message_layout.rs

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//! 64-byte message frame layout constants.
//!
//! This module is the single source of truth for the on-wire message
//! header layout shared between encoders, decoders, and zero-copy views.
//!
//! ```text
//! [checksum:8][id:16][offset:8][timestamp:8][origin_timestamp:8]
//! [user_headers_length:4][payload_length:4][reserved:8]
//! ```

/// Fixed-size message header on the wire (64 bytes).
pub const WIRE_MESSAGE_HEADER_SIZE: usize = 64;

/// Fixed-size index entry per message (16 bytes).
///
/// Layout: `[0x00000000:4][cumulative_size:u32_le:4][0x0000000000000000:8]`
pub const WIRE_MESSAGE_INDEX_SIZE: usize = 16;

// Field offsets within the 64-byte message header.
pub const MSG_CHECKSUM_OFFSET: usize = 0;
pub const MSG_ID_OFFSET: usize = 8;
pub const MSG_OFFSET_OFFSET: usize = 24;
pub const MSG_TIMESTAMP_OFFSET: usize = 32;
pub const MSG_ORIGIN_TIMESTAMP_OFFSET: usize = 40;
pub const MSG_USER_HEADERS_LEN_OFFSET: usize = 48;
pub const MSG_PAYLOAD_LEN_OFFSET: usize = 52;
pub const MSG_RESERVED_OFFSET: usize = 56;

// Compile-time verification that field offsets are contiguous and sum to the header size.
const _: () = {
    assert!(MSG_ID_OFFSET == MSG_CHECKSUM_OFFSET + 8);
    assert!(MSG_OFFSET_OFFSET == MSG_ID_OFFSET + 16);
    assert!(MSG_TIMESTAMP_OFFSET == MSG_OFFSET_OFFSET + 8);
    assert!(MSG_ORIGIN_TIMESTAMP_OFFSET == MSG_TIMESTAMP_OFFSET + 8);
    assert!(MSG_USER_HEADERS_LEN_OFFSET == MSG_ORIGIN_TIMESTAMP_OFFSET + 8);
    assert!(MSG_PAYLOAD_LEN_OFFSET == MSG_USER_HEADERS_LEN_OFFSET + 4);
    assert!(MSG_RESERVED_OFFSET == MSG_PAYLOAD_LEN_OFFSET + 4);
    assert!(MSG_RESERVED_OFFSET + 8 == WIRE_MESSAGE_HEADER_SIZE);
};