use std::collections::BTreeMap;
use std::fmt::{Display, Formatter};

use crate::permissions::{PermissionOutcome, PermissionPolicy, PermissionPrompter};
use crate::session::{ContentBlock, ConversationMessage, Session};

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ApiRequest {
    pub system_prompt: Vec<String>,
    pub messages: Vec<ConversationMessage>,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum AssistantEvent {
    TextDelta(String),
    ToolUse {
        id: String,
        name: String,
        input: String,
    },
    MessageStop,
}

pub trait ApiClient {
    fn stream(&mut self, request: ApiRequest) -> Result<Vec<AssistantEvent>, RuntimeError>;
}

pub trait ToolExecutor {
    fn execute(&mut self, tool_name: &str, input: &str) -> Result<String, ToolError>;
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ToolError {
    message: String,
}

impl ToolError {
    #[must_use]
    pub fn new(message: impl Into<String>) -> Self {
        Self {
            message: message.into(),
        }
    }
}

impl Display for ToolError {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.message)
    }
}

impl std::error::Error for ToolError {}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct RuntimeError {
    message: String,
}

impl RuntimeError {
    #[must_use]
    pub fn new(message: impl Into<String>) -> Self {
        Self {
            message: message.into(),
        }
    }
}

impl Display for RuntimeError {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.message)
    }
}

impl std::error::Error for RuntimeError {}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TurnSummary {
    pub assistant_messages: Vec<ConversationMessage>,
    pub tool_results: Vec<ConversationMessage>,
    pub iterations: usize,
}

pub struct ConversationRuntime<C, T> {
    session: Session,
    api_client: C,
    tool_executor: T,
    permission_policy: PermissionPolicy,
    system_prompt: Vec<String>,
    max_iterations: usize,
}

impl<C, T> ConversationRuntime<C, T>
where
    C: ApiClient,
    T: ToolExecutor,
{
    #[must_use]
    pub fn new(
        session: Session,
        api_client: C,
        tool_executor: T,
        permission_policy: PermissionPolicy,
        system_prompt: Vec<String>,
    ) -> Self {
        Self {
            session,
            api_client,
            tool_executor,
            permission_policy,
            system_prompt,
            max_iterations: 16,
        }
    }

    #[must_use]
    pub fn with_max_iterations(mut self, max_iterations: usize) -> Self {
        self.max_iterations = max_iterations;
        self
    }

    pub fn run_turn(
        &mut self,
        user_input: impl Into<String>,
        mut prompter: Option<&mut dyn PermissionPrompter>,
    ) -> Result<TurnSummary, RuntimeError> {
        self.session
            .messages
            .push(ConversationMessage::user_text(user_input.into()));

        let mut assistant_messages = Vec::new();
        let mut tool_results = Vec::new();
        let mut iterations = 0;

        loop {
            iterations += 1;
            if iterations > self.max_iterations {
                return Err(RuntimeError::new(
                    "conversation loop exceeded the maximum number of iterations",
                ));
            }

            let request = ApiRequest {
                system_prompt: self.system_prompt.clone(),
                messages: self.session.messages.clone(),
            };
            let events = self.api_client.stream(request)?;
            let assistant_message = build_assistant_message(events)?;
            let pending_tool_uses = assistant_message
                .blocks
                .iter()
                .filter_map(|block| match block {
                    ContentBlock::ToolUse { id, name, input } => {
                        Some((id.clone(), name.clone(), input.clone()))
                    }
                    _ => None,
                })
                .collect::<Vec<_>>();

            self.session.messages.push(assistant_message.clone());
            assistant_messages.push(assistant_message);

            if pending_tool_uses.is_empty() {
                break;
            }

            for (tool_use_id, tool_name, input) in pending_tool_uses {
                let permission_outcome = if let Some(prompt) = prompter.as_mut() {
                    self.permission_policy
                        .authorize(&tool_name, &input, Some(*prompt))
                } else {
                    self.permission_policy.authorize(&tool_name, &input, None)
                };

                let result_message = match permission_outcome {
                    PermissionOutcome::Allow => {
                        match self.tool_executor.execute(&tool_name, &input) {
                            Ok(output) => ConversationMessage::tool_result(
                                tool_use_id,
                                tool_name,
                                output,
                                false,
                            ),
                            Err(error) => ConversationMessage::tool_result(
                                tool_use_id,
                                tool_name,
                                error.to_string(),
                                true,
                            ),
                        }
                    }
                    PermissionOutcome::Deny { reason } => {
                        ConversationMessage::tool_result(tool_use_id, tool_name, reason, true)
                    }
                };
                self.session.messages.push(result_message.clone());
                tool_results.push(result_message);
            }
        }

        Ok(TurnSummary {
            assistant_messages,
            tool_results,
            iterations,
        })
    }

    #[must_use]
    pub fn session(&self) -> &Session {
        &self.session
    }

    #[must_use]
    pub fn into_session(self) -> Session {
        self.session
    }
}

fn build_assistant_message(
    events: Vec<AssistantEvent>,
) -> Result<ConversationMessage, RuntimeError> {
    let mut text = String::new();
    let mut blocks = Vec::new();
    let mut finished = false;

    for event in events {
        match event {
            AssistantEvent::TextDelta(delta) => text.push_str(&delta),
            AssistantEvent::ToolUse { id, name, input } => {
                flush_text_block(&mut text, &mut blocks);
                blocks.push(ContentBlock::ToolUse { id, name, input });
            }
            AssistantEvent::MessageStop => {
                finished = true;
            }
        }
    }

    flush_text_block(&mut text, &mut blocks);

    if !finished {
        return Err(RuntimeError::new(
            "assistant stream ended without a message stop event",
        ));
    }
    if blocks.is_empty() {
        return Err(RuntimeError::new("assistant stream produced no content"));
    }

    Ok(ConversationMessage::assistant(blocks))
}

fn flush_text_block(text: &mut String, blocks: &mut Vec<ContentBlock>) {
    if !text.is_empty() {
        blocks.push(ContentBlock::Text {
            text: std::mem::take(text),
        });
    }
}

type ToolHandler = Box<dyn FnMut(&str) -> Result<String, ToolError>>;

#[derive(Default)]
pub struct StaticToolExecutor {
    handlers: BTreeMap<String, ToolHandler>,
}

impl StaticToolExecutor {
    #[must_use]
    pub fn new() -> Self {
        Self::default()
    }

    #[must_use]
    pub fn register(
        mut self,
        tool_name: impl Into<String>,
        handler: impl FnMut(&str) -> Result<String, ToolError> + 'static,
    ) -> Self {
        self.handlers.insert(tool_name.into(), Box::new(handler));
        self
    }
}

impl ToolExecutor for StaticToolExecutor {
    fn execute(&mut self, tool_name: &str, input: &str) -> Result<String, ToolError> {
        self.handlers
            .get_mut(tool_name)
            .ok_or_else(|| ToolError::new(format!("unknown tool: {tool_name}")))?(input)
    }
}

#[cfg(test)]
mod tests {
    use super::{
        ApiClient, ApiRequest, AssistantEvent, ConversationRuntime, RuntimeError,
        StaticToolExecutor,
    };
    use crate::permissions::{
        PermissionMode, PermissionPolicy, PermissionPromptDecision, PermissionPrompter,
        PermissionRequest,
    };
    use crate::prompt::SystemPromptBuilder;
    use crate::session::{ContentBlock, MessageRole, Session};

    struct ScriptedApiClient {
        call_count: usize,
    }

    impl ApiClient for ScriptedApiClient {
        fn stream(&mut self, request: ApiRequest) -> Result<Vec<AssistantEvent>, RuntimeError> {
            self.call_count += 1;
            match self.call_count {
                1 => {
                    assert!(request
                        .messages
                        .iter()
                        .any(|message| message.role == MessageRole::User));
                    Ok(vec![
                        AssistantEvent::TextDelta("Let me calculate that.".to_string()),
                        AssistantEvent::ToolUse {
                            id: "tool-1".to_string(),
                            name: "add".to_string(),
                            input: "2,2".to_string(),
                        },
                        AssistantEvent::MessageStop,
                    ])
                }
                2 => {
                    let last_message = request
                        .messages
                        .last()
                        .expect("tool result should be present");
                    assert_eq!(last_message.role, MessageRole::Tool);
                    Ok(vec![
                        AssistantEvent::TextDelta("The answer is 4.".to_string()),
                        AssistantEvent::MessageStop,
                    ])
                }
                _ => Err(RuntimeError::new("unexpected extra API call")),
            }
        }
    }

    struct PromptAllowOnce;

    impl PermissionPrompter for PromptAllowOnce {
        fn decide(&mut self, request: &PermissionRequest) -> PermissionPromptDecision {
            assert_eq!(request.tool_name, "add");
            PermissionPromptDecision::Allow
        }
    }

    #[test]
    fn runs_user_to_tool_to_result_loop_end_to_end() {
        let api_client = ScriptedApiClient { call_count: 0 };
        let tool_executor = StaticToolExecutor::new().register("add", |input| {
            let total = input
                .split(',')
                .map(|part| part.parse::<i32>().expect("input must be valid integer"))
                .sum::<i32>();
            Ok(total.to_string())
        });
        let permission_policy = PermissionPolicy::new(PermissionMode::Prompt);
        let system_prompt = SystemPromptBuilder::new()
            .with_cwd("/tmp/project")
            .with_os("linux", "6.8")
            .with_date("2026-03-31")
            .build();
        let mut runtime = ConversationRuntime::new(
            Session::new(),
            api_client,
            tool_executor,
            permission_policy,
            system_prompt,
        );

        let summary = runtime
            .run_turn("what is 2 + 2?", Some(&mut PromptAllowOnce))
            .expect("conversation loop should succeed");

        assert_eq!(summary.iterations, 2);
        assert_eq!(summary.assistant_messages.len(), 2);
        assert_eq!(summary.tool_results.len(), 1);
        assert_eq!(runtime.session().messages.len(), 4);
        assert!(matches!(
            runtime.session().messages[1].blocks[1],
            ContentBlock::ToolUse { .. }
        ));
        assert!(matches!(
            runtime.session().messages[2].blocks[0],
            ContentBlock::ToolResult {
                is_error: false,
                ..
            }
        ));
    }

    #[test]
    fn records_denied_tool_results_when_prompt_rejects() {
        struct RejectPrompter;
        impl PermissionPrompter for RejectPrompter {
            fn decide(&mut self, _request: &PermissionRequest) -> PermissionPromptDecision {
                PermissionPromptDecision::Deny {
                    reason: "not now".to_string(),
                }
            }
        }

        struct SingleCallApiClient;
        impl ApiClient for SingleCallApiClient {
            fn stream(&mut self, request: ApiRequest) -> Result<Vec<AssistantEvent>, RuntimeError> {
                if request
                    .messages
                    .iter()
                    .any(|message| message.role == MessageRole::Tool)
                {
                    return Ok(vec![
                        AssistantEvent::TextDelta("I could not use the tool.".to_string()),
                        AssistantEvent::MessageStop,
                    ]);
                }
                Ok(vec![
                    AssistantEvent::ToolUse {
                        id: "tool-1".to_string(),
                        name: "blocked".to_string(),
                        input: "secret".to_string(),
                    },
                    AssistantEvent::MessageStop,
                ])
            }
        }

        let mut runtime = ConversationRuntime::new(
            Session::new(),
            SingleCallApiClient,
            StaticToolExecutor::new(),
            PermissionPolicy::new(PermissionMode::Prompt),
            vec!["system".to_string()],
        );

        let summary = runtime
            .run_turn("use the tool", Some(&mut RejectPrompter))
            .expect("conversation should continue after denied tool");

        assert_eq!(summary.tool_results.len(), 1);
        assert!(matches!(
            &summary.tool_results[0].blocks[0],
            ContentBlock::ToolResult { is_error: true, output, .. } if output == "not now"
        ));
    }
}