//! Retained crash-record codec primitives.
//!
//! This module defines and validates a fixed record only. It does not install a
//! panic handler, write retention memory, access flash, or transmit MAVLink.

use crate::KaptanError;

const MAGIC: u32 = u32::from_le_bytes(*b"KLOG");
const VERSION: u16 = 1;
const COMMITTED: u32 = u32::MAX;
/// Fixed endianness-independent retained record size.
pub const CRASH_RECORD_BYTES: usize = 32;

/// Stored reason for a previous abnormal or orderly runtime end.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
#[repr(u8)]
pub enum CrashKind {
    /// A panic was captured by a future target-specific handler.
    Panic = 1,
    /// A hard fault was captured by a future target-specific handler.
    HardFault = 2,
    /// A requested shutdown sequence was captured.
    Shutdown = 3,
    /// A platform reset cause was captured.
    Reset = 4,
}

impl CrashKind {
    const fn as_u8(self) -> u8 {
        self as u8
    }

    const fn from_u8(value: u8) -> Option<Self> {
        match value {
            1 => Some(Self::Panic),
            2 => Some(Self::HardFault),
            3 => Some(Self::Shutdown),
            4 => Some(Self::Reset),
            _ => None,
        }
    }
}

/// Validation failure for a retained record.
#[derive(Clone, Copy, Debug, Eq, PartialEq, KaptanError)]
pub enum CrashRecordError {
    /// Magic/version/commit marker did not identify a completed record.
    InvalidHeader,
    /// Record payload did not match its checksum.
    BadChecksum,
}

/// Fixed, commit-last retained record value.
///
/// A board integration writes every field except `commit`, writes `checksum`,
/// then writes `commit` last. This pure type only validates that protocol.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct CrashRecord {
    kind: CrashKind,
    boot_id: u64,
    detail: u64,
}

impl CrashRecord {
    /// Creates one completed record for tests or a future retention backend.
    pub fn new(kind: CrashKind, boot_id: u64, detail: u64) -> Self {
        Self {
            kind,
            boot_id,
            detail,
        }
    }

    /// Encodes an endian-independent record. A target writes the final four
    /// commit-marker bytes last; this function itself performs no I/O.
    pub fn encode(self) -> [u8; CRASH_RECORD_BYTES] {
        let mut bytes = [0_u8; CRASH_RECORD_BYTES];
        bytes[0..4].copy_from_slice(&MAGIC.to_le_bytes());
        bytes[4..6].copy_from_slice(&VERSION.to_le_bytes());
        bytes[6] = self.kind.as_u8();
        bytes[7] = 0;
        bytes[8..16].copy_from_slice(&self.boot_id.to_le_bytes());
        bytes[16..24].copy_from_slice(&self.detail.to_le_bytes());
        let record_checksum = checksum(&bytes[..24]);
        bytes[24..28].copy_from_slice(&record_checksum.to_le_bytes());
        bytes[28..32].copy_from_slice(&COMMITTED.to_le_bytes());
        bytes
    }

    /// Decodes and validates an explicitly little-endian retained record.
    /// Arbitrary retained bytes are never reinterpreted as Rust enums.
    pub fn decode(bytes: [u8; CRASH_RECORD_BYTES]) -> Result<Self, CrashRecordError> {
        if u32::from_le_bytes(bytes[0..4].try_into().unwrap()) != MAGIC
            || u16::from_le_bytes(bytes[4..6].try_into().unwrap()) != VERSION
            || bytes[7] != 0
            || u32::from_le_bytes(bytes[28..32].try_into().unwrap()) != COMMITTED
        {
            return Err(CrashRecordError::InvalidHeader);
        }
        if u32::from_le_bytes(bytes[24..28].try_into().unwrap()) != checksum(&bytes[..24]) {
            return Err(CrashRecordError::BadChecksum);
        }
        let Some(kind) = CrashKind::from_u8(bytes[6]) else {
            return Err(CrashRecordError::InvalidHeader);
        };
        Ok(Self {
            kind,
            boot_id: u64::from_le_bytes(bytes[8..16].try_into().unwrap()),
            detail: u64::from_le_bytes(bytes[16..24].try_into().unwrap()),
        })
    }

    /// Captured end reason.
    pub const fn kind(self) -> CrashKind {
        self.kind
    }

    /// Board-local boot identity.
    pub const fn boot_id(self) -> u64 {
        self.boot_id
    }

    /// Target-defined compact panic/fault/reset detail.
    pub const fn detail(self) -> u64 {
        self.detail
    }
}

const fn checksum(bytes: &[u8]) -> u32 {
    let mut state = 0xffff_ffff_u32;
    let mut index = 0;
    while index < bytes.len() {
        state = crc32c_byte(state, bytes[index]);
        index += 1;
    }
    !state
}

const fn crc32c_byte(mut state: u32, byte: u8) -> u32 {
    state ^= byte as u32;
    let mut index = 0;
    while index != 8 {
        state = if state & 1 != 0 {
            (state >> 1) ^ 0x82f6_3b78
        } else {
            state >> 1
        };
        index += 1;
    }
    state
}

#[cfg(test)]
mod tests {
    use super::{CrashKind, CrashRecord, CrashRecordError};
    use crate::ErrorReport;

    #[test]
    fn retained_record_rejects_torn_or_corrupt_payloads() {
        let record = CrashRecord::new(CrashKind::Panic, 7, 11);
        let bytes = record.encode();
        assert_eq!(CrashRecord::decode(bytes), Ok(record));
        let mut corrupt = bytes;
        corrupt[24] ^= 1;
        assert_eq!(
            CrashRecord::decode(corrupt),
            Err(CrashRecordError::BadChecksum)
        );
        let mut torn = bytes;
        torn[28..32].copy_from_slice(&0_u32.to_le_bytes());
        assert_eq!(
            CrashRecord::decode(torn),
            Err(CrashRecordError::InvalidHeader)
        );
    }

    #[test]
    fn validation_error_has_a_static_derived_message() {
        assert_eq!(
            CrashRecordError::BadChecksum.static_error().path(),
            "kaptan_log::crash::CrashRecordError::BadChecksum"
        );
    }
}