//! The shape rules a record meets before it is stored: a collection that
//! could name a lexicon, a `$type` that agrees with it, a body the atproto
//! data model can encode, and the lexicon itself when one is held.

use serde_json::Value;

use crate::Catalog;

/// Why a record's shape is refused.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
pub enum ShapeError {
    /// The collection is not a syntactically valid NSID.
    #[error("`{collection}` cannot be a collection: {source}")]
    MalformedCollection {
        /// The NSID the caller named.
        collection: String,
        /// Which of the NSID syntax rules it broke.
        #[source]
        source: didbot_lexicon::nsid_syntax::NsidError,
    },
    /// The explicit stance was asked for and no lexicon is held for the
    /// collection.
    #[error("`{collection}` has no lexicon here, and `validate: true` requires one")]
    NoSchemaForCollection {
        /// The NSID the caller named.
        collection: String,
    },
    /// The record cannot be represented in the atproto data model.
    #[error("record is not in the atproto data model: {source}")]
    UnrepresentableRecord {
        /// Where in the record the problem is, and what it was.
        #[source]
        source: didbot_data::DataError,
    },
    /// The record's `$type` disagrees with the collection it was written to.
    #[error("record `$type` is `{declared}` but the collection is `{collection}`")]
    TypeMismatch {
        /// The `$type` carried by the record.
        declared: String,
        /// The collection the caller asked to write into.
        collection: String,
    },
    /// The record's `$type` is present but is not a string.
    #[error("record `$type` must be a string")]
    TypeNotAString,
    /// The record does not satisfy the lexicon that defines its collection.
    #[error("record does not satisfy `{collection}`: `{path}` {reason}")]
    SchemaViolation {
        /// The collection whose lexicon refused the record.
        collection: String,
        /// The failing field's path within the record.
        path: String,
        /// What was wrong with it.
        reason: String,
    },
}

/// The first of [`validate_with`]'s rules, on its own.
///
/// Split out for the operations that name a collection and carry no record —
/// a `deleteRecord`, or the delete half of an `applyWrites` batch.
///
/// It asks only whether the string could name a lexicon. It deliberately does
/// *not* ask whether this deployment defines one: a personal data server holds
/// the account's repository, and the lexicons an account may write are the
/// world's rather than this project's.
///
/// ```
/// use didbot_schema::validate_collection;
/// assert!(validate_collection("com.example.thing").is_ok());
/// // A lexicon this project did not author, and does not hold.
/// assert!(validate_collection("app.bsky.feed.post").is_ok());
/// // Not an NSID at all, so it could never address anything.
/// assert!(validate_collection("post").is_err());
/// ```
pub fn validate_collection(collection: &str) -> Result<(), ShapeError> {
    didbot_lexicon::nsid_syntax::validate(collection).map_err(|source| {
        ShapeError::MalformedCollection {
            collection: collection.to_owned(),
            source,
        }
    })
}

/// Whether this server holds a lexicon that defines `collection`'s record.
///
/// The question the optimistic stance turns on, and the one a write response's
/// `validationStatus` reports: `valid` when this is true and the record passed,
/// `unknown` when it is false and the record was stored unchecked.
///
/// ```
/// use didbot_schema::schema_is_held;
/// assert!(schema_is_held("bot.did.registration"));
/// // A lexicon defined elsewhere, whether or not this server writes it.
/// assert!(!schema_is_held("com.example.thing"));
/// assert!(!schema_is_held("app.bsky.feed.post"));
/// ```
pub fn schema_is_held(collection: &str) -> bool {
    Catalog::embedded().defines_record(collection)
}

/// Which of the specification's three validation stances to take for one
/// write.
///
/// atproto's `validate` parameter is a tri-state and each state is one of the
/// stances the Lexicon specification names. Absent is the default and is the
/// one this server takes.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum Stance {
    /// Validate against the schema when one is held, and store faithfully when
    /// it is not. `validate` absent.
    #[default]
    Optimistic,
    /// Every record must resolve to a schema and satisfy it. `validate: true`.
    Explicit,
    /// Do not check the record against any schema. `validate: false`.
    Skip,
}

impl Stance {
    /// Reads the wire's tri-state `validate` parameter.
    pub fn from_validate(validate: Option<bool>) -> Self {
        match validate {
            None => Self::Optimistic,
            Some(true) => Self::Explicit,
            Some(false) => Self::Skip,
        }
    }
}

/// The shape rules under the optimistic stance, which is the one this server
/// takes when a caller says nothing. Authorship is not a property of the
/// bytes, so no rule here is about *who* may write.
pub fn validate_shape(collection: &str, record: &Value) -> Result<(), ShapeError> {
    validate_with(collection, record, Stance::Optimistic)
}

/// Checks a record against the collection it is being written to.
///
/// Four rules, in order.
///
/// 1. The collection must be a syntactically valid NSID — [`validate_collection`].
/// 2. A `$type` carried by the record must agree with it.
/// 3. The record must be representable in the atproto data model.
/// 4. The record must satisfy the lexicon that defines the collection, *when
///    this server holds one*.
///
/// The `$type` check is here because nothing else performs it. jacquard does
/// not verify `$type` on deserialize, so a record claiming to be a
/// record can be written into the memory collection and read back as a
/// memory, with the lie preserved inside the value. That is a data corruption
/// no later reader can detect, so it is refused at the only point where both
/// facts are in hand. It holds for a foreign collection too: `$type` and the
/// collection name the same lexicon in atproto, whoever wrote it.
///
/// A record with no `$type` is accepted: the collection already says what it
/// is, and atproto treats the field as redundant inside a repository.
///
/// # The data model check is the one that makes storage faithful
///
/// A record is stored as JSON and exported as DAG-CBOR inside a Merkle search
/// tree. JSON has values the data model does not — a float, most of all — and
/// under the closed collection set they could only arrive in an *undeclared*
/// field, which validation deliberately carries through unchecked. Opening the
/// collection set makes every field undeclared, so the check moves to the
/// boundary: a record that cannot be encoded is refused on write rather than
/// accepted and then breaking every export of the repository afterwards. What
/// the model *can* represent survives byte for byte, which is what the
/// `unknown` type exists for.
///
/// # Why the schema check is conditional
///
/// atproto's Lexicon specification names three stances a service may take:
/// *explicit*, where every record must resolve to a known schema and satisfy
/// it; *optimistic*, where a record is validated when a schema is at hand and
/// accepted when it is not; and *skip*. This server takes the optimistic one,
/// which is the stance a personal data server has to take, because a
/// repository is the account's and the lexicons are the world's. Validation is
/// explicit, and that choice held only while the collection set was closed. The
/// set is open now.
///
/// ```
/// use didbot_schema::validate_shape;
/// use serde_json::json;
///
/// // A record under a collection no schema is held for is accepted unchecked.
/// let foreign = json!({"text": "reading the MST encoder"});
/// assert!(validate_shape("com.example.thing", &foreign).is_ok());
///
/// // The one schema this deployment holds is enforced.
/// let bad = json!({"did": "did:web:a.example"});
/// assert!(validate_shape("bot.did.registration", &bad).is_err());
///
/// ```
pub fn validate_with(collection: &str, record: &Value, stance: Stance) -> Result<(), ShapeError> {
    validate_collection(collection)?;
    match record.get("$type") {
        None => {}
        Some(Value::String(declared)) if declared == collection => {}
        Some(Value::String(declared)) => {
            return Err(ShapeError::TypeMismatch {
                declared: declared.clone(),
                collection: collection.to_owned(),
            })
        }
        Some(_) => return Err(ShapeError::TypeNotAString),
    }
    // Before the schema, and unconditionally: a record no export can encode
    // is one this server must not acknowledge having stored, whatever any
    // lexicon says about it.
    didbot_data::Value::object_from_json(record)
        .map_err(|source| ShapeError::UnrepresentableRecord { source })?;
    if stance == Stance::Skip {
        return Ok(());
    }
    if !schema_is_held(collection) {
        if stance == Stance::Explicit {
            return Err(ShapeError::NoSchemaForCollection {
                collection: collection.to_owned(),
            });
        }
        tracing::debug!(
            collection,
            "no lexicon held for this collection; storing the record unchecked"
        );
        return Ok(());
    }
    Catalog::embedded()
        .validate_record(collection, record)
        .map_err(|invalid| ShapeError::SchemaViolation {
            collection: collection.to_owned(),
            path: invalid.path,
            reason: invalid.reason,
        })
}