// log.rs — Persistent conversation log
//
// Append-only JSONL file that records every message in the conversation.
// This is the permanent record — never truncated, never compacted.
// The in-memory message array is a view into this log; compaction
// builds that view by mixing raw recent messages with journal
// summaries of older ones.
//
// Each line is a JSON-serialized Message with its timestamp.
// The log survives session restarts, compactions, and crashes.

use anyhow::{Context, Result};
use std::fs::{File, OpenOptions};
use std::io::{BufRead, BufReader, Seek, SeekFrom, Write};
use std::path::{Path, PathBuf};

use crate::agent::types::Message;

pub struct ConversationLog {
    path: PathBuf,
}

impl ConversationLog {
    pub fn new(path: PathBuf) -> Result<Self> {
        // Ensure parent directory exists
        if let Some(parent) = path.parent() {
            std::fs::create_dir_all(parent)
                .with_context(|| format!("creating log dir {}", parent.display()))?;
        }
        Ok(Self { path })
    }

    /// Append a single message to the log.
    pub fn append(&self, msg: &Message) -> Result<()> {
        let mut file = OpenOptions::new()
            .create(true)
            .append(true)
            .open(&self.path)
            .with_context(|| format!("opening log {}", self.path.display()))?;

        let line = serde_json::to_string(msg)
            .context("serializing message for log")?;
        writeln!(file, "{}", line)
            .context("writing to conversation log")?;
        Ok(())
    }

    /// Read the tail of the log (last `max_bytes` bytes).
    /// Seeks to `file_len - max_bytes`, skips the first partial line,
    /// then parses forward. For logs smaller than `max_bytes`, reads everything.
    pub fn read_tail(&self, max_bytes: u64) -> Result<Vec<Message>> {
        if !self.path.exists() {
            return Ok(Vec::new());
        }
        let file = File::open(&self.path)
            .with_context(|| format!("opening log {}", self.path.display()))?;
        let file_len = file.metadata()?.len();
        let mut reader = BufReader::new(file);

        if file_len > max_bytes {
            reader.seek(SeekFrom::Start(file_len - max_bytes))?;
            // Skip partial first line
            let mut discard = String::new();
            reader.read_line(&mut discard)?;
        }

        let mut messages = Vec::new();
        for line in reader.lines() {
            let line = line.context("reading log tail")?;
            let line = line.trim();
            if line.is_empty() {
                continue;
            }
            match serde_json::from_str::<Message>(line) {
                Ok(msg) => messages.push(msg),
                Err(_) => {} // skip corrupt/partial lines
            }
        }
        Ok(messages)
    }

    /// Count messages in the log without loading content.
    #[allow(dead_code)]
    pub fn message_count(&self) -> Result<usize> {
        if !self.path.exists() {
            return Ok(0);
        }
        let file = File::open(&self.path)
            .with_context(|| format!("opening log {}", self.path.display()))?;
        let reader = BufReader::new(file);
        Ok(reader.lines()
            .filter(|l| l.as_ref().map_or(false, |s| !s.trim().is_empty()))
            .count())
    }

    /// Read all messages from the log. Returns empty vec if log doesn't exist.
    /// NOTE: Don't use this in hot paths — use read_tail() instead.
    #[allow(dead_code)]
    pub fn read_all(&self) -> Result<Vec<Message>> {
        if !self.path.exists() {
            return Ok(Vec::new());
        }
        let file = File::open(&self.path)
            .with_context(|| format!("opening log {}", self.path.display()))?;
        let reader = BufReader::new(file);
        let mut messages = Vec::new();

        for (i, line) in reader.lines().enumerate() {
            let line = line.with_context(|| format!("reading log line {}", i))?;
            let line = line.trim();
            if line.is_empty() {
                continue;
            }
            match serde_json::from_str::<Message>(line) {
                Ok(msg) => messages.push(msg),
                Err(e) => {
                    // Log corruption — skip bad lines rather than failing
                    eprintln!("warning: skipping corrupt log line {}: {}", i, e);
                }
            }
        }
        Ok(messages)
    }

    pub fn path(&self) -> &Path {
        &self.path
    }
}