use crate::config::Config;
use anyhow::{Context, Result, anyhow};
use futures_util::StreamExt;
use serde::{Deserialize, Serialize};
use serde_json::json;

#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum Role {
    System,
    User,
    Assistant,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ChatMessage {
    pub role: Role,
    pub content: String,
}

impl ChatMessage {
    pub fn system(content: impl Into<String>) -> Self {
        Self {
            role: Role::System,
            content: content.into(),
        }
    }
    pub fn user(content: impl Into<String>) -> Self {
        Self {
            role: Role::User,
            content: content.into(),
        }
    }
    pub fn assistant(content: impl Into<String>) -> Self {
        Self {
            role: Role::Assistant,
            content: content.into(),
        }
    }
}

#[derive(Debug, Deserialize)]
struct StreamChunk {
    choices: Option<Vec<StreamChoice>>,
    error: Option<ApiErrorBody>,
}

#[derive(Debug, Deserialize)]
struct StreamChoice {
    delta: Option<Delta>,
}

#[derive(Debug, Deserialize)]
struct Delta {
    content: Option<String>,
}

#[derive(Debug, Deserialize)]
struct ApiErrorBody {
    message: Option<String>,
}

/// Stream a chat completion; invoke `on_delta` for each text piece.
/// Returns the full assistant text.
pub async fn stream_chat(
    config: &Config,
    messages: &[ChatMessage],
    mut on_delta: impl FnMut(&str),
) -> Result<String> {
    let client = reqwest::Client::new();
    let url = format!("{}/chat/completions", config.base_url);
    let body = json!({
        "model": config.model,
        "messages": messages,
        "stream": true,
    });

    let response = client
        .post(&url)
        .bearer_auth(&config.api_key)
        .header("content-type", "application/json")
        .json(&body)
        .send()
        .await
        .context("request to chat completions failed")?;

    if !response.status().is_success() {
        let status = response.status();
        let text = response.text().await.unwrap_or_default();
        return Err(anyhow!("chat completions HTTP {status}: {text}"));
    }

    let mut full = String::new();
    let mut stream = response.bytes_stream();
    let mut buffer = String::new();

    while let Some(item) = stream.next().await {
        let chunk = item.context("reading SSE stream")?;
        buffer.push_str(&String::from_utf8_lossy(&chunk));

        while let Some(pos) = buffer.find('\n') {
            let mut line = buffer[..pos].to_string();
            buffer.drain(..=pos);
            if line.ends_with('\r') {
                line.pop();
            }
            let line = line.trim();
            if line.is_empty() {
                continue;
            }
            if !line.starts_with("data:") {
                continue;
            }
            let data = line[5..].trim();
            if data == "[DONE]" {
                return Ok(full);
            }
            let parsed: StreamChunk = match serde_json::from_str(data) {
                Ok(v) => v,
                Err(_) => continue,
            };
            if let Some(err) = parsed.error {
                return Err(anyhow!(
                    "provider error: {}",
                    err.message.unwrap_or_else(|| data.to_string())
                ));
            }
            if let Some(choices) = parsed.choices {
                for choice in choices {
                    if let Some(delta) = choice.delta.and_then(|d| d.content) {
                        if !delta.is_empty() {
                            on_delta(&delta);
                            full.push_str(&delta);
                        }
                    }
                }
            }
        }
    }

    Ok(full)
}

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

    #[test]
    fn messages_serialize_roles() {
        let msg = ChatMessage::user("hi");
        let v = serde_json::to_value(&msg).unwrap();
        assert_eq!(v["role"], "user");
        assert_eq!(v["content"], "hi");
    }
}