j2k-jpeg 0.6.1

JPEG inspect/decode and fallback encode support for j2k
Documentation 
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// SPDX-License-Identifier: Apache-2.0

//! APP14 "Adobe" color-transform detection.
//!
//! Layout per libjpeg-turbo's `jdmarker.c` and the Adobe DCT filters guide:
//!
//! ```text
//! APP14 payload:
//!   bytes[0..5]  = "Adobe" ASCII signature
//!   bytes[5..7]  = DCTEncodeVersion (unused)
//!   bytes[7..9]  = APP14Flags0
//!   bytes[9..11] = APP14Flags1
//!   bytes[11]    = ColorTransform:  0 = Unknown (no transform applied),
//!                                   1 = YCbCr,
//!                                   2 = YCCK
//! ```
//!
//! We return `None` for non-Adobe APP14 segments (any APP14 not beginning
//! with `"Adobe"` — those may be Adobe DCT but in an older container).

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum AdobeTransform {
    Unknown, // 0 — interpret as component-count heuristic (RGB for 3, CMYK for 4)
    YCbCr,   // 1
    Ycck,    // 2
}

pub(crate) fn parse_adobe_app14(payload: &[u8]) -> Option<AdobeTransform> {
    if payload.len() < 12 || &payload[0..5] != b"Adobe" {
        return None;
    }
    match payload[11] {
        0 => Some(AdobeTransform::Unknown),
        1 => Some(AdobeTransform::YCbCr),
        2 => Some(AdobeTransform::Ycck),
        _ => Some(AdobeTransform::Unknown), // emit warning at caller
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use alloc::vec;
    use alloc::vec::Vec;

    fn adobe_payload(transform: u8) -> Vec<u8> {
        let mut v = vec![b'A', b'd', b'o', b'b', b'e'];
        v.extend_from_slice(&[0x00, 0x64]); // DCTEncodeVersion 100
        v.extend_from_slice(&[0x00, 0x00]); // flags0
        v.extend_from_slice(&[0x00, 0x00]); // flags1
        v.push(transform);
        v
    }

    #[test]
    fn parses_adobe_ycbcr_transform() {
        assert_eq!(
            parse_adobe_app14(&adobe_payload(1)),
            Some(AdobeTransform::YCbCr)
        );
    }

    #[test]
    fn parses_adobe_ycck_transform() {
        assert_eq!(
            parse_adobe_app14(&adobe_payload(2)),
            Some(AdobeTransform::Ycck)
        );
    }

    #[test]
    fn parses_adobe_unknown_transform_zero() {
        assert_eq!(
            parse_adobe_app14(&adobe_payload(0)),
            Some(AdobeTransform::Unknown)
        );
    }

    #[test]
    fn parses_adobe_unknown_transform_out_of_range() {
        assert_eq!(
            parse_adobe_app14(&adobe_payload(99)),
            Some(AdobeTransform::Unknown)
        );
    }

    #[test]
    fn rejects_non_adobe_signature() {
        let mut p = adobe_payload(1);
        p[0] = b'X';
        assert_eq!(parse_adobe_app14(&p), None);
    }

    #[test]
    fn rejects_short_payload() {
        assert_eq!(parse_adobe_app14(b"Adobe"), None);
    }
}