urna-format 0.5.0

//! section payload encoding (raw / zstd / float16 / int8).
//!
//! Two orthogonal axes:
//!
//! - **Wire encoding** of a section payload (`SECTION_ENCODING_*` in
//!   `layout`): how the bytes are stored on disk. `raw` and `zstd`
//!   apply to any non-embedding section. `float16` and `int8` only
//!   apply to the embeddings section.
//!
//! - **Logical dtype** of an embeddings section (`Manifest::dtype`):
//!   how to interpret the bytes after wire decoding. `float32` is the
//!   recall-max baseline; `float16` and `int8` are smaller-but-lossy
//!   variants that always accumulate in `f32` at search time.
//!
//! Section checksums (`SectionEntry::checksum`) hash the **physical**
//! bytes as stored. `content_hash` hashes the **decoded** bytes so two
//! files with the same logical content but different wire encoding
//! still produce the same content_hash for non-quantized sections.

mod dedup;
mod float16;
mod fsst;
mod fsst_table;
mod int4;
mod int8;
mod intpack;
mod txt_streams;
mod zstd_codec;
mod zstd_dict;

pub use dedup::{
    DEDUP_MAP_V1, Deduped, decode_map as decode_dedup_map, dedup, encode_map as encode_dedup_map,
    expand as expand_dedup,
};
pub use float16::{f16_bytes_to_f32, f32_to_f16_bytes};
pub use fsst::{TXT_STREAMS_V3, decode as decode_fsst_payload, encode as encode_fsst};
pub use int4::{
    INT4_BLOCK, INT4_PAYLOAD_VERSION, INT4_PREFIX_SIZE, INT4_SCALE_KIND_PER_GROUP,
    Int4EmbeddingsView, encode_int4_embeddings, int4_blocks_per_row, nibble_to_i4, pack_nibbles,
    quantize_f32_to_i4,
};
pub use int8::{
    INT8_PAYLOAD_VERSION, INT8_PREFIX_SIZE, INT8_SCALE_KIND_PER_VECTOR, Int8EmbeddingsView,
    encode_int8_embeddings, quantize_f32_to_i8,
};
pub use intpack::{INTPACK_BLOCK, IntpackReader, pack_u64s, unpack_u64s};
pub use txt_streams::{
    TXT_STREAMS_V1, TxtStreams, decode as decode_txt_streams_payload, encode_txt_streams,
};
pub use zstd_codec::{DEFAULT_ZSTD_LEVEL, zstd_encode};
pub use zstd_dict::{
    MAX_DICT_BYTES, TXT_STREAMS_V2, decode as decode_zstd_dict_payload, encode as encode_zstd_dict,
    train_dict,
};

use crate::error::UrnaError;
use crate::layout::{
    SECTION_ENCODING_FLOAT16, SECTION_ENCODING_FSST, SECTION_ENCODING_INT4, SECTION_ENCODING_INT8,
    SECTION_ENCODING_INTPACK, SECTION_ENCODING_RAW, SECTION_ENCODING_TXT_STREAMS,
    SECTION_ENCODING_ZSTD, SECTION_ENCODING_ZSTD_DICT,
};
use std::borrow::Cow;

/// The context-free wire codecs, as a small registry. Decoding dispatches
/// through `WireCodec::from_id`, so adding a reserved codec is a localized
/// additive diff: a variant, a `from_id` arm, a `decode` arm, and its own
/// `<=300`-line module. Reserved-but-unimplemented ids (and the dict codec,
/// which needs section 0x0A) are deliberately ABSENT here so old and new
/// readers agree on the frozen wire format.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum WireCodec {
    Raw,
    Zstd,
    Float16Embeddings,
    Int8Embeddings,
    Int4Embeddings,
    Intpack,
    TxtStreams,
    Fsst,
}

impl WireCodec {
    fn from_id(encoding: u32) -> Option<Self> {
        match encoding {
            SECTION_ENCODING_RAW => Some(Self::Raw),
            SECTION_ENCODING_ZSTD => Some(Self::Zstd),
            SECTION_ENCODING_FLOAT16 => Some(Self::Float16Embeddings),
            SECTION_ENCODING_INT8 => Some(Self::Int8Embeddings),
            SECTION_ENCODING_INT4 => Some(Self::Int4Embeddings),
            SECTION_ENCODING_INTPACK => Some(Self::Intpack),
            SECTION_ENCODING_TXT_STREAMS => Some(Self::TxtStreams),
            SECTION_ENCODING_FSST => Some(Self::Fsst),
            _ => None,
        }
    }

    fn decode<'a>(self, bytes: &'a [u8]) -> crate::Result<Cow<'a, [u8]>> {
        // intpack / txt_streams / fsst all decode BYTE-IDENTICALLY to the raw
        // canonical payload, so content_hash and citations are unchanged; raw
        // and the embedding-only encodings ARE their canonical bytes (the
        // runtime dispatches on `dtype`).
        match self {
            Self::Raw | Self::Float16Embeddings | Self::Int8Embeddings | Self::Int4Embeddings => {
                Ok(Cow::Borrowed(bytes))
            }
            Self::Zstd => zstd_codec::zstd_decode(bytes).map(Cow::Owned),
            Self::Intpack => crate::sections::decode_intpack_repack(bytes).map(Cow::Owned),
            Self::TxtStreams => crate::sections::decode_txt_streams(bytes).map(Cow::Owned),
            Self::Fsst => fsst::decode(bytes).map(Cow::Owned),
        }
    }
}

/// Decode a section payload from its on-disk encoding to the logical bytes
/// a reader consumes, via the wire-codec registry. For `raw` this is a
/// borrow; for `zstd` an owned decompressed buffer. The `zstd_dict` (id 5)
/// codec needs the shared dictionary (section 0x0A) and is decoded via
/// [`decode_payload_with_dict`], so it is rejected here. Unknown or
/// reserved-but-unimplemented encodings are rejected.
pub fn decode_payload(encoding: u32, bytes: &[u8]) -> crate::Result<Cow<'_, [u8]>> {
    match WireCodec::from_id(encoding) {
        Some(codec) => codec.decode(bytes),
        None => Err(UrnaError::UnsupportedSectionEncoding {
            section_id: 0,
            encoding,
        }),
    }
}

/// Decode a chunks_canonical payload that MAY be dict-framed (`zstd_dict`,
/// id 5), supplying the shared dictionary from section 0x0A. all other
/// encodings ignore the dict and route through [`decode_payload`]. the dict
/// variant decodes BYTE-IDENTICALLY to the raw chunks_canonical payload, so
/// content_hash and citations are unchanged.
pub fn decode_payload_with_dict<'a>(
    encoding: u32,
    bytes: &'a [u8],
    dict: Option<&[u8]>,
) -> crate::Result<Cow<'a, [u8]>> {
    if encoding == SECTION_ENCODING_ZSTD_DICT {
        let dict = dict.ok_or_else(|| UrnaError::MalformedSectionPayload {
            section_id: 0,
            reason: "zstd_dict: dict-framed section but no dictionary (0x0A)".into(),
        })?;
        return zstd_dict::decode(bytes, dict).map(Cow::Owned);
    }
    decode_payload(encoding, bytes)
}

/// Encode `payload` with one non-embedding wire encoding (raw or zstd).
/// The embedding dtypes (float16/int8) are not general-purpose encoders
/// and are rejected here; they are chosen by preset on the embeddings
/// section directly.
fn encode_wire(encoding: u32, payload: &[u8]) -> crate::Result<Vec<u8>> {
    match encoding {
        SECTION_ENCODING_RAW => Ok(payload.to_vec()),
        SECTION_ENCODING_ZSTD => zstd_codec::zstd_encode(payload),
        other => Err(UrnaError::UnsupportedSectionEncoding {
            section_id: 0,
            encoding: other,
        }),
    }
}

/// Cost-driven encoder: try every candidate wire encoding and return the
/// `(encoding_id, bytes)` of the SMALLEST result, so the writer can record
/// the chosen id in the section entry. Ties break toward the EARLIEST
/// candidate (cheaper-to-decode wins an equal-size race). This only auto-
/// picks among non-embedding encodings; existing presets that name an
/// encoding explicitly are untouched, so the frozen output stays
/// byte-identical.
pub fn encode_smallest(candidates: &[u32], payload: &[u8]) -> crate::Result<(u32, Vec<u8>)> {
    let mut best: Option<(u32, Vec<u8>)> = None;
    for &enc in candidates {
        let bytes = encode_wire(enc, payload)?;
        let smaller = best.as_ref().is_none_or(|(_, b)| bytes.len() < b.len());
        if smaller {
            best = Some((enc, bytes));
        }
    }
    best.ok_or_else(|| UrnaError::InvalidInput("encode_smallest: no candidate encodings".into()))
}

/// Expected size of the embeddings section for a given dtype. `None` for an
/// unknown dtype AND when `n * dim` overflows: both come from an untrusted
/// header, and a wrapped product would let a hostile file claim a huge
/// corpus behind a tiny section (mutation-fuzz finding).
pub fn expected_embeddings_size(dtype: &str, n: usize, dim: usize) -> Option<usize> {
    let cells = n.checked_mul(dim)?;
    match dtype {
        "float32" => cells.checked_mul(4),
        "float16" => cells.checked_mul(2),
        "int8" => n
            .checked_mul(4)?
            .checked_add(cells)?
            .checked_add(INT8_PREFIX_SIZE),
        // prefix + f16 group scales (n * dim/64) + packed nibbles (n * dim/2).
        "int4" if dim % INT4_BLOCK == 0 => n
            .checked_mul(dim / INT4_BLOCK)?
            .checked_mul(2)?
            .checked_add(cells / 2)?
            .checked_add(INT4_PREFIX_SIZE),
        _ => None,
    }
}

#[cfg(test)]
mod tests;