portal/src/chain.rs

//! SHA-256 hashed answer chain. `parent_hashes` is DAG-shaped, but
//! nothing produces more than one parent yet - the shape is reserved
//! for follow-up questions posted back into a visitor's chain.
#![cfg(feature = "ssr")]

use sha2::{Digest, Sha256};

/// Hashes one chain node from its question, its parent hash(es), the
/// submitted responses, and a timestamp. Parents are sorted first so the
/// hash doesn't depend on the order multiple parents happened to arrive
/// in.
/// Where submitted chain nodes are indexed - hash → which question was
/// answered. Small on purpose (no responses), and shared by every
/// portal instance on the JetStream (hashes are globally unique, so
/// cross-site collisions can't happen). This is what lets
/// `requires_chain` pages verify a visitor's `?chain=` actually ends
/// at the question they claim to have answered.
pub const CHAIN_BUCKET: &str = "portal_chains";

#[derive(serde::Serialize, serde::Deserialize)]
pub struct ChainNode {
    pub question_id: String,
    pub timestamp_ms: i64,
}

/// Indexes one submitted node. Best-effort by design (the caller logs
/// and continues): the NATS event is the durable record, this is a
/// lookup convenience.
pub async fn record_node(
    js: &async_nats::jetstream::Context,
    chain_hash: &str,
    question_id: &str,
    timestamp_ms: i64,
) -> anyhow::Result<()> {
    let store = match js.get_key_value(CHAIN_BUCKET).await {
        Ok(store) => store,
        Err(_) => {
            js.create_key_value(async_nats::jetstream::kv::Config {
                bucket: CHAIN_BUCKET.to_string(),
                ..Default::default()
            })
            .await?
        }
    };
    let node = ChainNode {
        question_id: question_id.to_string(),
        timestamp_ms,
    };
    store.put(chain_hash, serde_json::to_vec(&node)?.into()).await?;
    Ok(())
}

/// Looks a chain hash up - `None` covers both "no such node" and
/// "bucket not created yet" (no submissions anywhere), which read the
/// same to a `requires_chain` check: the claimed lineage can't be
/// verified, so the gate stays shut.
pub async fn lookup_node(
    js: &async_nats::jetstream::Context,
    chain_hash: &str,
) -> Option<ChainNode> {
    let store = js.get_key_value(CHAIN_BUCKET).await.ok()?;
    let bytes = store.get(chain_hash).await.ok()??;
    serde_json::from_slice(&bytes).ok()
}

pub fn hash_node(
    question_id: &str,
    parent_hashes: &[String],
    responses: &serde_json::Value,
    timestamp_ms: i64,
) -> String {
    let mut parents = parent_hashes.to_vec();
    parents.sort();
    let payload = serde_json::json!({
        "questionId": question_id,
        "parentHashes": parents,
        "responses": responses,
        "timestamp": timestamp_ms,
    });
    let data = serde_json::to_vec(&payload).expect("json values always serialize");
    let mut hasher = Sha256::new();
    hasher.update(&data);
    format!("{:x}", hasher.finalize())
}