/*
  Copyright 2010 Larry Gritz and the other authors and contributors.
  All Rights Reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are
  met:
  * Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaimer.
  * Redistributions in binary form must reproduce the above copyright
    notice, this list of conditions and the following disclaimer in the
    documentation and/or other materials provided with the distribution.
  * Neither the name of the software's owners nor the names of its
    contributors may be used to endorse or promote products derived from
    this software without specific prior written permission.
  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

  (This is the Modified BSD License)
*/

#include <cmath>
#include <cstdio>
#include <cstdlib>

#include "dds_pvt.h"

#include <OpenImageIO/dassert.h>
#include <OpenImageIO/fmath.h>
#include <OpenImageIO/imageio.h>
#include <OpenImageIO/typedesc.h>

#include "squish/squish.h"

OIIO_PLUGIN_NAMESPACE_BEGIN

using namespace DDS_pvt;

// uncomment the following define to enable 3x2 cube map layout
//#define DDS_3X2_CUBE_MAP_LAYOUT

class DDSInput final : public ImageInput {
public:
    DDSInput() { init(); }
    virtual ~DDSInput() { close(); }
    virtual const char* format_name(void) const override { return "dds"; }
    virtual bool open(const std::string& name, ImageSpec& newspec) override;
    virtual bool close() override;
    virtual int current_subimage(void) const override
    {
        lock_guard lock(m_mutex);
        return m_subimage;
    }
    virtual int current_miplevel(void) const override
    {
        lock_guard lock(m_mutex);
        return m_miplevel;
    }
    virtual bool seek_subimage(int subimage, int miplevel) override;
    virtual bool read_native_scanline(int subimage, int miplevel, int y, int z,
                                      void* data) override;
    virtual bool read_native_tile(int subimage, int miplevel, int x, int y,
                                  int z, void* data) override;

private:
    std::string m_filename;            ///< Stash the filename
    FILE* m_file;                      ///< Open image handle
    std::vector<unsigned char> m_buf;  ///< Buffer the image pixels
    int m_subimage;
    int m_miplevel;
    int m_nchans;            ///< Number of colour channels in image
    int m_nfaces;            ///< Number of cube map sides in image
    int m_Bpp;               ///< Number of bytes per pixel
    int m_redL, m_redR;      ///< Bit shifts to extract red channel
    int m_greenL, m_greenR;  ///< Bit shifts to extract green channel
    int m_blueL, m_blueR;    ///< Bit shifts to extract blue channel
    int m_alphaL, m_alphaR;  ///< Bit shifts to extract alpha channel

    dds_header m_dds;  ///< DDS header

    /// Reset everything to initial state
    ///
    void init()
    {
        m_file     = NULL;
        m_subimage = -1;
        m_miplevel = -1;
        m_buf.clear();
    }

    /// Helper function: read the image as scanlines (all but cubemaps).
    ///
    bool readimg_scanlines();

    /// Helper function: read the image as tiles (cubemaps only).
    ///
    bool readimg_tiles();

    /// Helper function: calculate bit shifts to properly extract channel data
    ///
    inline void calc_shifts(int mask, int& left, int& right);

    /// Helper function: performs the actual file seeking.
    ///
    void internal_seek_subimage(int cubeface, int miplevel, unsigned int& w,
                                unsigned int& h, unsigned int& d);

    /// Helper function: performs the actual pixel decoding.
    bool internal_readimg(unsigned char* dst, int w, int h, int d);

    /// Helper: read, with error detection
    ///
    bool fread(void* buf, size_t itemsize, size_t nitems)
    {
        size_t n = ::fread(buf, itemsize, nitems, m_file);
        if (n != nitems)
            error("Read error");
        return n == nitems;
    }
};



// Obligatory material to make this a recognizeable imageio plugin:
OIIO_PLUGIN_EXPORTS_BEGIN

OIIO_EXPORT ImageInput*
dds_input_imageio_create()
{
    return new DDSInput;
}

OIIO_EXPORT int dds_imageio_version = OIIO_PLUGIN_VERSION;

OIIO_EXPORT const char*
dds_imageio_library_version()
{
    return nullptr;
}

OIIO_EXPORT const char* dds_input_extensions[] = { "dds", nullptr };

OIIO_PLUGIN_EXPORTS_END



bool
DDSInput::open(const std::string& name, ImageSpec& newspec)
{
    m_filename = name;

    m_file = Filesystem::fopen(name, "rb");
    if (!m_file) {
        error("Could not open file \"%s\"", name.c_str());
        return false;
    }

// due to struct packing, we may get a corrupt header if we just load the
// struct from file; to adress that, read every member individually
// save some typing
#define RH(memb)                                                               \
    if (!fread(&m_dds.memb, sizeof(m_dds.memb), 1))                            \
    return false

    RH(fourCC);
    RH(size);
    RH(flags);
    RH(height);
    RH(width);
    RH(pitch);
    RH(depth);
    RH(mipmaps);

    // advance the file pointer by 44 bytes (reserved fields)
    fseek(m_file, 44, SEEK_CUR);

    // pixel format struct
    RH(fmt.size);
    RH(fmt.flags);
    RH(fmt.fourCC);
    RH(fmt.bpp);
    RH(fmt.rmask);
    RH(fmt.gmask);
    RH(fmt.bmask);
    RH(fmt.amask);

    // caps
    RH(caps.flags1);
    RH(caps.flags2);

    // advance the file pointer by 8 bytes (reserved fields)
    fseek(m_file, 8, SEEK_CUR);
#undef RH
    if (bigendian()) {
        // DDS files are little-endian
        // only swap values which are not flags or bitmasks
        swap_endian(&m_dds.size);
        swap_endian(&m_dds.height);
        swap_endian(&m_dds.width);
        swap_endian(&m_dds.pitch);
        swap_endian(&m_dds.depth);
        swap_endian(&m_dds.mipmaps);

        swap_endian(&m_dds.fmt.size);
        swap_endian(&m_dds.fmt.bpp);
    }

    /*std::cerr << "[dds] fourCC: " << ((char *)&m_dds.fourCC)[0]
                                  << ((char *)&m_dds.fourCC)[1]
                                  << ((char *)&m_dds.fourCC)[2]
                                  << ((char *)&m_dds.fourCC)[3]
                                  << " (" << m_dds.fourCC << ")\n";
    std::cerr << "[dds] size: " << m_dds.size << "\n";
    std::cerr << "[dds] flags: " << m_dds.flags << "\n";
    std::cerr << "[dds] pitch: " << m_dds.pitch << "\n";
    std::cerr << "[dds] width: " << m_dds.width << "\n";
    std::cerr << "[dds] height: " << m_dds.height << "\n";
    std::cerr << "[dds] depth: " << m_dds.depth << "\n";
    std::cerr << "[dds] mipmaps: " << m_dds.mipmaps << "\n";
    std::cerr << "[dds] fmt.size: " << m_dds.fmt.size << "\n";
    std::cerr << "[dds] fmt.flags: " << m_dds.fmt.flags << "\n";
    
    std::cerr << "[dds] fmt.fourCC: " << ((char *)&m_dds.fmt.fourCC)[0]
                                      << ((char *)&m_dds.fmt.fourCC)[1]
                                      << ((char *)&m_dds.fmt.fourCC)[2]
                                      << ((char *)&m_dds.fmt.fourCC)[3]
                                      << " (" << m_dds.fmt.fourCC << ")\n";
    std::cerr << "[dds] fmt.bpp: " << m_dds.fmt.bpp << "\n";
    std::cerr << "[dds] caps.flags1: " << m_dds.caps.flags1 << "\n";
    std::cerr << "[dds] caps.flags2: " << m_dds.caps.flags2 << "\n";*/

    // sanity checks - valid 4CC, correct struct sizes and flags which should
    // be always present, regardless of the image type, size etc., also check
    // for impossible flag combinations
    if (m_dds.fourCC != DDS_MAKE4CC('D', 'D', 'S', ' ') || m_dds.size != 124
        || m_dds.fmt.size != 32 || !(m_dds.caps.flags1 & DDS_CAPS1_TEXTURE)
        || !(m_dds.flags & DDS_CAPS) || !(m_dds.flags & DDS_PIXELFORMAT)
        || (m_dds.caps.flags2 & DDS_CAPS2_VOLUME
            && !(m_dds.caps.flags1 & DDS_CAPS1_COMPLEX
                 && m_dds.flags & DDS_DEPTH))
        || (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP
            && !(m_dds.caps.flags1 & DDS_CAPS1_COMPLEX))) {
        error("Invalid DDS header, possibly corrupt file");
        return false;
    }

    // make sure all dimensions are > 0 and that we have at least one channel
    // (for uncompressed images)
    if (!(m_dds.flags & DDS_WIDTH) || !m_dds.width
        || !(m_dds.flags & DDS_HEIGHT) || !m_dds.height
        || ((m_dds.flags & DDS_DEPTH) && !m_dds.depth)
        || (!(m_dds.fmt.flags & DDS_PF_FOURCC)
            && !((m_dds.fmt.flags & DDS_PF_RGB)
                 | (m_dds.fmt.flags & DDS_PF_LUMINANCE)
                 | (m_dds.fmt.flags & DDS_PF_ALPHA)))) {
        error("Image with no data");
        return false;
    }

    // validate the pixel format
    // TODO: support DXGI and the "wackier" uncompressed formats
    if (m_dds.fmt.flags & DDS_PF_FOURCC && m_dds.fmt.fourCC != DDS_4CC_DXT1
        && m_dds.fmt.fourCC != DDS_4CC_DXT2 && m_dds.fmt.fourCC != DDS_4CC_DXT3
        && m_dds.fmt.fourCC != DDS_4CC_DXT4
        && m_dds.fmt.fourCC != DDS_4CC_DXT5) {
        error("Unsupported compression type");
        return false;
    }

    // determine the number of channels we have
    if (m_dds.fmt.flags & DDS_PF_FOURCC) {
        // squish decompresses everything to RGBA anyway
        /*if (m_dds.fmt.fourCC == DDS_4CC_DXT1)
            m_nchans = 3; // no alpha in DXT1
        else*/
        m_nchans = 4;
    } else {
        m_nchans = ((m_dds.fmt.flags & DDS_PF_LUMINANCE) ? 1 : 3)
                   + ((m_dds.fmt.flags & DDS_PF_ALPHA) ? 1 : 0);
        // also calculate bytes per pixel and the bit shifts
        m_Bpp = (m_dds.fmt.bpp + 7) >> 3;
        if (!(m_dds.fmt.flags & DDS_PF_LUMINANCE)) {
            calc_shifts(m_dds.fmt.rmask, m_redL, m_redR);
            calc_shifts(m_dds.fmt.gmask, m_greenL, m_greenR);
            calc_shifts(m_dds.fmt.bmask, m_blueL, m_blueR);
            calc_shifts(m_dds.fmt.amask, m_alphaL, m_alphaR);
        }
    }

    // fix depth, pitch and mipmaps for later use, if needed
    if (!(m_dds.fmt.flags & DDS_PF_FOURCC && m_dds.flags & DDS_PITCH))
        m_dds.pitch = m_dds.width * m_Bpp;
    if (!(m_dds.caps.flags2 & DDS_CAPS2_VOLUME))
        m_dds.depth = 1;
    if (!(m_dds.flags & DDS_MIPMAPCOUNT))
        m_dds.mipmaps = 1;
    // count cube map faces
    if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP) {
        m_nfaces = 0;
        for (int flag = DDS_CAPS2_CUBEMAP_POSITIVEX;
             flag <= DDS_CAPS2_CUBEMAP_NEGATIVEZ; flag <<= 1) {
            if (m_dds.caps.flags2 & flag)
                m_nfaces++;
        }
    } else
        m_nfaces = 1;

    seek_subimage(0, 0);
    newspec = spec();
    return true;
}



inline void
DDSInput::calc_shifts(int mask, int& left, int& right)
{
    if (mask == 0) {
        left = right = 0;
        return;
    }

    int i, tmp = mask;
    for (i = 0; i < 32; i++, tmp >>= 1) {
        if (tmp & 1)
            break;
    }
    right = i;

    for (i = 0; i < 8; i++, tmp >>= 1) {
        if (!(tmp & 1))
            break;
    }
    left = 8 - i;
}



// NOTE: This function has no sanity checks! It's a private method and relies
// on the input being correct and valid!
void
DDSInput::internal_seek_subimage(int cubeface, int miplevel, unsigned int& w,
                                 unsigned int& h, unsigned int& d)
{
    // early out for cubemaps that don't contain the requested face
    if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP
        && !(m_dds.caps.flags2 & (DDS_CAPS2_CUBEMAP_POSITIVEX << cubeface))) {
        w = h = d = 0;
        return;
    }
    // we can easily calculate the offsets because both compressed and
    // uncompressed images have predictable length
    // calculate the offset; start with after the header
    unsigned int ofs = 128;
    unsigned int len;
    // this loop is used to iterate over cube map sides, or run once in the
    // case of ordinary 2D or 3D images
    for (int j = 0; j <= cubeface; j++) {
        w = m_dds.width;
        h = m_dds.height;
        d = m_dds.depth;
        // skip subimages preceding the one we're seeking to
        // if we have no mipmaps, the modulo formula doesn't work and we
        // don't skip at all, so just add the offset and continue
        if (m_dds.mipmaps < 2) {
            if (j > 0) {
                if (m_dds.fmt.flags & DDS_PF_FOURCC)
                    // only check for DXT1 - all other formats have same block
                    // size
                    len = squish::GetStorageRequirements(w, h,
                                                         m_dds.fmt.fourCC
                                                                 == DDS_4CC_DXT1
                                                             ? squish::kDxt1
                                                             : squish::kDxt5);
                else
                    len = w * h * d * m_Bpp;
                ofs += len;
            }
            continue;
        }
        for (int i = 0; i < miplevel; i++) {
            if (m_dds.fmt.flags & DDS_PF_FOURCC)
                // only check for DXT1 - all other formats have same block size
                len = squish::GetStorageRequirements(w, h,
                                                     m_dds.fmt.fourCC
                                                             == DDS_4CC_DXT1
                                                         ? squish::kDxt1
                                                         : squish::kDxt5);
            else
                len = w * h * d * m_Bpp;
            ofs += len;
            w >>= 1;
            if (!w)
                w = 1;
            h >>= 1;
            if (!h)
                h = 1;
            d >>= 1;
            if (!d)
                d = 1;
        }
    }
    // seek to the offset we've found
    fseek(m_file, ofs, SEEK_SET);
}



bool
DDSInput::seek_subimage(int subimage, int miplevel)
{
    if (subimage != 0)
        return false;

    // early out
    if (subimage == current_subimage() && miplevel == current_miplevel()) {
        return true;
    }

    // don't seek if the image doesn't contain mipmaps, isn't 3D or a cube map,
    // and don't seek out of bounds
    if (miplevel < 0
        || (!(m_dds.caps.flags1 & DDS_CAPS1_COMPLEX) && miplevel != 0)
        || (unsigned int)miplevel >= m_dds.mipmaps)
        return false;

    // clear buffer so that readimage is called
    m_buf.clear();

    // for cube maps, the seek will be performed when reading a tile instead
    unsigned int w = 0, h = 0, d = 0;
    if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP) {
        // calc sizes separately for cube maps
        w = m_dds.width;
        h = m_dds.height;
        d = m_dds.depth;
        for (int i = 1; i < miplevel; i++) {
            w >>= 1;
            if (w < 1)
                w = 1;
            h >>= 1;
            if (h < 1)
                h = 1;
            d >>= 1;
            if (d < 1)
                d = 1;
        }
        // create imagespec for the 3x2 cube map layout
#ifdef DDS_3X2_CUBE_MAP_LAYOUT
        m_spec = ImageSpec(w * 3, h * 2, m_nchans, TypeDesc::UINT8);
#else   // 1x6 layout
        m_spec = ImageSpec(w, h * 6, m_nchans, TypeDesc::UINT8);
#endif  // DDS_3X2_CUBE_MAP_LAYOUT
        m_spec.depth      = d;
        m_spec.tile_width = m_spec.full_width = w;
        m_spec.tile_height = m_spec.full_height = h;
        m_spec.tile_depth = m_spec.full_depth = d;
    } else {
        internal_seek_subimage(0, miplevel, w, h, d);
        // create imagespec
        m_spec       = ImageSpec(w, h, m_nchans, TypeDesc::UINT8);
        m_spec.depth = d;
    }

    // fill the imagespec
    if (m_dds.fmt.flags & DDS_PF_FOURCC) {
        std::string tempstr = "";
        tempstr += ((char*)&m_dds.fmt.fourCC)[0];
        tempstr += ((char*)&m_dds.fmt.fourCC)[1];
        tempstr += ((char*)&m_dds.fmt.fourCC)[2];
        tempstr += ((char*)&m_dds.fmt.fourCC)[3];
        m_spec.attribute("compression", tempstr);
    }
    m_spec.attribute("oiio:BitsPerSample", m_dds.fmt.bpp);
    m_spec.default_channel_names();

    // detect texture type
    if (m_dds.caps.flags2 & DDS_CAPS2_VOLUME) {
        m_spec.attribute("texturetype", "Volume Texture");
        m_spec.attribute("textureformat", "Volume Texture");
    } else if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP) {
        m_spec.attribute("texturetype", "Environment");
        m_spec.attribute("textureformat", "CubeFace Environment");
        // check available cube map sides
        std::string sides = "";
        if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP_POSITIVEX)
            sides += "+x";
        if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP_NEGATIVEX) {
            if (sides.size())
                sides += " ";
            sides += "-x";
        }
        if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP_POSITIVEY) {
            if (sides.size())
                sides += " ";
            sides += "+y";
        }
        if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP_NEGATIVEY) {
            if (sides.size())
                sides += " ";
            sides += "-y";
        }
        if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP_POSITIVEZ) {
            if (sides.size())
                sides += " ";
            sides += "+z";
        }
        if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP_NEGATIVEZ) {
            if (sides.size())
                sides += " ";
            sides += "-z";
        }
        m_spec.attribute("dds:CubeMapSides", sides);
    } else {
        m_spec.attribute("texturetype", "Plain Texture");
        m_spec.attribute("textureformat", "Plain Texture");
    }

    m_subimage = subimage;
    m_miplevel = miplevel;
    return true;
}



bool
DDSInput::internal_readimg(unsigned char* dst, int w, int h, int d)
{
    if (m_dds.fmt.flags & DDS_PF_FOURCC) {
        // compressed image
        int flags = 0;
        switch (m_dds.fmt.fourCC) {
        case DDS_4CC_DXT1: flags = squish::kDxt1; break;
        // DXT2 and 3 are the same, only 2 has pre-multiplied alpha
        case DDS_4CC_DXT2:
        case DDS_4CC_DXT3: flags = squish::kDxt3; break;
        // DXT4 and 5 are the same, only 4 has pre-multiplied alpha
        case DDS_4CC_DXT4:
        case DDS_4CC_DXT5: flags = squish::kDxt5; break;
        }
        // create source buffer
        std::vector<squish::u8> tmp(
            squish::GetStorageRequirements(w, h, flags));
        // load image into buffer
        if (!fread(&tmp[0], tmp.size(), 1))
            return false;
        // decompress image
        squish::DecompressImage(dst, w, h, &tmp[0], flags);
        tmp.clear();
        // correct pre-multiplied alpha, if necessary
        if (m_dds.fmt.fourCC == DDS_4CC_DXT2
            || m_dds.fmt.fourCC == DDS_4CC_DXT4) {
            int k;
            for (int y = 0; y < h; y++) {
                for (int x = 0; x < w; x++) {
                    k          = (y * w + x) * 4;
                    dst[k + 0] = (unsigned char)((int)dst[k + 0] * 255
                                                 / (int)dst[k + 3]);
                    dst[k + 1] = (unsigned char)((int)dst[k + 1] * 255
                                                 / (int)dst[k + 3]);
                    dst[k + 2] = (unsigned char)((int)dst[k + 2] * 255
                                                 / (int)dst[k + 3]);
                }
            }
        }
    } else {
        // uncompressed image

        // HACK: shortcut for luminance
        if (m_dds.fmt.flags & DDS_PF_LUMINANCE) {
            return fread(dst, w * m_Bpp, h);
        }

        int k, pixel = 0;
        for (int z = 0; z < d; z++) {
            for (int y = 0; y < h; y++) {
                for (int x = 0; x < w; x++) {
                    if (!fread(&pixel, 1, m_Bpp))
                        return false;
                    k          = (z * h * w + y * w + x) * m_spec.nchannels;
                    dst[k + 0] = ((pixel & m_dds.fmt.rmask) >> m_redR)
                                 << m_redL;
                    dst[k + 1] = ((pixel & m_dds.fmt.gmask) >> m_greenR)
                                 << m_greenL;
                    dst[k + 2] = ((pixel & m_dds.fmt.bmask) >> m_blueR)
                                 << m_blueL;
                    if (m_dds.fmt.flags & DDS_PF_ALPHA)
                        dst[k + 3] = ((pixel & m_dds.fmt.amask) >> m_alphaR)
                                     << m_alphaL;
                }
            }
        }
    }
    return true;
}



bool
DDSInput::readimg_scanlines()
{
    //std::cerr << "[dds] readimg: " << ftell (m_file) << "\n";
    // resize destination buffer
    m_buf.resize(m_spec.scanline_bytes() * m_spec.height * m_spec.depth
                 /*/ (1 << m_miplevel)*/);

    return internal_readimg(&m_buf[0], m_spec.width, m_spec.height,
                            m_spec.depth);
}



bool
DDSInput::readimg_tiles()
{
    // resize destination buffer
    m_buf.resize(m_spec.tile_bytes());

    return internal_readimg(&m_buf[0], m_spec.tile_width, m_spec.tile_height,
                            m_spec.tile_depth);
}



bool
DDSInput::close()
{
    if (m_file) {
        fclose(m_file);
        m_file = NULL;
    }

    init();  // Reset to initial state
    return true;
}



bool
DDSInput::read_native_scanline(int subimage, int miplevel, int y, int z,
                               void* data)
{
    lock_guard lock(m_mutex);
    if (!seek_subimage(subimage, miplevel))
        return false;

    // don't proceed if a cube map - use tiles then instead
    if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP)
        return false;
    if (m_buf.empty())
        readimg_scanlines();

    size_t size = spec().scanline_bytes();
    memcpy(data, &m_buf[0] + z * m_spec.height * size + y * size, size);
    return true;
}



bool
DDSInput::read_native_tile(int subimage, int miplevel, int x, int y, int z,
                           void* data)
{
    lock_guard lock(m_mutex);
    if (!seek_subimage(subimage, miplevel))
        return false;

    // static ints to keep track of the current cube face and re-seek and
    // re-read face
    static int lastx = -1, lasty = -1, lastz = -1;
    // don't proceed if not a cube map - use scanlines then instead
    if (!(m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP))
        return false;
    // make sure we get the right dimensions
    if (x % m_spec.tile_width || y % m_spec.tile_height
        || z % m_spec.tile_width)
        return false;
    if (m_buf.empty() || x != lastx || y != lasty || z != lastz) {
        lastx          = x;
        lasty          = y;
        lastz          = z;
        unsigned int w = 0, h = 0, d = 0;
#ifdef DDS_3X2_CUBE_MAP_LAYOUT
        internal_seek_subimage(((x / m_spec.tile_width) << 1)
                                   + y / m_spec.tile_height,
                               m_miplevel, w, h, d);
#else   // 1x6 layout
        internal_seek_subimage(y / m_spec.tile_height, m_miplevel, w, h, d);
#endif  // DDS_3X2_CUBE_MAP_LAYOUT
        if (!w && !h && !d)
            // face not present in file, black-pad the image
            memset(&m_buf[0], 0, m_spec.tile_bytes());
        else
            readimg_tiles();
    }

    memcpy(data, &m_buf[0], m_spec.tile_bytes());
    return true;
}

OIIO_PLUGIN_NAMESPACE_END