practical o(n) {⍵[⍋⍵]} if {645=⎕dr⍵}

magic.c

void magic(float * x, size_t n) {
    uint32_t * result = malloc(n * sizeof(uint32_t)),
             * buckets = malloc(0x100*sizeof(uint32_t)),
             * si = malloc(0x100*sizeof(uint32_t)),
             * y = (uint32_t *) x, key, temp;
    for (size_t i = 0; i < n; i++) y[i] ^= 0x80000000;
    for (int i = 0; i < n; ++i) ++buckets[y[i] & 0xFF];
    si[0] = 0;
    for (int j = 1; j < 0x100; ++j) si[j] = si[j - 1] + buckets[j - 1];
    for (int i = n-1; i >= 0; --i) key = y[i] & 0xFF, result[si[key] + --buckets[key]] = y[i];
    memcpy(y,result,n*sizeof(uint32_t));
    for (int i = 0; i < n; ++i) ++buckets[(y[i] & 0xFF00) >> 8];
    si[0] = 0;
    for (int j = 1; j < 0x100; ++j) si[j] = si[j - 1] + buckets[j - 1];
    for (int i = n-1; i >= 0; --i) key = (y[i] & 0xFF00) >> 8, result[si[key] + --buckets[key]] = y[i];
    memcpy(y,result,n*sizeof(uint32_t));
    for (int i = 0; i < n; ++i) ++buckets[(y[i] & 0xFF0000) >> 16];
    si[0] = 0;
    for (int j = 1; j < 0x100; ++j) si[j] = si[j - 1] + buckets[j - 1];
    for (int i = n-1; i >= 0; --i) key = (y[i] & 0xFF0000) >> 16, result[si[key] + --buckets[key]] = y[i];
    memcpy(y,result,n*sizeof(uint32_t));
    for (int i = 0; i < n; ++i) ++buckets[(y[i] & 0xFF000000) >> 24];
    si[0] = 0;
    for (int j = 1; j < 0x100; ++j) si[j] = si[j - 1] + buckets[j - 1];
    for (int i = n-1; i >= 0; --i) key = (y[i] & 0xFF000000) >> 24, result[si[key] + --buckets[key]] = y[i];
    memcpy(y,result,n*sizeof(uint32_t));
    free(result);
    free(buckets);
    free(si);
    for (size_t i = 0; i < n; i++) y[i] ^= 0x80000000;
    key = 0;
    for (size_t i = 0; i < n; i++) if (y[i] & 0x80000000) key++;
    for (size_t i = 0; i < key / 2; i++) temp = y[i], y[i] = y[key - i - 1], y[key - i - 1] = temp;
}