total data size, which fits in 2048.

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