truelayer-interview/src/clients/funtranslations.rs

use crate::{clients::body, domain::TranslationKind, error::AppError};
use serde::Deserialize;
use std::time::Duration;
use tracing::instrument;

#[derive(Clone)]
pub struct TranslationClient {
    base_url: String,
    http: reqwest::Client,
}

impl TranslationClient {
    pub fn new(base_url: impl Into<String>, timeout: Duration) -> Result<Self, AppError> {
        let http = reqwest::Client::builder()
            .timeout(timeout)
            .build()
            .map_err(|error| AppError::Upstream(error.to_string()))?;

        Ok(Self {
            base_url: trim_trailing_slash(&base_url.into()),
            http,
        })
    }

    #[instrument(skip(self, text), fields(translation.kind = ?kind))]
    pub async fn translate(&self, kind: TranslationKind, text: &str) -> Result<String, AppError> {
        let url = format!("{}/translate/{}", self.base_url, endpoint_for(kind));
        let response = self
            .http
            .post(url)
            .json(&TranslationRequest { text })
            .send()
            .await
            .map_err(|error| map_reqwest_error(&error))?;
        let status = response.status();

        if !status.is_success() {
            return Err(AppError::Upstream(format!(
                "FunTranslations returned status {status}"
            )));
        }

        let payload = body::read_json::<TranslationResponse>(response, "FunTranslations").await?;
        if payload.contents.translated.trim().is_empty() {
            Err(AppError::InvalidUpstreamData(
                "FunTranslations returned an empty translation".to_owned(),
            ))
        } else {
            Ok(payload.contents.translated)
        }
    }
}

#[derive(serde::Serialize)]
struct TranslationRequest<'a> {
    text: &'a str,
}

#[derive(Debug, Deserialize)]
struct TranslationResponse {
    contents: TranslationContents,
}

#[derive(Debug, Deserialize)]
struct TranslationContents {
    translated: String,
}

const fn endpoint_for(kind: TranslationKind) -> &'static str {
    match kind {
        TranslationKind::Shakespeare => "shakespeare",
        TranslationKind::Yoda => "yoda",
    }
}

fn trim_trailing_slash(value: &str) -> String {
    value.trim_end_matches('/').to_owned()
}

fn map_reqwest_error(error: &reqwest::Error) -> AppError {
    if error.is_timeout() {
        AppError::Timeout
    } else {
        AppError::Upstream(error.to_string())
    }
}

#[cfg(test)]
mod tests {
    use super::{TranslationClient, endpoint_for};
    use crate::domain::TranslationKind;
    use std::time::Duration;
    use wiremock::{Mock, MockServer, ResponseTemplate, matchers};

    #[test]
    fn translation_kind_resolves_funtranslations_endpoint() {
        assert_eq!(endpoint_for(TranslationKind::Yoda), "yoda");
        assert_eq!(endpoint_for(TranslationKind::Shakespeare), "shakespeare");
    }

    #[tokio::test]
    async fn posts_text_to_the_requested_translation_endpoint() {
        let server = MockServer::start().await;
        Mock::given(matchers::method("POST"))
            .and(matchers::path("/translate/yoda"))
            .and(matchers::body_string_contains("Created"))
            .respond_with(ResponseTemplate::new(200).set_body_json(serde_json::json!({
                "contents": { "translated": "Created, it was." }
            })))
            .expect(1)
            .mount(&server)
            .await;
        let client = client_for(&server);

        let translated = client
            .translate(TranslationKind::Yoda, "Created")
            .await
            .expect("translation should succeed");

        assert_eq!(translated, "Created, it was.");
    }

    #[tokio::test]
    async fn rejects_empty_translations_so_callers_can_fallback() {
        let server = MockServer::start().await;
        Mock::given(matchers::method("POST"))
            .and(matchers::path("/translate/yoda"))
            .respond_with(ResponseTemplate::new(200).set_body_json(serde_json::json!({
                "contents": { "translated": "   " }
            })))
            .mount(&server)
            .await;
        let client = client_for(&server);

        let error = client
            .translate(TranslationKind::Yoda, "hello")
            .await
            .expect_err("empty translation should fail");

        assert!(error.to_string().contains("empty translation"));
    }

    #[tokio::test]
    async fn maps_translation_http_failures_to_upstream_errors() {
        let server = MockServer::start().await;
        Mock::given(matchers::method("POST"))
            .and(matchers::path("/translate/shakespeare"))
            .respond_with(ResponseTemplate::new(429))
            .mount(&server)
            .await;
        let client = client_for(&server);

        let error = client
            .translate(TranslationKind::Shakespeare, "hello")
            .await
            .expect_err("translation should fail");

        assert!(error.to_string().contains("FunTranslations"));
    }

    fn client_for(server: &MockServer) -> TranslationClient {
        TranslationClient::new(server.uri(), Duration::from_secs(1)).expect("client should build")
    }
}