use anyhow::Result;
use reqwest::Client;
use serde_json::json;

/// Rerank response item (bare array, not OpenAI envelope).
#[derive(serde::Deserialize, Debug)]
pub struct RerankScore {
    pub index: usize,
    pub score: f32,
}

/// Rerank client (BAAI/bge-reranker-base via TEI).
pub struct RerankClient {
    base_url: String,
    api_key: String,
    model: String,
    timeout_secs: u64,
}

impl RerankClient {
    /// Create rerank client.
    pub fn new(base_url: &str, api_key: &str, model: &str) -> Result<Self> {
        Ok(Self {
            base_url: base_url.to_string(),
            api_key: api_key.to_string(),
            model: model.to_string(),
            timeout_secs: 300,
        })
    }

    /// Rerank query against texts, return scored items in score order.
    /// Returns Vec<(index, score)> mapping back to input positions.
    pub async fn rerank(&self, query: &str, texts: &[&str]) -> Result<Vec<(usize, f32)>> {
        // Empty input returns empty without request
        if texts.is_empty() {
            return Ok(vec![]);
        }

        let url = format!("{}/rerank", self.base_url);

        let client = Client::builder()
            .timeout(std::time::Duration::from_secs(self.timeout_secs))
            .build()?;

        let payload = json!({
            "query": query,
            "texts": texts,
        });

        let response = client
            .post(&url)
            .header("apikey", &self.api_key)
            .header("Content-Type", "application/json")
            .json(&payload)
            .send()
            .await?;

        if !response.status().is_success() {
            return Err(anyhow::anyhow!("Rerank failed: {}", response.status()));
        }

        // Parse bare array (not OpenAI envelope)
        let scores: Vec<RerankScore> = response.json().await?;

        // Map back to input positions and scores
        let mut results: Vec<(usize, f32)> = scores
            .into_iter()
            .map(|s| (s.index, s.score))
            .collect();

        // Sort by score descending (highest first)
        results.sort_by(|a, b| b.1.partial_cmp(&a.1).unwrap());

        Ok(results)
    }
}