//! Agent state machine — tracks lifecycle of a single AI agent invocation.
//!
//! States: Idle → Thinking → ToolCall → Thinking → ... → Answering | Error
//! The Thinking ↔ ToolCall cycle repeats until max tool depth or final answer.

use chrono::{DateTime, Utc};
use serde::{Deserialize, Serialize};

/// Current phase of an agent's execution lifecycle.
#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
pub enum AgentState {
    /// Agent is idle, waiting for input
    Idle,
    /// Agent is reasoning/thinking (may produce thinking chunks)
    Thinking {
        started_at: DateTime<Utc>,
        tool_depth: u32,
    },
    /// Agent is executing a tool call
    ToolCall {
        tool_name: String,
        started_at: DateTime<Utc>,
    },
    /// Agent is returning the final answer
    Answering {
        /// Accumulated answer content so far
        content_chars: u64,
        started_at: DateTime<Utc>,
    },
    /// Agent encountered a non-recoverable error
    Error { message: String, tool_depth: u32 },
}

impl AgentState {
    pub fn is_terminal(&self) -> bool {
        matches!(
            self,
            AgentState::Answering { .. } | AgentState::Error { .. }
        )
    }

    pub fn is_idle(&self) -> bool {
        matches!(self, AgentState::Idle)
    }

    pub fn current_phase(&self) -> &'static str {
        match self {
            AgentState::Idle => "idle",
            AgentState::Thinking { .. } => "thinking",
            AgentState::ToolCall { .. } => "tool_call",
            AgentState::Answering { .. } => "answering",
            AgentState::Error { .. } => "error",
        }
    }
}

/// State machine for agent lifecycle transitions.
pub struct AgentRuntime {
    state: AgentState,
    max_tool_depth: u32,
    current_depth: u32,
}

impl AgentRuntime {
    pub fn new(max_tool_depth: u32) -> Self {
        Self {
            state: AgentState::Idle,
            max_tool_depth,
            current_depth: 0,
        }
    }

    pub fn state(&self) -> &AgentState {
        &self.state
    }

    /// Transition from Idle → Thinking
    pub fn start_thinking(&mut self) {
        debug_assert!(self.state.is_idle(), "must be Idle to start thinking");
        self.current_depth = 0;
        self.state = AgentState::Thinking {
            started_at: Utc::now(),
            tool_depth: 0,
        };
    }

    /// Transition from Thinking → ToolCall (increments tool depth)
    pub fn start_tool_call(&mut self, tool_name: String) -> Result<(), &'static str> {
        if !matches!(self.state, AgentState::Thinking { .. }) {
            return Err("must be Thinking to start tool call");
        }
        if self.current_depth >= self.max_tool_depth {
            return Err("max tool depth reached");
        }
        self.state = AgentState::ToolCall {
            tool_name,
            started_at: Utc::now(),
        };
        Ok(())
    }

    /// Transition from ToolCall → Thinking (back after tool result)
    pub fn complete_tool_call(&mut self) -> Result<(), &'static str> {
        if !matches!(self.state, AgentState::ToolCall { .. }) {
            return Err("must be ToolCall to complete");
        }
        self.current_depth += 1;
        self.state = AgentState::Thinking {
            started_at: Utc::now(),
            tool_depth: self.current_depth,
        };
        Ok(())
    }

    /// Transition to Answering (terminal)
    pub fn start_answer(&mut self) {
        self.state = AgentState::Answering {
            content_chars: 0,
            started_at: Utc::now(),
        };
    }

    pub fn append_answer(&mut self, content: &str) {
        if let AgentState::Answering { content_chars, .. } = &mut self.state {
            *content_chars += content.len() as u64;
        }
    }

    /// Transition to Error (terminal)
    pub fn fail(&mut self, message: String) {
        self.state = AgentState::Error {
            message,
            tool_depth: self.current_depth,
        };
    }
}

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

    #[test]
    fn test_starts_idle() {
        let rt = AgentRuntime::new(10);
        assert!(rt.state().is_idle());
        assert_eq!(rt.state().current_phase(), "idle");
    }

    #[test]
    fn test_idle_to_thinking() {
        let mut rt = AgentRuntime::new(10);
        rt.start_thinking();
        assert_eq!(rt.state().current_phase(), "thinking");
        assert!(!rt.state().is_terminal());
    }

    #[test]
    fn test_thinking_to_tool_call_and_back() {
        let mut rt = AgentRuntime::new(10);
        rt.start_thinking();
        rt.start_tool_call("search".into()).unwrap();
        assert_eq!(rt.state().current_phase(), "tool_call");
        rt.complete_tool_call().unwrap();
        assert_eq!(rt.state().current_phase(), "thinking");
    }

    #[test]
    fn test_thinking_to_answer() {
        let mut rt = AgentRuntime::new(10);
        rt.start_thinking();
        rt.start_answer();
        assert_eq!(rt.state().current_phase(), "answering");
        assert!(rt.state().is_terminal());
    }

    #[test]
    fn test_append_answer_tracks_chars() {
        let mut rt = AgentRuntime::new(10);
        rt.start_thinking();
        rt.start_answer();
        rt.append_answer("hello");
        if let AgentState::Answering { content_chars, .. } = rt.state() {
            assert_eq!(*content_chars, 5);
        } else {
            panic!("expected Answering state");
        }
    }

    #[test]
    fn test_error_is_terminal() {
        let mut rt = AgentRuntime::new(10);
        rt.start_thinking();
        rt.fail("something broke".into());
        assert_eq!(rt.state().current_phase(), "error");
        assert!(rt.state().is_terminal());
    }

    #[test]
    fn test_transition_from_wrong_state() {
        let mut rt = AgentRuntime::new(10);
        // Can't start tool call from Idle
        assert!(rt.start_tool_call("tool".into()).is_err());
        // Can't complete tool call from Idle
        assert!(rt.complete_tool_call().is_err());
    }

    #[test]
    fn test_max_depth_rejected() {
        let mut rt = AgentRuntime::new(2);
        rt.start_thinking();
        rt.start_tool_call("tool1".into()).unwrap();
        rt.complete_tool_call().unwrap();
        rt.start_tool_call("tool2".into()).unwrap();
        rt.complete_tool_call().unwrap();
        assert!(rt.start_tool_call("tool3".into()).is_err());
    }
}