Function write_coefficients 

pub fn write_coefficients(
    jpeg: &[u8],
    coeffs: &JpegCoefficients,
) -> Result<Vec<u8>, DctError>

Re-encode a JPEG with modified DCT coefficients.

Takes the original JPEG bytes and a JpegCoefficients (typically obtained from read_coefficients and then modified), and produces a new JPEG byte stream with the updated coefficients re-encoded using the same Huffman tables as the original.

The output is a valid JPEG. All non-entropy-coded segments (EXIF, ICC profile, quantization tables, etc.) are preserved verbatim.

Safety note

The output is only as valid as the input JPEG’s Huffman tables permit. If you set a coefficient to a value whose (run, category) symbol does not exist in the original Huffman table, encoding will return DctError::CorruptEntropy. Stick to modifying the LSB of coefficients with |v| >= 2 (JSteg-style) to stay safely within the table.

Errors

Returns DctError::Incompatible if coeffs has a different number of components, a different block count, or mismatched component IDs compared to the original JPEG. Returns DctError for any parse or encoding failure.