// RD-03E frame scanner — pure logic, host-testable. See rd03e_parse.h.

#include "rd03e_parse.h"

bool rd03e_parse_latest(const uint8_t *buf, size_t len, rd03e_frame_t *out) {
    if (buf == NULL || out == NULL || len < RD03E_FRAME_LEN) {
        return false;
    }

    bool parsed_any = false;
    rd03e_frame_t latest = {0};

    // Scan for the newest complete, validated frame in whatever arrived
    // this cycle; keep overwriting `latest` so we report the freshest one.
    //
    // The `i + RD03E_FRAME_LEN <= len` bound is what makes a truncated
    // trailing frame get ignored rather than read past the buffer: a
    // partial frame at the end simply never satisfies the bound.
    for (size_t i = 0; i + RD03E_FRAME_LEN <= len; i++) {
        if (buf[i] != RD03E_FRAME_HEADER) {
            continue;
        }
        // Footer lives at [4] and [5] of the frame. If this offset only
        // *looks* like a header (a 0xAA that is really a distance byte, a
        // gesture code, or line noise), the footer check rejects it and
        // the scan resynchronises on the next byte.
        if (buf[i + 4] != RD03E_FRAME_FOOTER0 || buf[i + 5] != RD03E_FRAME_FOOTER1) {
            continue; // not a real header byte, or a corrupted frame
        }
        latest.gesture = buf[i + 1];
        // Little-endian: low byte first. Getting this backwards, or
        // letting the footer bytes bleed into the high byte, is exactly
        // the bug this unit exists to make testable.
        latest.distance_cm = (uint16_t)((uint16_t)buf[i + 2] | ((uint16_t)buf[i + 3] << 8));
        parsed_any = true;
        i += RD03E_FRAME_LEN - 1; // loop's i++ moves past this frame
    }

    if (parsed_any) {
        *out = latest;
    }
    return parsed_any;
}