median.c

#include "median.h"
#include "hex.h"
#include <strings.h>
#include <stdio.h>

// The structure of the data buffer is this:
// ----------------------------------------------------------------------------
// | Metadata | BlockData | Data * nMembers | BlockData | Data * nMembers .....
// ----------------------------------------------------------------------------
// Where the number of blocks is nLevels and stored in Metadata, and nMembers
// is stored in Metadata as well.
// The buffer is cast into this struct.
typedef struct metadata {
  unsigned int gid;        // The next available global id
  size_t nLevels;          // Number of buffers available.
  size_t nMembers;         // The number of members in each block.
  size_t currentBufferIdx; // The index of the currently active buffer.
  unsigned char data[0];   // The start of the data.
} metadata_t;

typedef struct bufferdata {
  unsigned int id;           // Block identity.
  size_t level;              // The level of this buffer.
  size_t occupancy;          // Number of occupants for this buffer.
  median_data_t data[0];     // The data for this buffer.
} blockdata_t;


// ----------------------------------------------------------------------------
// Helper functions.
//
// This turns epsilon into a suggested size for each block.
size_t calculate_nMembers(double epsilon) {
  return (size_t) ((1.0 / epsilon) + 1);
}

// This turns maxN into the number of levels needed. It's ceiling(log2(maxN)).
size_t calculate_nLevels(size_t maxN) {
  size_t nLevels = 1;
  while((maxN = (maxN >> 1))) nLevels++;
  return nLevels;
}

// This returns the blocksize.
size_t blocksize(metadata_t * m) {
  return sizeof(blockdata_t) + m->nMembers * sizeof(median_data_t);
}

// This returns the size of the entire buffer.
size_t buffersize(metadata_t * m) {
  return sizeof(metadata_t) + m->nLevels * blocksize(m);
}

// Returns the first byte address which is out of bounds.
unsigned char * out_of_bounds(metadata_t * m) {
  return m->data + buffersize(m) - sizeof(blockdata_t);
}

// Get the current block.
blockdata_t * get_current_block(metadata_t *m) {
  size_t offset = m->currentBufferIdx * blocksize(m);
  return (blockdata_t *)(m->data + offset);
}
// Does the current block have space?
unsigned int current_block_has_space(metadata_t* m) {
  return (get_current_block(m)->occupancy < m->nMembers);
}

// This will only be called in a safe situation. It's an internal function.
// Does not bound check!
// Do not call it unless you know what you are doing.
void insert_data_into_current_block(metadata_t *m, median_data_t data) {
  blockdata_t * b = get_current_block(m);
  b->data[b->occupancy++] = data;
}

// Check if there is space in some block and if there is, then
// make that block the current block.
unsigned int occupy_available_empty_block(metadata_t * m) {
  size_t idx = 0;
  for(unsigned char * ptr = (unsigned char *) m->data; // Start of the data blocks.
      ptr <= (m->data + buffersize(m) - blocksize(m)); // The next block will fit into the buffer.
      ptr += blocksize(m)) { // Advance by a block
    blockdata_t * b = (blockdata_t *)ptr;
    if(b->occupancy < m->nMembers) {
      // Empty block found!
      m->currentBufferIdx = idx;
      return 1;
    } else {
      idx++;
    }
  }
  return 0;
}

unsigned int create_empty_block(metadata_t * m) {
  // TODO
  return 0;
}

// Shift the current block.
unsigned int shift_current_block(metadata_t * m) {
  if(!occupy_available_empty_block(m)) {
    if(create_empty_block(m)) {
      // Now there should be an empty block!
      // So occupy it.
      occupy_available_empty_block(m);
    } else {
      // Stuck we are, block not.
      return 0;
    }
  }
  return 1;
}


// ----------------------------------------------------------------------------
// Implementation of the interface functions (specified in median.h).
median_error_t median_suggest_buffer_size(double epsilon, size_t maxN, size_t *suggested_size){
  // First check for sane parameters.

  // Check that maxN is not zero.
  if (maxN <= 0) return MEDIAN_ERROR_INVALID_PARAMETERS;

  // If n is too large, (i.e. epsilon is too small), you are looking for too fine an approximation.
  // This is not the right algorithm. Maybe consider random sampling instead.
  if (calculate_nMembers(epsilon) > 100000) return MEDIAN_ERROR_INVALID_PARAMETERS;

  // Calculate the required buffer size for the right nLevels and nMembers
  // value.
  metadata_t m;
  m.nLevels = calculate_nLevels(maxN);
  m.nMembers = calculate_nMembers(epsilon);
  *suggested_size = buffersize(&m);

  // Things worked. Return MEDIAN_OK
  return MEDIAN_OK;
}
median_error_t median_init_buffer(void * buffer, size_t buffer_size, double epsilon, size_t maxN, median_buffer_t *initialized_buffer){

  // Initialize.
  size_t expectedSize = 0;
  median_error_t error = MEDIAN_OK;

  // First run median_suggest_buffer_size
  if ((error = median_suggest_buffer_size(epsilon,maxN,&expectedSize)) != MEDIAN_OK) return error;
  // Ensure that the buffer is large enough.
  if (expectedSize > buffer_size) return MEDIAN_ERROR_BUFFER_TOO_SMALL;
  // Cast the buffer as metadata_t so that we can initialize the metadata block.
  metadata_t * m = (metadata_t *)buffer;
  // Initialize the metadata block.
  m->gid = 1;
  m->nMembers = calculate_nMembers(epsilon);
  m->nLevels = calculate_nLevels(maxN);
  m->currentBufferIdx = 0;
  // Zero all the data following the metadata block.
  // Strictly this should not be necessary, but I'm a coward!
  bzero(m->data, buffersize(m) - sizeof(metadata_t));

  // cast the data part as an array of blockdata_t fronted blocks.
  // and initialize each of the blocks.
  for(unsigned char * ptr = (unsigned char *) m->data; // Start of the data blocks.
      ptr <= (m->data + buffersize(m) - blocksize(m)); // The next block will fit into the buffer.
      ptr += blocksize(m)) { // Advance by a block
    blockdata_t * t = (blockdata_t *) ptr;
    t->id = (m->gid)++;
    t->level  = 0;
    t->occupancy = 0;
  }

  // Populate the return value.
  *initialized_buffer = (median_buffer_t)buffer;
  return error;
}


// Insert data into the buffer.
// We could have made this more concise and pretty if we used short-circuit
// execution of the boolean condition..
//
// if (current_block_has_space(m) || shift_current_block(m)) {
//   insert_data_into_current_block(m,data);
//   return MEDIAN_OK;
// } else {
//   return MEDIAN_ERROR_MAX_N_EXCEEDED;
// }
//
// But this code is harder to read, because of the side
// effect issue.
median_error_t median_insert_data(median_buffer_t buffer,
                                  median_data_t data){
  metadata_t * m = (metadata_t *) buffer;
  // If there is space in the current block..
  if(current_block_has_space(m)) {
    // Insert data into the current block. This
    // call is to an unchecked method, which will
    // never fail.
    insert_data_into_current_block(m,data);
  } else {
    // Current block is full. So try and shift it.
    if(shift_current_block(m)) {
      // The current block has shifted successfully.
      // So we can now call the unchecked method again.
      insert_data_into_current_block(m,data);
    } else {
      // Failed to shift block. Stuck we are.
      return MEDIAN_ERROR_MAX_N_EXCEEDED;
    }
  }
  // Everything worked. So return MEDIAN_OK.
  return MEDIAN_OK;
}

median_error_t median_output_median(const median_buffer_t buffer,
                                    median_data_t *approximate_median){
  return MEDIAN_OK;
}

// Dump the state to stderr.
median_error_t median_dump_stderr(const median_buffer_t buffer){
  // First print out all the metadata.
  metadata_t * m = (metadata_t *) buffer;
  blockdata_t * current = get_current_block(m);
  fprintf(stderr, "Metadata:\n Next GID:%d\n nMembers:%d\n nLevels:%d\n CurrentBlock:%d\n",m->gid,(unsigned)m->nMembers,(unsigned)m->nLevels, current->id);

  // Now dump the blocks.
  for(unsigned char * ptr = (unsigned char *) m->data; // Start of the data blocks.
      ptr <= (m->data + buffersize(m) - blocksize(m)); // The next block will fit into the buffer.
      ptr += blocksize(m)) { // Advance by a block
    blockdata_t * b = (blockdata_t *)ptr;
    fprintf(stderr, "Block:%d\n Level:%d\n Occupancy:%d\n", b->id,(unsigned)b->level,(unsigned)b->occupancy);
    // For every block, dump the first 8 keys in the block.
    stderr_hexdmp("Data:",(unsigned char *)(b->data),8*sizeof(median_data_t));
  }

  // Done, return MEDIAN_OK.
  return MEDIAN_OK;
}