core/fxcrt/fx_basic_util.cpp


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// Copyright 2014 PDFium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com

#include <algorithm>

#include "core/fxcrt/fx_system.h"

uint32_t GetBits32(const uint8_t* pData, int bitpos, int nbits) {
  ASSERT(0 < nbits && nbits <= 32);
  const uint8_t* dataPtr = &pData[bitpos / 8];
  int bitShift;
  int bitMask;
  int dstShift;
  int bitCount = bitpos & 0x07;
  if (nbits < 8 && nbits + bitCount <= 8) {
    bitShift = 8 - nbits - bitCount;
    bitMask = (1 << nbits) - 1;
    dstShift = 0;
  } else {
    bitShift = 0;
    int bitOffset = 8 - bitCount;
    bitMask = (1 << std::min(bitOffset, nbits)) - 1;
    dstShift = nbits - bitOffset;
  }
  uint32_t result =
      static_cast<uint32_t>((*dataPtr++ >> bitShift & bitMask) << dstShift);
  while (dstShift >= 8) {
    dstShift -= 8;
    result |= *dataPtr++ << dstShift;
  }
  if (dstShift > 0) {
    bitShift = 8 - dstShift;
    bitMask = (1 << dstShift) - 1;
    result |= *dataPtr++ >> bitShift & bitMask;
  }
  return result;
}