//! Minimal Anthropic Messages API client with a hand-rolled agentic loop.
//!
//! Structured output is obtained by forcing a tool choice: we register an
//! "emit_result" tool whose input schema is the caller's desired output
//! type (derived via `schemars`), then require the model to call it.

use std::sync::Arc;

use anyhow::{bail, Context, Result};
use serde::de::DeserializeOwned;
use serde_json::Value;

use super::messages::{ContentBlock, Message, RequestBody, ResponseBody, ToolChoice, ToolSpec};
use super::tool::Tool;

const API_URL: &str = "https://api.anthropic.com/v1/messages";
const API_VERSION: &str = "2023-06-01";
const DEFAULT_MAX_TOKENS: u32 = 2048;
const MAX_AGENT_TURNS: usize = 6;

pub struct AnthropicClient {
    http: reqwest::Client,
    api_key: String,
    model: String,
}

impl AnthropicClient {
    pub fn new(api_key: impl Into<String>, model: impl Into<String>) -> Self {
        Self {
            http: reqwest::Client::new(),
            api_key: api_key.into(),
            model: model.into(),
        }
    }

    /// A single structured-output call with no tools beyond the forced
    /// output tool. Used by agents that don't need to call external tools
    /// (e.g. the policy-compliance agent).
    pub async fn structured_call<T: DeserializeOwned>(
        &self,
        system: &str,
        user_message: &str,
        output_schema: Value,
    ) -> Result<T> {
        self.run_agentic(system, user_message, &[], "emit_result", output_schema)
            .await
    }

    /// Runs a tool-calling loop: the model may call any of `tools` any
    /// number of times, and must eventually call `output_tool_name` (whose
    /// schema is `output_schema`) to finish. Bounded by `MAX_AGENT_TURNS` so
    /// a confused model can't loop forever.
    pub async fn run_agentic<T: DeserializeOwned>(
        &self,
        system: &str,
        user_message: &str,
        tools: &[Arc<dyn Tool>],
        output_tool_name: &str,
        output_schema: Value,
    ) -> Result<T> {
        let mut tool_specs: Vec<ToolSpec> = tools
            .iter()
            .map(|t| ToolSpec {
                name: t.name().to_string(),
                description: t.description().to_string(),
                input_schema: t.input_schema(),
            })
            .collect();
        tool_specs.push(ToolSpec {
            name: output_tool_name.to_string(),
            description: "Emit the final structured result for this task.".to_string(),
            input_schema: output_schema,
        });

        let mut messages = vec![Message::user_text(user_message)];

        for _ in 0..MAX_AGENT_TURNS {
            let tool_choice = if tools.is_empty() {
                ToolChoice::Tool {
                    name: output_tool_name.to_string(),
                }
            } else {
                ToolChoice::Auto
            };

            let response = self
                .send(system, &messages, &tool_specs, tool_choice)
                .await?;

            let mut tool_results = Vec::new();
            let mut output_value = None;

            for block in &response.content {
                let ContentBlock::ToolUse { id, name, input } = block else {
                    continue;
                };
                if name == output_tool_name {
                    output_value = Some(input.clone());
                    continue;
                }
                let Some(tool) = tools.iter().find(|t| t.name() == name) else {
                    tool_results.push(ContentBlock::ToolResult {
                        tool_use_id: id.clone(),
                        content: format!("error: unknown tool '{name}'"),
                    });
                    continue;
                };
                let result = match tool.execute(input.clone()).await {
                    Ok(v) => v.to_string(),
                    Err(e) => format!("error: {e}"),
                };
                tool_results.push(ContentBlock::ToolResult {
                    tool_use_id: id.clone(),
                    content: result,
                });
            }

            messages.push(Message {
                role: "assistant".into(),
                content: response.content,
            });

            if let Some(value) = output_value {
                return serde_json::from_value(value)
                    .context("deserializing model's structured output");
            }

            if tool_results.is_empty() {
                bail!(
                    "model produced no tool call and no structured output (stop_reason: {:?})",
                    response.stop_reason
                );
            }

            messages.push(Message {
                role: "user".into(),
                content: tool_results,
            });
        }

        bail!("exceeded {MAX_AGENT_TURNS} agent turns without a structured result")
    }

    async fn send(
        &self,
        system: &str,
        messages: &[Message],
        tools: &[ToolSpec],
        tool_choice: ToolChoice,
    ) -> Result<ResponseBody> {
        let body = RequestBody {
            model: &self.model,
            max_tokens: DEFAULT_MAX_TOKENS,
            system,
            messages,
            tools,
            tool_choice,
        };

        let response = self
            .http
            .post(API_URL)
            .header("x-api-key", &self.api_key)
            .header("anthropic-version", API_VERSION)
            .json(&body)
            .send()
            .await
            .context("sending request to Anthropic API")?;

        let status = response.status();
        let text = response.text().await.context("reading Anthropic response body")?;
        if !status.is_success() {
            bail!("Anthropic API error ({status}): {text}");
        }

        serde_json::from_str(&text).context("parsing Anthropic response body")
    }
}