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//! `#[br(seek = <bits>)]` — read a field at an absolute bit offset (following a
//! pointer), and `#[br(dbg)]` — trace a field as it decodes.
//!
//! `seek` is a read-side primitive (the writer is append-only). Paired with
//! `restore_position` it reads at an offset and returns, so later fields continue from
//! where they were — the classic offset-table / pointer-follow shape. Because it seeks,
//! `decode_from` is bound on `SeekSource`; the slice entry points always qualify.
use bnb::{bin, prelude::*};
#[bin(big)]
#[derive(Debug, PartialEq)]
struct Record {
/// Byte offset of `target` within the buffer.
ptr: u8,
/// Read the byte at `ptr`, then rewind so `next` reads right after `ptr`.
#[br(seek = ptr.bytes(), restore_position)]
target: u8,
next: u8,
}
#[test]
fn seek_follows_a_pointer_then_restore_position_returns() {
// [ptr=3][next=0x11][filler=0x22][target=0xAB]
let buf = [0x03, 0x11, 0x22, 0xAB];
// `peek` doesn't require full consumption — seek/restore leave bytes 2.. untouched.
let p = Record::peek(&buf).unwrap();
assert_eq!(p.ptr, 3);
assert_eq!(p.target, 0xAB); // read from byte offset 3 via seek
assert_eq!(p.next, 0x11); // restore_position rewound, so `next` is byte 1
}
#[test]
fn seek_reads_at_an_offset_without_restore() {
// Without restore_position the cursor stays past the seeked read; useful when the
// pointee is the tail of the message. Here `target` lands exactly at the end.
#[bin(big)]
#[derive(Debug, PartialEq)]
struct Tail {
off: u8,
#[br(seek = off.bytes())]
target: u8,
}
let t = Tail::peek(&[0x02, 0x00, 0x7E]).unwrap();
assert_eq!(t.off, 2);
assert_eq!(t.target, 0x7E); // byte offset 2
}
// `dbg` is a read-side diagnostic: it emits a `tracing` event but is otherwise inert —
// decode and encode are unaffected (enable with `RUST_LOG=bnb::dbg=trace`).
#[bin(big)]
#[derive(Debug, PartialEq)]
struct Dbg {
a: u8,
#[br(dbg)]
b: u16,
}
#[test]
fn dbg_is_inert_for_the_codec() {
let m = Dbg::decode_exact(&[0x01, 0xCA, 0xFE]).unwrap();
assert_eq!(m, Dbg { a: 1, b: 0xCAFE });
assert_eq!(m.to_bytes().unwrap(), [0x01, 0xCA, 0xFE]); // write side untouched
}