// Transcript JSONL parsing utilities.
//
// Provides mmap-based backward scanning of Claude Code transcript files
// and compaction detection. Used by memory-search (hook mode) and
// parse-claude-conversation (debug tool).

use memchr::memrchr3;
use memmap2::Mmap;
use serde_json::Value;
use std::fs;
use std::path::Path;

/// Scan backwards through mmap'd bytes, yielding byte slices of complete
/// top-level JSON objects (outermost { to matching }).
///
/// Uses memrchr3 (SIMD) to jump between structurally significant bytes
/// ({, }, ") instead of scanning byte-by-byte. Tracks brace depth,
/// skipping braces inside JSON strings. Returns objects in reverse order
/// (newest first).
pub struct JsonlBackwardIter<'a> {
    data: &'a [u8],
    pos: usize,
}

impl<'a> JsonlBackwardIter<'a> {
    pub fn new(data: &'a [u8]) -> Self {
        Self { data, pos: data.len() }
    }
}

impl<'a> Iterator for JsonlBackwardIter<'a> {
    type Item = &'a [u8];

    fn next(&mut self) -> Option<Self::Item> {
        // Find the closing } of the next object
        let close = loop {
            let p = memrchr3(b'{', b'}', b'"', &self.data[..self.pos])?;
            self.pos = p;
            if self.data[p] == b'}' {
                break p;
            }
            // Skip past any { or " that aren't our closing brace
        };

        // Track brace depth to find matching {
        let mut depth: usize = 1;
        let mut in_string = false;

        loop {
            let p = memrchr3(b'{', b'}', b'"', &self.data[..self.pos])?;
            self.pos = p;
            let ch = self.data[p];

            if in_string {
                if ch == b'"' {
                    // Check for escaped quote (count preceding backslashes)
                    let mut bs = 0;
                    while p > bs + 1 && self.data[p - 1 - bs] == b'\\' {
                        bs += 1;
                    }
                    if bs % 2 == 0 {
                        in_string = false;
                    }
                }
                // { and } inside strings don't affect depth
                continue;
            }

            match ch {
                b'"' => { in_string = true; }
                b'}' => { depth += 1; }
                b'{' => {
                    depth -= 1;
                    if depth == 0 {
                        return Some(&self.data[self.pos..=close]);
                    }
                }
                _ => {}
            }
        }
    }
}

/// Find the byte offset of the last compaction summary in mmap'd transcript data.
///
/// Scans backward for a user-type message whose content starts with
/// "This session is being continued". Returns the byte offset of the
/// JSON object's opening brace.
pub fn find_last_compaction(data: &[u8]) -> Option<usize> {
    let marker = b"This session is being continued";

    for obj_bytes in JsonlBackwardIter::new(data) {
        // Quick byte check before parsing
        if !contains_bytes(obj_bytes, marker) {
            continue;
        }

        let obj: Value = match serde_json::from_slice(obj_bytes) {
            Ok(v) => v,
            Err(_) => continue,
        };

        if obj.get("type").and_then(|v| v.as_str()) != Some("user") {
            continue;
        }

        if let Some(content) = obj.get("message")
            .and_then(|m| m.get("content"))
            .and_then(|c| c.as_str())
            && content.starts_with("This session is being continued") {
                let offset = obj_bytes.as_ptr() as usize - data.as_ptr() as usize;
                return Some(offset);
            }
    }

    None
}

/// Find the byte offset of the last compaction in a transcript file.
/// Returns None if the file can't be opened or has no compaction.
pub fn find_last_compaction_in_file(path: &str) -> Option<u64> {
    if path.is_empty() { return None; }

    let file = fs::File::open(path).ok()?;
    let meta = file.metadata().ok()?;
    if meta.len() == 0 { return None; }

    let mmap = unsafe { Mmap::map(&file).ok()? };
    find_last_compaction(&mmap).map(|off| off as u64)
}

/// Mmap a transcript file. Returns (Mmap, File) to keep both alive.
pub fn mmap_transcript(path: &str) -> Option<(Mmap, fs::File)> {
    let file = fs::File::open(path).ok()?;
    let meta = file.metadata().ok()?;
    if meta.len() == 0 { return None; }
    let mmap = unsafe { Mmap::map(&file).ok()? };
    Some((mmap, file))
}

fn contains_bytes(haystack: &[u8], needle: &[u8]) -> bool {
    haystack.windows(needle.len()).any(|w| w == needle)
}

/// Reverse-scan a transcript file, collecting user/assistant messages
/// until `max_tokens` tokens (~4 chars each) are accumulated. Stops at
/// the last compaction boundary. Returns messages in chronological order.
pub fn tail_messages(path: &str, max_bytes: usize) -> Vec<(String, String, String)> {
    let (mmap, _file) = match mmap_transcript(path) {
        Some(v) => v,
        None => return Vec::new(),
    };

    let mut messages: Vec<(String, String, String)> = Vec::new();
    let mut total_bytes = 0;

    for obj_bytes in JsonlBackwardIter::new(&mmap) {
        if total_bytes >= max_bytes { break; }

        // Quick byte check: skip objects that aren't user/assistant
        // (avoids parsing large tool_result / system objects)
        if !contains_bytes(obj_bytes, b"\"user\"")
            && !contains_bytes(obj_bytes, b"\"assistant\"") {
            continue;
        }

        let obj: Value = match serde_json::from_slice(obj_bytes) {
            Ok(v) => v,
            Err(_) => continue,
        };

        let msg_type = obj.get("type").and_then(|v| v.as_str()).unwrap_or("");
        if msg_type != "user" && msg_type != "assistant" { continue; }

        let msg = obj.get("message").unwrap_or(&obj);
        let text = match msg.get("content") {
            Some(Value::String(s)) => s.clone(),
            Some(Value::Array(arr)) => {
                arr.iter()
                    .filter(|b| b.get("type").and_then(|v| v.as_str()) == Some("text"))
                    .filter_map(|b| b.get("text").and_then(|v| v.as_str()))
                    .collect::<Vec<_>>()
                    .join(" ")
            }
            _ => continue,
        };
        if text.is_empty() { continue; }

        // Stop at compaction boundary
        if msg_type == "user" && text.starts_with("This session is being continued") {
            break;
        }

        let timestamp = obj.get("timestamp")
            .and_then(|v| v.as_str())
            .unwrap_or("")
            .to_string();

        total_bytes += text.len();
        messages.push((msg_type.to_string(), text, timestamp));
    }

    messages.reverse();
    messages
}

/// Get the timestamp of the compaction message at a given byte offset.
/// Returns a human-readable datetime string, or None if unavailable.
pub fn compaction_timestamp(path: &str, offset: u64) -> Option<String> {
    let (mmap, _file) = mmap_transcript(path)?;
    let start = offset as usize;
    if start >= mmap.len() { return None; }

    // Find the end of this JSONL line
    let end = mmap[start..].iter().position(|&b| b == b'\n')
        .map(|p| start + p)
        .unwrap_or(mmap.len());

    let obj: Value = serde_json::from_slice(&mmap[start..end]).ok()?;

    // Claude Code transcript entries have a "timestamp" field (ISO 8601)
    if let Some(ts) = obj.get("timestamp").and_then(|v| v.as_str()) {
        return Some(ts.to_string());
    }

    // Fallback: try "createdAt" or similar fields
    for field in &["createdAt", "created_at", "time"] {
        if let Some(ts) = obj.get(*field).and_then(|v| v.as_str()) {
            return Some(ts.to_string());
        }
    }

    None
}

/// Detect whether a compaction has occurred since the last check.
///
/// Compares the current compaction offset against a saved value in
/// `state_dir/compaction-{session_id}`. Returns true if a new
/// compaction was found. Updates the saved offset.
pub fn detect_new_compaction(
    state_dir: &Path,
    session_id: &str,
    transcript_path: &str,
) -> bool {
    let offset = find_last_compaction_in_file(transcript_path);

    let save_path = state_dir.join(format!("compaction-{}", session_id));
    let saved: Option<u64> = fs::read_to_string(&save_path)
        .ok()
        .and_then(|s| s.trim().parse().ok());

    let is_new = match (offset, saved) {
        (Some(cur), Some(prev)) => cur != prev,
        (Some(_), None) => true,
        _ => false,
    };

    // Save current offset
    if let Some(off) = offset {
        fs::write(&save_path, off.to_string()).ok();
    }

    is_new
}