//! A deliberately deaf JSON-RPC client.
//!
//! The Codex app-server can send requests *to* its client, including
//! `account/chatgptAuthTokens/refresh`, which returns a bare access token.
//! AGENTS.md forbids Lumbridge from scraping a harness's private credentials.
//! Choosing not to call that method would be a convention; this module makes
//! it unrepresentable instead.
//!
//! There is no free-form method string anywhere in this crate. Outbound
//! requests come from a two-variant enum, and every inbound server request is
//! answered with `-32601 method not found` regardless of what it asks for.
//! Classification exists only so a refusal can be counted and named, which
//! makes "we were asked for a credential and refused" an auditable event
//! rather than an absence.

use serde_json::{Value, json};

/// JSON-RPC's "method not found". The only reply this client ever sends.
const METHOD_NOT_FOUND: i64 = -32601;
/// Codex returns this for a request that needs an account it does not have.
pub(crate) const INVALID_REQUEST: i64 = -32600;

/// Every request this crate is able to send.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(crate) enum OutboundRequest {
    Initialize,
    ReadAccountRateLimits,
}

impl OutboundRequest {
    pub(crate) const fn method(self) -> &'static str {
        match self {
            Self::Initialize => "initialize",
            Self::ReadAccountRateLimits => "account/rateLimits/read",
        }
    }
}

/// The notifications this client accepts. Anything else is ignored.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(crate) enum AcceptedNotification {
    AccountRateLimitsUpdated,
}

impl AcceptedNotification {
    pub(crate) fn from_method(method: &str) -> Option<Self> {
        match method {
            "account/rateLimits/updated" => Some(Self::AccountRateLimitsUpdated),
            _ => None,
        }
    }
}

/// What a server-to-client request was asking for.
///
/// Purely descriptive. Every class is refused identically; the distinction
/// exists so a probe can report that it declined a credential request.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum ServerRequestClass {
    /// A request that would hand Lumbridge a credential. Always refused.
    CredentialRefresh,
    /// A request to run something on the harness's behalf.
    Execution,
    /// A request for a human approval decision.
    Approval,
    /// A request for ambient information such as the current time.
    Ambient,
    /// Anything this client does not recognise.
    Unrecognised,
}

impl ServerRequestClass {
    pub(crate) fn classify(method: &str) -> Self {
        match method {
            "account/chatgptAuthTokens/refresh" => Self::CredentialRefresh,
            "item/tool/call" | "attestation/generate" => Self::Execution,
            "item/permissions/requestApproval" => Self::Approval,
            "currentTime/read" => Self::Ambient,
            _ => Self::Unrecognised,
        }
    }

    #[must_use]
    pub const fn label(self) -> &'static str {
        match self {
            Self::CredentialRefresh => "credential refresh",
            Self::Execution => "execution",
            Self::Approval => "approval",
            Self::Ambient => "ambient information",
            Self::Unrecognised => "unrecognised request",
        }
    }
}

/// A classified inbound line.
#[derive(Clone, Debug)]
pub(crate) enum Inbound {
    /// A successful response to one of our two requests.
    Result { id: u64, result: Value },
    /// A failed response to one of our two requests.
    Failure { id: u64, code: i64 },
    /// An accepted push notification.
    Notification {
        kind: AcceptedNotification,
        params: Value,
    },
    /// The server asked us for something. It will be refused.
    ServerRequest {
        id: Value,
        class: ServerRequestClass,
    },
    /// A well-formed frame with nothing for us in it.
    Ignored,
}

/// Encodes one of the two permitted requests.
pub(crate) fn encode_request(id: u64, request: OutboundRequest, params: &Value) -> String {
    json!({
        "jsonrpc": "2.0",
        "id": id,
        "method": request.method(),
        "params": params,
    })
    .to_string()
}

pub(crate) fn encode_notification(method: &'static str, params: &Value) -> String {
    json!({ "jsonrpc": "2.0", "method": method, "params": params }).to_string()
}

/// The refusal sent for every server-to-client request.
pub(crate) fn encode_refusal(id: &Value) -> String {
    json!({
        "jsonrpc": "2.0",
        "id": id,
        "error": { "code": METHOD_NOT_FOUND, "message": "method not found" },
    })
    .to_string()
}

/// Classifies one line of JSON-RPC without interpreting its payload.
///
/// Returns `None` when the line is not a JSON object at all, which the caller
/// reports as [`crate::HarnessError::Malformed`].
pub(crate) fn classify(line: &str) -> Option<Inbound> {
    let frame: Value = serde_json::from_str(line).ok()?;
    let object = frame.as_object()?;
    let method = object.get("method").and_then(Value::as_str);
    let id = object.get("id");

    match (method, id) {
        // A method with an id is the server asking us for something.
        (Some(method), Some(id)) => Some(Inbound::ServerRequest {
            id: id.clone(),
            class: ServerRequestClass::classify(method),
        }),
        // A method without an id is a notification.
        (Some(method), None) => Some(AcceptedNotification::from_method(method).map_or(
            Inbound::Ignored,
            |kind| Inbound::Notification {
                kind,
                params: object.get("params").cloned().unwrap_or(Value::Null),
            },
        )),
        // An id without a method is a response to one of ours.
        (None, Some(id)) => {
            let id = id.as_u64()?;
            if let Some(error) = object.get("error") {
                let code = error.get("code").and_then(Value::as_i64).unwrap_or(0);
                return Some(Inbound::Failure { id, code });
            }
            Some(Inbound::Result {
                id,
                result: object.get("result").cloned().unwrap_or(Value::Null),
            })
        }
        (None, None) => Some(Inbound::Ignored),
    }
}

#[cfg(test)]
mod tests {
    use super::{
        AcceptedNotification, Inbound, OutboundRequest, ServerRequestClass, classify,
        encode_refusal, encode_request,
    };
    use serde_json::{Value, json};

    #[test]
    fn only_two_methods_can_be_sent() {
        assert_eq!(OutboundRequest::Initialize.method(), "initialize");
        assert_eq!(
            OutboundRequest::ReadAccountRateLimits.method(),
            "account/rateLimits/read"
        );
    }

    #[test]
    fn a_credential_refresh_request_is_classified_and_refused() {
        let line = r#"{"jsonrpc":"2.0","id":9,"method":"account/chatgptAuthTokens/refresh"}"#;
        let Some(Inbound::ServerRequest { id, class }) = classify(line) else {
            panic!("a method with an id is a server request");
        };
        assert_eq!(class, ServerRequestClass::CredentialRefresh);
        let refusal: Value =
            serde_json::from_str(&encode_refusal(&id)).expect("the refusal is valid JSON");
        assert_eq!(refusal["error"]["code"], json!(-32601));
        assert!(
            refusal.get("result").is_none(),
            "a refusal must never carry a result"
        );
    }

    #[test]
    fn every_server_request_class_is_refused_the_same_way() {
        for method in [
            "account/chatgptAuthTokens/refresh",
            "item/tool/call",
            "item/permissions/requestApproval",
            "currentTime/read",
            "something/entirely/new",
        ] {
            let line = format!(r#"{{"jsonrpc":"2.0","id":1,"method":"{method}"}}"#);
            let Some(Inbound::ServerRequest { id, .. }) = classify(&line) else {
                panic!("{method} must classify as a server request");
            };
            let refusal: Value = serde_json::from_str(&encode_refusal(&id)).expect("valid JSON");
            assert_eq!(refusal["error"]["code"], json!(-32601));
        }
    }

    #[test]
    fn only_the_rate_limit_notification_is_accepted() {
        let accepted = classify(
            r#"{"jsonrpc":"2.0","method":"account/rateLimits/updated","params":{"rateLimits":{}}}"#,
        );
        assert!(matches!(
            accepted,
            Some(Inbound::Notification {
                kind: AcceptedNotification::AccountRateLimitsUpdated,
                ..
            })
        ));
        let ignored =
            classify(r#"{"jsonrpc":"2.0","method":"thread/tokenUsage/updated","params":{}}"#);
        assert!(matches!(ignored, Some(Inbound::Ignored)));
    }

    #[test]
    fn responses_and_failures_are_separated() {
        assert!(matches!(
            classify(r#"{"jsonrpc":"2.0","id":7,"result":{"rateLimits":{}}}"#),
            Some(Inbound::Result { id: 7, .. })
        ));
        assert!(matches!(
            classify(r#"{"jsonrpc":"2.0","id":7,"error":{"code":-32600,"message":"nope"}}"#),
            Some(Inbound::Failure {
                id: 7,
                code: -32600
            })
        ));
    }

    #[test]
    fn malformed_and_non_object_lines_are_rejected() {
        for line in ["", "not json", "[1,2,3]", "\"a string\"", "{"] {
            assert!(classify(line).is_none(), "{line:?} must not classify");
        }
    }

    #[test]
    fn a_request_encodes_without_a_free_form_method() {
        let encoded = encode_request(1, OutboundRequest::ReadAccountRateLimits, &json!({}));
        let frame: Value = serde_json::from_str(&encoded).expect("valid JSON");
        assert_eq!(frame["method"], json!("account/rateLimits/read"));
        assert_eq!(frame["jsonrpc"], json!("2.0"));
    }
}