// The CUBE store's format, in one file, used by both sides.
//
// `cube-store-raw` (and through it `cube-cli`, `cube-image`) and this kernel's driver have to agree
// about the store's bytes to the byte: the digest line is diffed between a kernel-written image and
// a userspace-written one, a listing taken through `cube(2)` is compared with one taken by
// userspace, and a store the kernel folds has to be a store userspace can read. They were two
// implementations of one format, and when v3 landed the kernel moved and userspace did not — an
// hour of "unsupported-version" and a README's worth of careful duplication.
//
// Two build systems cannot share a crate: the kernel's Rust build compiles what is in its own
// module tree, and a cargo crate is not that. So they share a *file*, instead: this one, which the
// driver declares as a module and `crates/cube-format` includes by path. There is no copy to drift
// from, which is the only arrangement that cannot go stale.
//
// Everything here is a pure function over slices: no allocation, no I/O, no `std`, no logging. The
// kernel reads into buffers with its own allocator and userspace already has the whole image; what
// they must agree on is the layout, and that is what lives here.
//
// The constants are `pub` because both sides need them (a reader computing a stride, a writer
// sizing an image), and `#[allow(dead_code)]` is applied per item where one side does not use it —
// an unused `pub` constant in a kernel module is a warning, and a warning-free build is how the
// gates stay readable.

/// Every image starts with these four bytes.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const MAGIC: &[u8; 4] = b"CUBE";

/// The original format: magic, version, curve, then records until zero padding.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const VERSION_V1: u8 = 1;
/// The packed format: the same, plus the image's byte extent and record count.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const VERSION_V2: u8 = 2;
/// The addressed format: the same, plus a space table, a fixed-size index, and packed values.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const VERSION_V3: u8 = 3;

#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const HEADER_LEN_V1: usize = 6;
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const HEADER_LEN_V2: usize = 6 + 8 + 8;
/// magic(4) version(1) curve(1) image_bytes(8) record_count(8) space_count(8) index_off(8)
/// values_off(8).
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const HEADER_LEN_V3: usize = 4 + 1 + 1 + 8 + 8 + 8 + 8 + 8;

#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const SPACE_ID_LEN: usize = 32;
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const RAW_KEY_LEN: usize = 24;
/// A packed record's fixed part: `space | key | value_len(u64)`, with the value behind it.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const RECORD_FIXED: usize = SPACE_ID_LEN + RAW_KEY_LEN + 8;
/// A v3 index entry: `key | value_off(u64) | value_len(u64)`.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const INDEX_ENTRY: usize = RAW_KEY_LEN + 8 + 8;
/// A v3 space-table row: `space | first index(u64) | records(u64)`.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const SPACE_ENTRY: usize = SPACE_ID_LEN + 8 + 8;

/// The log's framing, which the fold and the readers both parse.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const WAL_MAGIC: &[u8; 4] = b"CUBW";
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const WAL_VERSION: u8 = 1;
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const WAL_HEADER_LEN: usize = 6;
/// `op(1) | crc(4) | space(32) | key(24) | len(4)`, with the value behind it.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const ENTRY_FIXED: usize = 1 + 4 + SPACE_ID_LEN + RAW_KEY_LEN + 4;

/// `CUBE_OP_PUT`: an entry that stores a value.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const WAL_OP_WRITE: u8 = 1;
/// `CUBE_OP_DEL`: an entry that removes one.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const WAL_OP_DELETE: u8 = 2;

/// What an image's header says, in any version.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub struct Header {
    pub version: u8,
    pub curve: u8,
    /// The image's byte length, header included. `None` in v1, which states no extent.
    pub image_bytes: Option<u64>,
    /// How many records follow the header. `None` in v1.
    pub record_count: Option<u64>,
}

impl Header {
    /// Where the records start.
    pub fn records_off(&self) -> usize {
        if self.version == VERSION_V1 {
            HEADER_LEN_V1
        } else if self.version == VERSION_V3 {
            HEADER_LEN_V3
        } else {
            HEADER_LEN_V2
        }
    }

    /// The image's extent within `bytes`: what the header promises, or all of it for v1.
    pub fn extent(&self, bytes_len: usize) -> usize {
        match self.image_bytes {
            Some(n) => core::cmp::min(n as usize, bytes_len),
            None => bytes_len,
        }
    }
}

/// Why an image was refused. A reader that guesses at an extent can read somebody else's bytes, so
/// these are all refusals rather than defaults.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub enum Bad {
    /// Not a CUBE image at all.
    Magic,
    /// A version this build does not know.
    Version(u8),
    /// A header that does not fit, or an extent that does not cover its own header.
    Extent,
    /// A v3 table that does not sit where the header says it does.
    Tables,
}

/// Read and validate an image header.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub fn parse_header(bytes: &[u8]) -> Result<Header, Bad> {
    if bytes.len() < 5 || &bytes[0..4] != MAGIC {
        return Err(Bad::Magic);
    }
    let version = bytes[4];
    let curve = bytes[5];
    match version {
        VERSION_V1 => Ok(Header {
            version,
            curve,
            image_bytes: None,
            record_count: None,
        }),
        VERSION_V2 => {
            if bytes.len() < HEADER_LEN_V2 {
                return Err(Bad::Magic);
            }
            let image_bytes = le_u64(bytes, 6);
            let record_count = le_u64(bytes, 14);
            if (image_bytes as usize) < HEADER_LEN_V2 {
                return Err(Bad::Extent);
            }
            Ok(Header {
                version,
                curve,
                image_bytes: Some(image_bytes),
                record_count: Some(record_count),
            })
        }
        VERSION_V3 => {
            if bytes.len() < HEADER_LEN_V3 {
                return Err(Bad::Magic);
            }
            let geometry = V3::decode(bytes)?;
            Ok(Header {
                version,
                curve,
                image_bytes: Some(le_u64(bytes, 6)),
                record_count: Some(geometry.record_count),
            })
        }
        other => Err(Bad::Version(other)),
    }
}

/// The v3 tables' geometry: where the index is, where the values start, and how many of each.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub struct V3 {
    pub record_count: u64,
    pub space_count: u64,
    pub index_off: u64,
    pub values_off: u64,
}

impl V3 {
    /// Read the tables' geometry, refusing anything that does not add up.
    ///
    /// The three equalities here are the whole point of the format: a reader computes a record's
    /// place as `index_off + i * INDEX_ENTRY`, and that is only a place if the index really starts
    /// there and really is that wide.
    pub fn decode(bytes: &[u8]) -> Result<Self, Bad> {
        if bytes.len() < HEADER_LEN_V3 || &bytes[0..4] != MAGIC || bytes[4] != VERSION_V3 {
            return Err(Bad::Magic);
        }
        let geometry = V3 {
            record_count: le_u64(bytes, 14),
            space_count: le_u64(bytes, 22),
            index_off: le_u64(bytes, 30),
            values_off: le_u64(bytes, 38),
        };
        let image_bytes = le_u64(bytes, 6);
        if image_bytes < HEADER_LEN_V3 as u64 {
            return Err(Bad::Extent);
        }
        let table_end = HEADER_LEN_V3 as u64 + geometry.space_count * SPACE_ENTRY as u64;
        let index_end = geometry.index_off + geometry.record_count * INDEX_ENTRY as u64;
        if geometry.index_off != table_end
            || geometry.values_off != index_end
            || geometry.values_off > image_bytes
        {
            return Err(Bad::Tables);
        }
        Ok(geometry)
    }

    /// Write the header this geometry describes. `image_bytes` is filled in by the caller's
    /// arithmetic, since only it knows how long the values are.
    pub fn encode(&self, out: &mut [u8], curve: u8, image_bytes: u64) -> Result<usize, Bad> {
        if out.len() < HEADER_LEN_V3 {
            return Err(Bad::Extent);
        }
        out[0..4].copy_from_slice(MAGIC);
        out[4] = VERSION_V3;
        out[5] = curve;
        out[6..14].copy_from_slice(&image_bytes.to_le_bytes());
        out[14..22].copy_from_slice(&self.record_count.to_le_bytes());
        out[22..30].copy_from_slice(&self.space_count.to_le_bytes());
        out[30..38].copy_from_slice(&self.index_off.to_le_bytes());
        out[38..46].copy_from_slice(&self.values_off.to_le_bytes());
        Ok(HEADER_LEN_V3)
    }

    /// One space-table row: the space, where its records start in the index, and how many.
    pub fn space_row<'a>(&self, bytes: &'a [u8], row: u64) -> Option<(&'a [u8; SPACE_ID_LEN], u64, u64)> {
        if row >= self.space_count {
            return None;
        }
        let at = HEADER_LEN_V3 + row as usize * SPACE_ENTRY;
        if at + SPACE_ENTRY > bytes.len() {
            return None;
        }
        let space: &[u8; SPACE_ID_LEN] = bytes[at..at + SPACE_ID_LEN].try_into().ok()?;
        Some((
            space,
            le_u64(bytes, at + SPACE_ID_LEN),
            le_u64(bytes, at + SPACE_ID_LEN + 8),
        ))
    }

    /// One index entry, by its position in the index.
    pub fn index_entry<'a>(&self, bytes: &'a [u8], at: u64) -> Option<IndexEntry<'a>> {
        if at >= self.record_count {
            return None;
        }
        let off = self.index_off as usize + at as usize * INDEX_ENTRY;
        if off + INDEX_ENTRY > bytes.len() {
            return None;
        }
        let entry = IndexEntry {
            key: bytes[off..off + RAW_KEY_LEN].try_into().ok()?,
            value_off: le_u64(bytes, off + RAW_KEY_LEN),
            value_len: le_u64(bytes, off + RAW_KEY_LEN + 8),
        };
        // A value that is not inside the image is a truncated image, not an empty one.
        if entry.value_off < self.values_off
            || entry.value_off + entry.value_len > bytes.len() as u64
        {
            return None;
        }
        Some(entry)
    }

    /// The value an index entry points at.
    pub fn value<'a>(&self, bytes: &'a [u8], entry: &IndexEntry<'_>) -> Option<&'a [u8]> {
        let at = entry.value_off as usize;
        let len = entry.value_len as usize;
        if at < self.values_off as usize || at + len > bytes.len() {
            return None;
        }
        Some(&bytes[at..at + len])
    }
}

/// A v3 index entry: the key, and where its value lies.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub struct IndexEntry<'a> {
    pub key: &'a [u8; RAW_KEY_LEN],
    pub value_off: u64,
    pub value_len: u64,
}

/// One packed record, and where the next one starts.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub struct PackedRecord<'a> {
    pub space: &'a [u8; SPACE_ID_LEN],
    pub key: &'a [u8; RAW_KEY_LEN],
    pub value: &'a [u8],
    /// The offset of the record after this one.
    pub next: usize,
}

/// Read one packed record at `off`, if there is a whole one before `end`.
///
/// This is the v1/v2 layout's only parsing rule, and it is shared for the same reason the constants
/// are: a reader that disagrees about a frame's length reads every following record wrong.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub fn packed_record(bytes: &[u8], off: usize, end: usize) -> Option<PackedRecord<'_>> {
    if off + RECORD_FIXED > end || end > bytes.len() {
        return None;
    }
    let frame = &bytes[off..off + RECORD_FIXED];
    let value_len = le_u64(frame, SPACE_ID_LEN + RAW_KEY_LEN) as usize;
    let value_at = off + RECORD_FIXED;
    if value_at + value_len > end {
        return None;
    }
    Some(PackedRecord {
        space: frame[..SPACE_ID_LEN].try_into().ok()?,
        key: frame[SPACE_ID_LEN..SPACE_ID_LEN + RAW_KEY_LEN].try_into().ok()?,
        value: &bytes[value_at..value_at + value_len],
        next: value_at + value_len,
    })
}

/// One log entry, and where the next one starts.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub struct WalEntry<'a> {
    pub space: &'a [u8; SPACE_ID_LEN],
    pub key: &'a [u8; RAW_KEY_LEN],
    pub op: u8,
    pub value: &'a [u8],
    pub next: usize,
}

/// Read one log entry at `off`.
///
/// The checksum covers space, key, length and value, so a torn tail is stopped at rather than
/// applied — the rule the fold and every reader share.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub fn wal_entry(log: &[u8], off: usize) -> Option<WalEntry<'_>> {
    if off + ENTRY_FIXED > log.len() {
        return None;
    }
    let op = log[off];
    if op != WAL_OP_WRITE && op != WAL_OP_DELETE {
        return None;
    }
    let crc = le_u32(log, off + 1);
    let len = le_u32(log, off + 61) as usize;
    let frame_end = off + ENTRY_FIXED + len;
    if frame_end > log.len() {
        return None;
    }
    if crc32(&log[off + 5..frame_end]) != crc {
        return None;
    }
    Some(WalEntry {
        space: log[off + 5..off + 37].try_into().ok()?,
        key: log[off + 37..off + 61].try_into().ok()?,
        op,
        value: &log[off + ENTRY_FIXED..frame_end],
        next: frame_end,
    })
}

/// A little-endian `u64` at `at`, or zero if it does not fit — callers bound-check first, and the
/// alternative is a panic in a reader that exists to refuse rather than guess.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub fn le_u64(bytes: &[u8], at: usize) -> u64 {
    let mut w = [0u8; 8];
    if at + 8 <= bytes.len() {
        w.copy_from_slice(&bytes[at..at + 8]);
    }
    u64::from_le_bytes(w)
}

/// A little-endian `u32` at `at`, with the same rule.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub fn le_u32(bytes: &[u8], at: usize) -> u32 {
    let mut w = [0u8; 4];
    if at + 4 <= bytes.len() {
        w.copy_from_slice(&bytes[at..at + 4]);
    }
    u32::from_le_bytes(w)
}

/// CRC-32 (IEEE 802.3), bitwise.
///
/// A corruption check, not a security check: it catches a torn write or a flipped bit, and says
/// nothing about whether anyone tampered with the bytes — that is `cube-crypt`'s job, and it belongs
/// on the record rather than on the framing.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub fn crc32(bytes: &[u8]) -> u32 {
    let mut crc = 0xFFFF_FFFFu32;
    for byte in bytes {
        crc ^= *byte as u32;
        for _ in 0..8 {
            let mask = (crc & 1).wrapping_neg();
            crc = (crc >> 1) ^ (0xEDB8_8320 & mask);
        }
    }
    !crc
}

/// The store's digest hash: FNV-1a, 64-bit.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub const FNV_OFFSET: u64 = 0xcbf2_9ce4_8422_2325;

/// Fold more bytes into an FNV-1a hash.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub fn fnv1a64(bytes: &[u8], mut h: u64) -> u64 {
    for byte in bytes {
        h ^= *byte as u64;
        h = h.wrapping_mul(0x0000_0100_0000_01b3);
    }
    h
}

/// The digest line, as both sides print it.
///
/// `bytes` is the records' logical size — the same number whatever layout holds them — because this
/// line is what a kernel-written store and a userspace-written one are compared by, and a figure
/// that changed with the layout would make one store look like two.
#[allow(dead_code)] // one build uses it and the other does not: neither subset is dead
pub struct Digest {
    pub version: u8,
    pub curve: u8,
    pub records: u64,
    pub value_bytes: u64,
    pub hash: u64,
    pub errors: u64,
}

impl Digest {
    /// The logical size of the store these records describe.
    pub fn logical_bytes(records: u64, value_bytes: u64) -> u64 {
        records * RECORD_FIXED as u64 + value_bytes
    }

    /// Fold one record in.
    pub fn add(&mut self, space: &[u8], key: &[u8], value: &[u8]) {
        self.hash = fnv1a64(space, self.hash);
        self.hash = fnv1a64(key, self.hash);
        self.hash = fnv1a64(&(value.len() as u64).to_le_bytes(), self.hash);
        self.hash = fnv1a64(value, self.hash);
        self.records += 1;
        self.value_bytes += value.len() as u64;
    }

    pub fn new(version: u8, curve: u8) -> Self {
        Digest {
            version,
            curve,
            records: 0,
            value_bytes: 0,
            hash: FNV_OFFSET,
            errors: 0,
        }
    }
}