codec-rs 0.4.1

// SPDX-License-Identifier: MIT
//! Client-side helpers for the Codec compression contract.
//!
//! Rust twin of `codecai.compression` (Python): the server emits
//! `Codec-Zstd-Dict: sha256:<hex>` on every zstd response, the client
//! validates that header against locally-loaded dicts before
//! decompressing. See `spec/PROTOCOL.md` "Codec-Zstd-Dict response
//! header" (stable since v0.2) for the full contract.
//!
//! The actual zstd decompression is intentionally out of scope here —
//! callers usually already have an HTTP stack and pick their own
//! zstd binding (`zstd` crate, `zstd-safe`, an FFI wrapper, etc.).
//! This module just gives you the small piece that's specific to
//! Codec: matching a response's declared dict hash to the dict you've
//! loaded, with the case-insensitive header lookup and fail-fast
//! semantics the spec mandates.
//!
//! Wrong-dict decompression produces garbage bytes that downstream
//! msgpack / protobuf parsers will silently misinterpret — fail fast
//! at the dict-select boundary instead.

use std::collections::HashMap;
use std::fmt;

use sha2::{Digest, Sha256};

/// Compute the canonical `Codec-Zstd-Dict` hash for `dict_bytes`.
///
/// Returns `sha256:<lowercase hex>` — same shape as the server-side
/// header value and the `hash` field in tokenizer-map
/// `zstd_dictionaries[]` entries.
pub fn hash_zstd_dict(dict_bytes: &[u8]) -> String {
    let mut hasher = Sha256::new();
    hasher.update(dict_bytes);
    let digest = hasher.finalize();
    format!("sha256:{}", hex::encode(digest))
}

/// Raised when the server's `Codec-Zstd-Dict` header doesn't match any
/// dict the client has loaded, or is missing on a zstd response.
///
/// A wrong-dict decompression would produce garbage bytes that
/// downstream parsers would misinterpret — fail fast instead.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum CodecZstdDictError {
    /// Response was `Content-Encoding: zstd` but the server omitted
    /// the `Codec-Zstd-Dict` header. Per spec/PROTOCOL.md the server
    /// MUST name the dict it used; we refuse to guess.
    MissingHeader,
    /// `Codec-Zstd-Dict` value was not in the canonical
    /// `sha256:<64 hex chars>` shape. The wrapped string is the raw
    /// header value (trimmed) for diagnostics.
    MalformedHash(String),
    /// `Codec-Zstd-Dict` named a dict we haven't loaded. The caller
    /// should fetch it from the tokenizer map's `zstd_dictionaries[]`
    /// entry (the one whose `hash` matches) or retry the request with
    /// `Accept-Encoding: gzip` to downgrade to a no-dict path. The
    /// wrapped string is the declared hash.
    UnknownHash(String),
}

impl fmt::Display for CodecZstdDictError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            CodecZstdDictError::MissingHeader => write!(
                f,
                "Response is Content-Encoding: zstd but no Codec-Zstd-Dict \
                 header was present. Per spec/PROTOCOL.md the server MUST \
                 name the dict it used. Refusing to guess."
            ),
            CodecZstdDictError::MalformedHash(value) => write!(
                f,
                "Malformed Codec-Zstd-Dict value: {value:?}. Expected \
                 'sha256:<64 hex chars>'."
            ),
            CodecZstdDictError::UnknownHash(hash) => write!(
                f,
                "Server used zstd dict {hash} but it isn't loaded \
                 locally. Fetch it from the tokenizer map's \
                 zstd_dictionaries[] entry (the entry whose hash \
                 matches), or send Accept-Encoding: gzip to downgrade."
            ),
        }
    }
}

impl std::error::Error for CodecZstdDictError {}

/// Pick the zstd dict to decompress this response with.
///
/// # Parameters
///
/// - `response_headers`: HTTP response header map. Keys are looked up
///   case-insensitively, matching the way `reqwest::HeaderMap` and most
///   HTTP libraries treat headers on the wire.
/// - `loaded_dicts`: `{ "sha256:<hex>" -> dict bytes }`. Populate this
///   from your tokenizer map's `zstd_dictionaries[]` entries; keys
///   follow the same canonical shape the server emits.
///
/// # Returns
///
/// - `Ok(Some(&dict_bytes))` when the response is
///   `Content-Encoding: zstd` and the server's `Codec-Zstd-Dict`
///   header points at a loaded dict — pass these bytes to your zstd
///   decoder (e.g. `zstd::stream::Decoder::with_dictionary`).
/// - `Ok(None)` when the response isn't zstd. The caller should pass
///   the body through identity / let its HTTP stack auto-decompress
///   gzip / brotli.
///
/// # Errors
///
/// Returns `CodecZstdDictError` when the response is zstd but the
/// header is missing, malformed, or names a dict the client hasn't
/// loaded. Wrong-dict decompression is never attempted — see the
/// spec rationale at `spec/PROTOCOL.md`.
pub fn select_zstd_dict_for_response<'a>(
    response_headers: &HashMap<String, String>,
    loaded_dicts: &'a HashMap<String, Vec<u8>>,
) -> Result<Option<&'a [u8]>, CodecZstdDictError> {
    let enc = header(response_headers, "content-encoding");
    match enc.map(|v| v.trim().to_ascii_lowercase()) {
        Some(ref v) if v == "zstd" => {}
        _ => return Ok(None), // caller's HTTP stack handles gzip/br/identity
    }

    let declared = match header(response_headers, "codec-zstd-dict") {
        Some(v) => v.trim().to_string(),
        None => return Err(CodecZstdDictError::MissingHeader),
    };
    if declared.is_empty() {
        return Err(CodecZstdDictError::MissingHeader);
    }

    if !is_canonical_sha256(&declared) {
        return Err(CodecZstdDictError::MalformedHash(declared));
    }

    match loaded_dicts.get(&declared) {
        Some(bytes) => Ok(Some(bytes.as_slice())),
        None => Err(CodecZstdDictError::UnknownHash(declared)),
    }
}

/// Case-insensitive header lookup. Most idiomatic HTTP libraries
/// (`reqwest::HeaderMap`, `http::HeaderMap`) already treat header
/// names case-insensitively; this is the defensive fallback for
/// callers that hand us a plain `HashMap<String, String>` lifted
/// out of their own request plumbing.
fn header<'a>(headers: &'a HashMap<String, String>, name: &str) -> Option<&'a str> {
    if let Some(v) = headers.get(name) {
        return Some(v.as_str());
    }
    let lower = name.to_ascii_lowercase();
    for (k, v) in headers.iter() {
        if k.to_ascii_lowercase() == lower {
            return Some(v.as_str());
        }
    }
    None
}

fn is_canonical_sha256(value: &str) -> bool {
    const PREFIX: &str = "sha256:";
    if !value.starts_with(PREFIX) {
        return false;
    }
    let hex = &value[PREFIX.len()..];
    hex.len() == 64 && hex.bytes().all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'f'))
}

// ── unit tests ────────────────────────────────────────────────────────────

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn hash_zstd_dict_matches_python_reference() {
        // "hello world" sha256 — same digest both languages produce.
        let got = hash_zstd_dict(b"hello world");
        assert_eq!(
            got,
            "sha256:b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
        );
    }

    #[test]
    fn select_returns_none_when_not_zstd() {
        let mut headers = HashMap::new();
        headers.insert("content-encoding".into(), "gzip".into());
        let dicts: HashMap<String, Vec<u8>> = HashMap::new();
        assert_eq!(
            select_zstd_dict_for_response(&headers, &dicts).unwrap(),
            None
        );
    }

    #[test]
    fn select_returns_none_when_no_encoding() {
        let headers: HashMap<String, String> = HashMap::new();
        let dicts: HashMap<String, Vec<u8>> = HashMap::new();
        assert_eq!(
            select_zstd_dict_for_response(&headers, &dicts).unwrap(),
            None
        );
    }

    #[test]
    fn select_missing_header_is_error() {
        let mut headers = HashMap::new();
        headers.insert("Content-Encoding".into(), "zstd".into());
        let dicts: HashMap<String, Vec<u8>> = HashMap::new();
        assert_eq!(
            select_zstd_dict_for_response(&headers, &dicts),
            Err(CodecZstdDictError::MissingHeader)
        );
    }

    #[test]
    fn select_malformed_hash_is_error() {
        let mut headers = HashMap::new();
        headers.insert("content-encoding".into(), "zstd".into());
        headers.insert("codec-zstd-dict".into(), "md5:abc".into());
        let dicts: HashMap<String, Vec<u8>> = HashMap::new();
        match select_zstd_dict_for_response(&headers, &dicts) {
            Err(CodecZstdDictError::MalformedHash(v)) => assert_eq!(v, "md5:abc"),
            other => panic!("expected MalformedHash, got {other:?}"),
        }
    }
}