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#include <unistd.h>
#include <streambuf>
#include <ios>
#include <algorithm>
#include <cstring>
//CLASS
class FdUnget: public std::streambuf
{
int d_fd;
size_t d_bufsize;
size_t d_reserved;
char *d_buffer;
char *d_base;
public:
FdUnget(int fd, size_t bufsz, size_t unget);
~FdUnget() override;
private:
int underflow() override;
};
//=
//CONS
FdUnget::FdUnget(int fd, size_t bufsz, size_t unget)
:
d_fd(fd),
d_reserved(unget)
{
size_t allocate =
bufsz > d_reserved ?
bufsz
:
d_reserved + 1;
d_buffer = new char[allocate];
d_base = d_buffer + d_reserved;
setg(d_base, d_base, d_base);
d_bufsize = allocate - d_reserved;
}
//=
//DESTR
inline FdUnget::~FdUnget()
{
delete[] d_buffer;
}
//=
//UNDERFLOW
int FdUnget::underflow()
{
size_t ungetsize = gptr() - eback();
size_t move = std::min(ungetsize, d_reserved);
memcpy(d_base - move, egptr() - move, move);
int nread = read(d_fd, d_base, d_bufsize);
if (nread <= 0) // none read -> return EOF
return EOF;
setg(d_base - move, d_base, d_base + nread);
return static_cast<unsigned char>(*gptr());
}
//=
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