ClickHouse/dbms/IO/ReadBuffer.h

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#pragma once
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#include <cstring>
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#include <algorithm>
#include <memory>
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#include <Common/Exception.h>
#include <IO/BufferBase.h>
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namespace DB
{
namespace ErrorCodes
{
extern const int ATTEMPT_TO_READ_AFTER_EOF;
extern const int CANNOT_READ_ALL_DATA;
}
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/** A simple abstract class for buffered data reading (char sequences) from somewhere.
* Unlike std::istream, it provides access to the internal buffer,
* and also allows you to manually manage the position inside the buffer.
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*
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* Note! `char *`, not `const char *` is used
* (so that you can take out the common code into BufferBase, and also so that you can fill the buffer in with new data).
* This causes inconveniences - for example, when using ReadBuffer to read from a chunk of memory const char *,
* you have to use const_cast.
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*
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* Derived classes must implement the nextImpl() method.
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*/
class ReadBuffer : public BufferBase
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{
public:
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/** Creates a buffer and sets a piece of available data to read to zero size,
* so that the next() function is called to load the new data portion into the buffer at the first try.
*/
ReadBuffer(Position ptr, size_t size) : BufferBase(ptr, size, 0) { working_buffer.resize(0); }
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/** Used when the buffer is already full of data that can be read.
* (in this case, pass 0 as an offset)
*/
ReadBuffer(Position ptr, size_t size, size_t offset) : BufferBase(ptr, size, offset) {}
// FIXME: behavior differs greately from `BufferBase::set()` and it's very confusing.
void set(Position ptr, size_t size) { BufferBase::set(ptr, size, 0); working_buffer.resize(0); }
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/** read next data and fill a buffer with it; set position to the beginning;
* return `false` in case of end, `true` otherwise; throw an exception, if something is wrong
*/
bool next()
{
bytes += offset();
bool res = nextImpl();
if (!res)
working_buffer.resize(0);
pos = working_buffer.begin() + working_buffer_offset;
working_buffer_offset = 0;
return res;
}
inline void nextIfAtEnd()
{
if (!hasPendingData())
next();
}
virtual ~ReadBuffer() {}
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/** Unlike std::istream, it returns true if all data was read
* (and not in case there was an attempt to read after the end).
* If at the moment the position is at the end of the buffer, it calls the next() method.
* That is, it has a side effect - if the buffer is over, then it updates it and set the position to the beginning.
*
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* Try to read after the end should throw an exception.
*/
bool ALWAYS_INLINE eof()
{
return !hasPendingData() && !next();
}
void ignore()
{
if (!eof())
++pos;
else
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throwReadAfterEOF();
}
void ignore(size_t n)
{
while (n != 0 && !eof())
{
size_t bytes_to_ignore = std::min(static_cast<size_t>(working_buffer.end() - pos), n);
pos += bytes_to_ignore;
n -= bytes_to_ignore;
}
if (n)
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throwReadAfterEOF();
}
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/// You could call this method `ignore`, and `ignore` call `ignoreStrict`.
size_t tryIgnore(size_t n)
{
size_t bytes_ignored = 0;
while (bytes_ignored < n && !eof())
{
size_t bytes_to_ignore = std::min(static_cast<size_t>(working_buffer.end() - pos), n - bytes_ignored);
pos += bytes_to_ignore;
bytes_ignored += bytes_to_ignore;
}
return bytes_ignored;
}
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/** Reads a single byte. */
bool ALWAYS_INLINE read(char & c)
{
if (eof())
return false;
c = *pos++;
return true;
}
void ALWAYS_INLINE readStrict(char & c)
{
if (read(c))
return;
throwReadAfterEOF();
}
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/** Reads as many as there are, no more than n bytes. */
size_t read(char * to, size_t n)
{
size_t bytes_copied = 0;
while (bytes_copied < n && !eof())
{
size_t bytes_to_copy = std::min(static_cast<size_t>(working_buffer.end() - pos), n - bytes_copied);
::memcpy(to + bytes_copied, pos, bytes_to_copy);
pos += bytes_to_copy;
bytes_copied += bytes_to_copy;
}
return bytes_copied;
}
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/** Reads n bytes, if there are less - throws an exception. */
void readStrict(char * to, size_t n)
{
auto read_bytes = read(to, n);
if (n != read_bytes)
throw Exception("Cannot read all data. Bytes read: " + std::to_string(read_bytes) + ". Bytes expected: " + std::to_string(n) + ".", ErrorCodes::CANNOT_READ_ALL_DATA);
}
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/** A method that can be more efficiently implemented in derived classes, in the case of reading large enough blocks.
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* The implementation can read data directly into `to`, without superfluous copying, if in `to` there is enough space for work.
* For example, a CompressedReadBuffer can decompress the data directly into `to`, if the entire decompressed block fits there.
* By default - the same as read.
* Don't use for small reads.
*/
virtual size_t readBig(char * to, size_t n)
{
return read(to, n);
}
protected:
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/// The number of bytes to ignore from the initial position of `working_buffer` buffer.
size_t working_buffer_offset = 0;
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private:
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/** Read the next data and fill a buffer with it.
* Return `false` in case of the end, `true` otherwise.
* Throw an exception if something is wrong.
*/
virtual bool nextImpl() { return false; }
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[[noreturn]] void throwReadAfterEOF()
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{
throw Exception("Attempt to read after eof", ErrorCodes::ATTEMPT_TO_READ_AFTER_EOF);
}
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};
using ReadBufferPtr = std::shared_ptr<ReadBuffer>;
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}