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#include <stdio.h>#include <stdint.h>struct chunk {  uint64_t n;  uint8_t data[CHUNK_SIZE];};void xor_buf(uint8_t out, uint8_t a, uint8_t b){  long n = CHUNK_SIZE;  while(n >= 8) {    *(uint64_t*)out = *(uint64_t*)a ^ *(uint64_t*)b;    out += 8;    a += 8;    b += 8;    n -= 8;  }  while(n) {    *out++ = *a++ ^ *b++;    n--;  }}long hamming_weight(uint8_t in){  long sum = 0;  long n = CHUNK_SIZE;  while(n >= sizeof(unsigned long long)) {    sum += __builtin_popcountll(*(unsigned long long*)in);    n -= sizeof(unsigned long long);    in += sizeof(unsigned long long);  }  while(n) {    sum += __builtin_popcount(*in);    n--;    in++;  }  return sum;}void compress(long nchunks){  struct chunk * chunks;  long have_chunks;  uint64_t read_chunks;  long best_chunk_index, best_chunk_weight, this_chunk_weight;  long i;  uint8_t temp[CHUNK_SIZE];  chunks = malloc(sizeof(struct chunk) * nchunks);  if(!chunks) {    perror("malloc");    exit(1);  }    have_chunks = 0;  read_chunks = 0;  while(have_chunks < nchunks && (fread(chunks[have_chunks].data, CHUNK_SIZE, 1, stdin) == 1)) {    chunks[have_chunks].n = read_chunks;    have_chunks++;    read_chunks++;  }  fwrite(chunks[0].data, CHUNK_SIZE, 1, stdout);  while(have_chunks) {    best_chunk_index = 0;    for(i = 1; i < have_chunks; i++) {      xor_buf(swap, chunks[0].data, chunks[i].data);      this_chunk_weight = hamming_weight(swap);      if(best_chunk_index == 0 || best_chunk_weight > this_chunk_weight) {        best_chunk_index = i;        best_chunk_weight = this_chunk_weight;        if(best_chunk_weight == 0)          break;      }    }    fwrite(chunks[0].n, sizeof(chunks[0].n), 1, stdout);    fwrite(chunks[best_chunk_index].n, sizeof(chunks[0].n), 1, stdout);    fwrite(swap, CHUNK_SIZE, 1, stdout);    chunks[0] = chunks[best_chunk_index];    if(!fread(chunks[best_chunk_index].data, CHUNK_SIZE, 1, stdin)) {      chunks[best_chunk_index] = chunks[--have_chunks];    } else {       chunks[best_chunk_index].n = read_chunks++;    }  }}