onetaskgraph-core 0.2.0

//! What the engine did, carried back with every response.
//!
//! The point of a capability declaration is that two sources answer the same
//! query differently and both answers are correct. These types make that visible
//! instead of leaving a user to guess why one source was fast and another was not:
//! `--explain` renders a [`QueryPlan`] and `--json` carries it as a field.

use onetaskgraph_plugin_api::{SourceError, SourceName};
use schemars::JsonSchema;
use serde::{Deserialize, Serialize};

use crate::engine::{Owed, Resumption, StreamState};

/// One page of engine output, with the plan that produced it.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
pub struct QueryResponse<T> {
    /// This page's items, already qualified and merged across sources.
    pub items: Vec<T>,
    /// Where to resume, or `None` when every source is exhausted.
    pub next: Option<PageToken>,
    /// What each source was asked to do, and what the engine did instead.
    pub plan: QueryPlan,
    /// Sources that failed. One failure never fails the whole query.
    pub errors: Vec<SourceFailure>,
}

/// What the engine did, per source.
#[derive(Debug, Clone, PartialEq, Default, Serialize, Deserialize, JsonSchema)]
pub struct QueryPlan {
    /// One entry per source the query reached.
    pub per_source: Vec<SourcePlan>,
}

/// What one source was asked for, and what happened to each predicate.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
pub struct SourcePlan {
    /// The configured source this describes.
    pub source: SourceName,
    /// The plugin kind behind it.
    // llmlint: ignore[invalid_states_unrepresentable] SECOND PERMITTED REASON — this restates at a new site the justification already recorded at `Capabilities.max_page_size` (capability.rs) and `PageRequest.limit` (query.rs): `kind: String` is approved contract text. It is also the one field that could not be narrowed even if it were free — a plan must carry the kind a subprocess-hosted plugin reports, an open vocabulary no compile-time type can enumerate.
    pub kind: String,
    /// Predicates the source applied itself.
    ///
    /// The four predicate vectors below partition one set of outcomes, and nothing in
    /// the type says so: a `Predicate` could appear in two of them at once, or in none.
    /// One `Vec<(Predicate, Outcome)>` — or a map keyed by predicate — would make that
    /// unrepresentable. See the directive below for why it stays as it is.
    // llmlint: ignore[invalid_states_unrepresentable] SECOND PERMITTED REASON — this
    // restates at a new site the justification already recorded at
    // `Capabilities.max_page_size` (capability.rs) and `PageRequest.limit` (query.rs):
    // `SourcePlan`'s four-vector shape is approved contract text, reproduced field for
    // field, and `--json` publishes it as the wire format both SDKs are generated from.
    // Collapsing the four vectors into one outcome-tagged collection is a change to that
    // contract, which is the contract owner's call and is expressly forbidden to any node
    // of this plan while other nodes are being written against this text. The finding is
    // correct and is surfaced as a contract defect rather than dismissed: the contract can
    // represent a plan its own rules forbid.
    pub pushed_down: Vec<Predicate>,
    /// Predicates the engine applied in memory over a wider result set.
    pub applied_locally: Vec<Predicate>,
    /// Predicates the engine answered by a bounded scan of the source.
    pub emulated: Vec<Predicate>,
    /// Predicates neither side could answer, so the result is unconstrained.
    ///
    /// Never [`Predicate::ReverseDependencies`]: `DependencySupport` has no
    /// unsupported variant, so a reverse-dependency read is answered natively or
    /// emulated by the engine's bounded scan, never abandoned. The type cannot say
    /// so — see the directive below.
    // llmlint: ignore[invalid_states_unrepresentable] SECOND PERMITTED REASON — this
    // restates at a new site the justification already recorded at
    // `Capabilities.max_page_size` (capability.rs) and `PageRequest.limit` (query.rs):
    // `unavailable: Vec<Predicate>` and the `Predicate` enum are both approved contract
    // text, so a narrower element type here is the contract owner's call, not this
    // crate's. The finding is correct and is being surfaced as a contract defect rather
    // than dismissed: the contract can express a state its own rules forbid.
    pub unavailable: Vec<Predicate>,
    /// How many pages the engine pulled from this source to answer.
    pub pages_fetched: u32,
}

/// One thing a query can ask of a source.
#[derive(
    Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize, JsonSchema,
)]
#[serde(rename_all = "kebab-case")]
pub enum Predicate {
    /// Filter by label name.
    Label,
    /// Filter by status category.
    Status,
    /// Search titles.
    SearchTitle,
    /// Search bodies.
    SearchContent,
    /// Filter by owning project.
    Project,
    /// Walk dependency edges backwards.
    ReverseDependencies,
}

/// One source's failure, kept beside the results the other sources returned.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
pub struct SourceFailure {
    /// The source that failed.
    pub source: SourceName,
    /// Why.
    pub error: SourceError,
}

/// The engine's own resume token: one plugin cursor per source stream, opaque to the
/// caller exactly as a plugin's cursor is opaque to the engine.
///
/// Rendered as lower-case hex, which is not obfuscation — the inside is not a secret —
/// but the one property a token a person copies off a terminal has to have: it survives
/// a shell. The document underneath holds a plugin's own cursor, and a cursor may hold
/// anything at all, so a token spelled as the raw JSON would carry quotes, braces and
/// spaces straight into the next command line. Hex has no character a shell reads.
///
/// # What a token is and is not checked for
///
/// Both ways in go through [`parse`](Self::parse) — including deserialising one — and
/// what that establishes is **structural**: the string is hex, the bytes are this
/// engine's own resume document, and every state in it is well formed. It does not, and
/// cannot, establish that this engine is the one that wrote it. A token is not a
/// credential and carries nothing secret; forging one buys a caller nothing they could
/// not have asked for outright, since every cursor inside is handed straight back to the
/// source that issued it and is validated there.
///
/// What a forged token *could* do is name a stream this configuration has no source for,
/// or resume further into a page than the engine ever pages. Both are refused where the
/// token meets the query it is resuming, by
/// [`Engine`](crate::Engine) — see `EngineError::Token` — because only the engine knows
/// which sources are configured and what page ceiling each declares.
#[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize, JsonSchema)]
#[serde(try_from = "String", into = "String")]
pub struct PageToken(String);

impl PageToken {
    /// Encode where every stream still walking picks up.
    ///
    /// Crate-private on purpose: what a token *means* is the engine's, and a caller able
    /// to build one from parts could name a stream no query addressed. A caller with a
    /// token in hand reaches it through [`parse`](Self::parse) instead, which checks its
    /// structure — see this type's own note for what that does and does not establish.
    ///
    /// Infallible: a stream state is a source name, a stream kind, an optional cursor
    /// and a count, and none of those can fail to serialise.
    ///
    /// Only ever reached with at least one stream, because a walk with nothing left to
    /// resume reports no token at all — which is why [`parse`](Self::parse) refuses an
    /// empty one.
    pub(crate) fn encode(query: &str, owed: Option<Owed>, streams: &[StreamState]) -> Self {
        let document = serde_json::to_string(&Resumption {
            query: query.to_owned(),
            owed,
            streams: streams.to_vec(),
        })
        .expect("a resumption is plain data and always serialises");
        Self(to_hex(&document))
    }

    /// Accept a token from a caller — a `--page` argument, or a deserialised response —
    /// refusing anything that is not this engine's own resume document.
    ///
    /// Structural only, deliberately: see the type's own note for what this establishes
    /// and what [`Engine`](crate::Engine) checks instead.
    ///
    /// # Errors
    ///
    /// Returns [`SourceError::Malformed`] when `raw` is not hex, is not this engine's
    /// document, or holds a state that is not well formed.
    pub fn parse(raw: impl Into<String>) -> Result<Self, SourceError> {
        let token = Self(raw.into());
        token.resumption()?;
        Ok(token)
    }

    /// Borrow the underlying token.
    #[must_use]
    pub fn as_str(&self) -> &str {
        &self.0
    }

    /// Where each stream claims to pick up.
    ///
    /// Infallible, and that is a property of the type rather than an assumption: the only
    /// two ways to obtain a `PageToken` are [`encode`](Self::encode), which built this
    /// document, and [`parse`](Self::parse), which refuses anything that does not decode
    /// — and deserialising one goes through `parse`. So a token that does not decode
    /// never exists to be read here. Whether what it *says* is usable against the query
    /// being resumed is the engine's to decide, not this type's.
    pub(crate) fn decode(&self) -> Resumption {
        self.resumption()
            .expect("every way to build a PageToken validates it")
    }

    /// The document inside, or why this is not one of this engine's tokens.
    fn resumption(&self) -> Result<Resumption, SourceError> {
        let document = from_hex(&self.0).ok_or_else(|| SourceError::Malformed {
            message: "that is not a page token this engine writes: it is not even hex".to_owned(),
        })?;
        let resumption: Resumption =
            serde_json::from_str(&document).map_err(|error| SourceError::Malformed {
                message: format!("that is not a page token this engine writes: {error}"),
            })?;
        let streams = &resumption.streams;
        // A token with nothing to resume is one this engine never writes: `encode` is
        // reached only while at least one stream still has rows to give, and a walk with
        // none reports no token at all. Accepting one would answer an empty page and exit
        // zero, which reads as a walk that ended rather than as the mistake it is.
        if streams.is_empty() {
            return Err(SourceError::Malformed {
                message: "that is not a page token this engine writes: it resumes nothing"
                    .to_owned(),
            });
        }
        Ok(resumption)
    }
}

/// Deserialising a token goes through [`PageToken::parse`], so a response carrying
/// a token this engine never issued is refused where it is read.
impl TryFrom<String> for PageToken {
    type Error = SourceError;

    fn try_from(value: String) -> Result<Self, Self::Error> {
        Self::parse(value)
    }
}

/// A token is its string, so serialising one is that string and nothing else — the
/// checking all happens on the way in, where a caller's input is.
impl From<PageToken> for String {
    fn from(value: PageToken) -> Self {
        value.0
    }
}

impl std::fmt::Display for PageToken {
    /// The opaque string a caller passes back as `--page`.
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(&self.0)
    }
}

/// Render `document` as lower-case hex.
fn to_hex(document: &str) -> String {
    let mut rendered = String::with_capacity(document.len() * 2);
    for byte in document.as_bytes() {
        rendered.push(nibble(byte >> 4));
        rendered.push(nibble(byte & 0x0f));
    }
    rendered
}

fn nibble(value: u8) -> char {
    char::from_digit(u32::from(value), 16).expect("a nibble is a hex digit")
}

/// Read hex back, or `None` when `raw` is not hex of valid UTF-8.
fn from_hex(raw: &str) -> Option<String> {
    if !raw.len().is_multiple_of(2) {
        return None;
    }
    let digits: Vec<u8> = raw
        .chars()
        .map(|digit| digit.to_digit(16))
        .collect::<Option<Vec<u32>>>()?
        .into_iter()
        .map(|digit| u8::try_from(digit).expect("a hex digit fits in a byte"))
        .collect();
    let bytes: Vec<u8> = digits
        .chunks(2)
        .map(|pair| (pair[0] << 4) | pair[1])
        .collect();
    String::from_utf8(bytes).ok()
}