io.h

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#ifndef RABIT_INTERNAL_IO_H_
#define RABIT_INTERNAL_IO_H_
 
#include <algorithm>
#include <cstddef>  // for size_t
#include <cstdio>
#include <cstring>  // for memcpy
#include <limits>
#include <numeric>
#include <string>
#include <vector>
 
#include "rabit/internal/utils.h"
#include "rabit/serializable.h"
 
namespace rabit::utils {
using SeekStream = dmlc::SeekStream;
struct MemoryFixSizeBuffer : public SeekStream {
 public:
  // similar to SEEK_END in libc
  static std::size_t constexpr kSeekEnd = std::numeric_limits<std::size_t>::max();
 
 public:
  MemoryFixSizeBuffer(void *p_buffer, std::size_t buffer_size)
      : p_buffer_(reinterpret_cast<char *>(p_buffer)), buffer_size_(buffer_size) {}
  ~MemoryFixSizeBuffer() override = default;
 
  std::size_t Read(void *ptr, std::size_t size) override {
    std::size_t nread = std::min(buffer_size_ - curr_ptr_, size);
    if (nread != 0) std::memcpy(ptr, p_buffer_ + curr_ptr_, nread);
    curr_ptr_ += nread;
    return nread;
  }
  void Write(const void *ptr, std::size_t size) override {
    if (size == 0) return;
    CHECK_LE(curr_ptr_ + size, buffer_size_);
    std::memcpy(p_buffer_ + curr_ptr_, ptr, size);
    curr_ptr_ += size;
  }
  void Seek(std::size_t pos) override {
    if (pos == kSeekEnd) {
      curr_ptr_ = buffer_size_;
    } else {
      curr_ptr_ = static_cast<std::size_t>(pos);
    }
  }
  std::size_t Tell() override { return curr_ptr_; }
  [[nodiscard]] virtual bool AtEnd() const { return curr_ptr_ == buffer_size_; }
 
 protected:
  char *p_buffer_{nullptr};
  std::size_t buffer_size_{0};
  std::size_t curr_ptr_{0};
};
 
struct MemoryBufferStream : public SeekStream {
 public:
  explicit MemoryBufferStream(std::string *p_buffer)
      : p_buffer_(p_buffer) {
    curr_ptr_ = 0;
  }
  ~MemoryBufferStream() override = default;
  size_t Read(void *ptr, size_t size) override {
    utils::Assert(curr_ptr_ <= p_buffer_->length(),
                  "read can not have position excceed buffer length");
    size_t nread = std::min(p_buffer_->length() - curr_ptr_, size);
    if (nread != 0) std::memcpy(ptr, &(*p_buffer_)[0] + curr_ptr_, nread);
    curr_ptr_ += nread;
    return nread;
  }
  void Write(const void *ptr, size_t size) override {
    if (size == 0) return;
    if (curr_ptr_ + size > p_buffer_->length()) {
      p_buffer_->resize(curr_ptr_+size);
    }
    std::memcpy(&(*p_buffer_)[0] + curr_ptr_, ptr, size);
    curr_ptr_ += size;
  }
  void Seek(size_t pos) override {
    curr_ptr_ = static_cast<size_t>(pos);
  }
  size_t Tell() override {
    return curr_ptr_;
  }
  virtual bool AtEnd() const {
    return curr_ptr_ == p_buffer_->length();
  }
 
 private:
  std::string *p_buffer_;
  size_t curr_ptr_;
};  // class MemoryBufferStream
}  // namespace rabit::utils
#endif  // RABIT_INTERNAL_IO_H_