// Set most of the fields

width(w),

height(h),

output(out),

positionX(0),

positionY(0),

deflateFilled(0),

adler(1) {

// Check arguments

if (width == 0 || height == 0)

throw std::domain_error("Zero width or height");

// Compute and check data siezs

uint64_t lineSz = static_cast<uint64_t>(width) * 3 + 1;

if (lineSz > UINT32_MAX)

throw std::length_error("Image too large");

lineSize = static_cast<uint32_t>(lineSz);

uint64_t uncompRm = lineSize * height;

if (uncompRm > UINT32_MAX)

throw std::length_error("Image too large");

uncompRemain = static_cast<uint32_t>(uncompRm);

uint32_t numBlocks = uncompRemain / DEFLATE_MAX_BLOCK_SIZE;

if (uncompRemain % DEFLATE_MAX_BLOCK_SIZE != 0)

numBlocks++; // Round up

// 5 bytes per DEFLATE uncompressed block header, 2 bytes for zlib header, 4 bytes for zlib Adler-32 footer

uint64_t idatSize = static_cast<uint64_t>(numBlocks) * 5 + 6;

idatSize += uncompRemain;

if (idatSize > static_cast<uint32_t>(INT32_MAX))

throw std::length_error("Image too large");

// Write header (not a pure header, but a couple of things concatenated together)

uint8_t header[] = { // 43 bytes long

// PNG header

0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A,

// IHDR chunk

0x00, 0x00, 0x00, 0x0D,

0x49, 0x48, 0x44, 0x52,

0, 0, 0, 0, // 'width' placeholder

0, 0, 0, 0, // 'height' placeholder

0x08, 0x02, 0x00, 0x00, 0x00,

0, 0, 0, 0, // IHDR CRC-32 placeholder

// IDAT chunk

0, 0, 0, 0, // 'idatSize' placeholder

0x49, 0x44, 0x41, 0x54,

// DEFLATE data

0x08, 0x1D,

};

putBigUint32(width, &header[16]);

putBigUint32(height, &header[20]);

putBigUint32(idatSize, &header[33]);

crc = 0;

crc32(&header[12], 17);

putBigUint32(crc, &header[29]);

write(header);

crc = 0;

crc32(&header[37], 6); // 0xD7245B6B

if (count > SIZE_MAX / 3)

throw std::length_error("Invalid argument");

count *= 3; // Convert pixel count to byte count

while (count > 0) {

if (pixels == nullptr)

throw std::invalid_argument("Null pointer");

if (positionY >= height)

throw std::logic_error("All image pixels already written");

if (deflateFilled == 0) { // Start DEFLATE block

uint16_t size = DEFLATE_MAX_BLOCK_SIZE;

if (uncompRemain < size)

size = static_cast<uint16_t>(uncompRemain);

const uint8_t header[] = { // 5 bytes long

static_cast<uint8_t>(uncompRemain <= DEFLATE_MAX_BLOCK_SIZE ? 1 : 0),

static_cast<uint8_t>(size >> 0),

static_cast<uint8_t>(size >> 8),

static_cast<uint8_t>((size >> 0) ^ 0xFF),

static_cast<uint8_t>((size >> 8) ^ 0xFF),

};

write(header);

crc32(header, sizeof(header) / sizeof(header[0]));

}

assert(positionX < lineSize && deflateFilled < DEFLATE_MAX_BLOCK_SIZE);

if (positionX == 0) { // Beginning of line - write filter method byte

uint8_t b[] = {0};

write(b);

crc32(b, 1);

adler32(b, 1);

positionX++;

uncompRemain--;

deflateFilled++;

} else { // Write some pixel bytes for current line

uint16_t n = DEFLATE_MAX_BLOCK_SIZE - deflateFilled;

if (lineSize - positionX < n)

n = static_cast<uint16_t>(lineSize - positionX);

if (count < n)

n = static_cast<uint16_t>(count);

if (static_cast<std::make_unsigned<std::streamsize>::type>(std::numeric_limits<std::streamsize>::max()) < std::numeric_limits<decltype(n)>::max())

n = std::min(n, static_cast<decltype(n)>(std::numeric_limits<std::streamsize>::max()));

assert(n > 0);

output.write(reinterpret_cast<const char*>(pixels), static_cast<std::streamsize>(n));

// Update checksums

crc32(pixels, n);

adler32(pixels, n);

// Increment positions

count -= n;

pixels += n;

positionX += n;

uncompRemain -= n;

deflateFilled += n;

}

if (deflateFilled >= DEFLATE_MAX_BLOCK_SIZE)

deflateFilled = 0; // End current block

if (positionX == lineSize) { // Increment line

positionX = 0;

positionY++;

if (positionY == height) { // Reached end of pixels

uint8_t footer[] = { // 20 bytes long

0, 0, 0, 0, // DEFLATE Adler-32 placeholder

0, 0, 0, 0, // IDAT CRC-32 placeholder

// IEND chunk

0x00, 0x00, 0x00, 0x00,

0x49, 0x45, 0x4E, 0x44,

0xAE, 0x42, 0x60, 0x82,

};

putBigUint32(adler, &footer[0]);

crc32(&footer[0], 4);

putBigUint32(crc, &footer[4]);

write(footer);

}

}

}

crc = ~crc;

for (size_t i = 0; i < len; i++) {

for (int j = 0; j < 8; j++) { // Inefficient bitwise implementation, instead of table-based

uint32_t bit = (crc ^ (data[i] >> j)) & 1;

crc = (crc >> 1) ^ ((-bit) & UINT32_C(0xEDB88320));

}

}

crc = ~crc;

uint32_t s1 = adler & 0xFFFF;

uint32_t s2 = adler >> 16;

for (size_t i = 0; i < len; i++) {

s1 = (s1 + data[i]) % 65521;

s2 = (s2 + s1) % 65521;

}

adler = s2 << 16 | s1;