Can someone explain me what is he doing in this code by map_stream?

1

#include <bits/stdc++.h>
#include <sys/mman.h>
#include <sys/stat.h>
using namespace std;

struct RdMap {
char* p, * last;
};
static RdMap rdmap_;

void map_stream(FILE* stream)
{
struct stat sb;
int fd = fileno(stream);

fstat(fd, &sb);
void* addr = mmap(0, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
assert(addr != MAP_FAILED);

rdmap_.p = static_cast<char*>(addr);
rdmap_.last = rdmap_.p + sb.st_size;

}

inline int readchar()
{
if (builtin_expect(rdmap.p != rdmap.last, 1))
return *rdmap_.p++;
return EOF;
}

long int readlong()
{
long int v = 0, m = 1;
int c;

while ( isspace(c = readchar()) )
    ;

if (c == '-') {
    c = readchar();
    m = -1;
}

c -= '0';
while ( static_cast<unsigned int>(c) < 10U ) {
    v = v * 10 + c;
    c  = readchar() - '0';
}
return v * m;

}

#define FOR(i, a, b)
for ( i = (a); i < (b); i++ )
#define ROF(i, a, b)
for ( i = (b); (a) < i–; )

constexpr int NMAX = 5*pow(10, 5);

void rsort(int a[], int n)
{
static int t[NMAX];
int* b = t, i, j;

FOR(i, 0, 4) {
    auto mask = [i](unsigned int x)
    { return (x >> i*8) % 256; };
    
    int h[256]{};
    FOR(j, 0, n)
        h[mask(a[j])]++;
    FOR(j, 1, 256)
        h[j] += h[j-1];
    ROF(j, 0, n)
        b[--h[mask(a[j])]] = a[j];
    swap(a, b);
}

}

int main(int argc, char* argv[])
{
map_stream(stdin);

int n = readlong();
int i;

static int v[NMAX];

FOR(i, 0, n) {
    v[i] = readlong();
}

rsort(v, n);

long int res = 0;

FOR(i, 0, n) {
    res = max(res, static_cast<long int>(v[i]) * (n - i));
}

printf("%ld\n", res);
return 0;

}