//! Turning an [`Error`] into the response a Hub client expects.
//!
//! `huggingface_hub` branches on the `X-Error-Code` header to raise typed
//! exceptions, so emitting it is not cosmetic. The status alone would collapse
//! "no such repo", "no such revision" and "no such file" into one 404 that the
//! client cannot tell apart.

use axum::response::{IntoResponse, Response};
use http::{StatusCode, header};
use rustingface_entities::error::Error;

/// Header name the client reads to classify a failure.
pub const X_ERROR_CODE: &str = "x-error-code";
/// Header the Hub uses to carry a human-readable reason.
pub const X_ERROR_MESSAGE: &str = "x-error-message";

/// The exact `X-Error-Message` the client matches to raise `DisabledRepoError`.
///
/// Verified against the pinned `huggingface_hub` by the conformance suite:
/// unlike the others, a disabled repo is dispatched on the *message*, not on
/// `X-Error-Code`, so emitting only the code would have the client raise a
/// generic `HfHubHTTPError` instead.
pub const DISABLED_REPO_MESSAGE: &str = "Access to this resource is disabled.";

/// Wrapper giving [`Error`] an axum response.
pub struct ApiError(pub Error);

impl From<Error> for ApiError {
    fn from(err: Error) -> Self {
        Self(err)
    }
}

impl IntoResponse for ApiError {
    fn into_response(self) -> Response {
        let ApiError(err) = self;
        let status =
            StatusCode::from_u16(err.status()).unwrap_or(StatusCode::INTERNAL_SERVER_ERROR);
        let message = err.to_string();
        // A disabled repo is dispatched by the client on this exact message
        // rather than on X-Error-Code; everything else carries its own reason.
        let wire_message = match &err {
            Error::DisabledRepo(_) => DISABLED_REPO_MESSAGE.to_owned(),
            _ => sanitise(&message),
        };

        let mut response =
            (status, axum::Json(serde_json::json!({ "error": message }))).into_response();

        if let Some(code) = err.code()
            && let Ok(value) = header::HeaderValue::from_str(code.as_str())
        {
            response.headers_mut().insert(X_ERROR_CODE, value);
        }

        // Sent for correctness rather than in hope: `huggingface_hub` ignores
        // it and sleeps its own exponential backoff. A client that does read
        // it should not hammer a transfer that takes minutes.
        if matches!(err, Error::FetchInProgress(_)) {
            response
                .headers_mut()
                .insert(header::RETRY_AFTER, header::HeaderValue::from_static("5"));
        }
        // Header values must be visible ASCII; a path or upstream detail could
        // carry anything, so a message that will not encode is simply dropped
        // rather than replacing the response with a 500.
        if let Ok(value) = header::HeaderValue::from_str(&wire_message) {
            response.headers_mut().insert(X_ERROR_MESSAGE, value);
        }

        if err.status() >= 500 {
            tracing::error!(%err, "request failed");
        } else {
            tracing::debug!(%err, "request refused");
        }
        response
    }
}

/// Reduce a message to characters a header value can carry.
fn sanitise(message: &str) -> String {
    message
        .chars()
        .map(|c| if (' '..='~').contains(&c) { c } else { ' ' })
        .take(512)
        .collect()
}

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

    #[test]
    fn a_missing_entry_carries_its_code_and_a_404() {
        let response = ApiError(Error::EntryNotFound("config.json".into())).into_response();
        assert_eq!(response.status(), StatusCode::NOT_FOUND);
        assert_eq!(response.headers()[X_ERROR_CODE], "EntryNotFound");
    }

    #[test]
    fn a_gated_repo_is_a_403_with_its_own_code() {
        let response = ApiError(Error::GatedRepo("terms".into())).into_response();
        assert_eq!(response.status(), StatusCode::FORBIDDEN);
        assert_eq!(response.headers()[X_ERROR_CODE], "GatedRepo");
    }

    #[test]
    fn an_upstream_auth_failure_is_a_401_with_no_code() {
        let response = ApiError(Error::UpstreamUnauthorized("bad token".into())).into_response();
        assert_eq!(response.status(), StatusCode::UNAUTHORIZED);
        assert!(!response.headers().contains_key(X_ERROR_CODE));
    }

    #[test]
    fn a_disabled_repo_carries_the_exact_message_the_client_matches_on() {
        // The client dispatches DisabledRepoError on this string rather than
        // on X-Error-Code, so it must go out verbatim.
        let response = ApiError(Error::DisabledRepo("upstream said so".into())).into_response();
        assert_eq!(response.status(), StatusCode::FORBIDDEN);
        assert_eq!(response.headers()[X_ERROR_MESSAGE], DISABLED_REPO_MESSAGE);
    }

    #[test]
    fn a_message_with_a_newline_still_produces_a_valid_header() {
        let response = ApiError(Error::EntryNotFound("a\nb\u{1f600}".into())).into_response();
        let value = response.headers()[X_ERROR_MESSAGE].to_str().unwrap();
        assert!(!value.contains('\n'), "{value:?}");
    }
}