codex/codex-rs/core/src/thread_rollout_truncation.rs

//! Helpers for truncating rollouts based on "user turn" boundaries.
//!
//! In core, "user turns" are detected by scanning `ResponseItem::Message` items and
//! interpreting them via `event_mapping::parse_turn_item(...)`.

use crate::context_manager::is_user_turn_boundary;
use crate::event_mapping;
use codex_protocol::items::TurnItem;
use codex_protocol::models::ResponseItem;
use codex_protocol::protocol::EventMsg;
use codex_protocol::protocol::InitialHistory;
use codex_protocol::protocol::InterAgentCommunication;
use codex_protocol::protocol::RolloutItem;

pub(crate) fn initial_history_has_prior_user_turns(conversation_history: &InitialHistory) -> bool {
    conversation_history.scan_rollout_items(rollout_item_is_user_turn_boundary)
}

fn rollout_item_is_user_turn_boundary(item: &RolloutItem) -> bool {
    match item {
        RolloutItem::ResponseItem(item) => is_user_turn_boundary(item),
        RolloutItem::InterAgentCommunication(_) => true,
        _ => false,
    }
}

/// Return the indices of user message boundaries in a rollout.
///
/// A user message boundary is a `RolloutItem::ResponseItem(ResponseItem::Message { .. })`
/// whose parsed turn item is `TurnItem::UserMessage`.
///
/// Rollouts can contain `ThreadRolledBack` markers. Those markers indicate that the
/// last N user turns were removed from the effective thread history; we apply them here so
/// indexing uses the post-rollback history rather than the raw stream.
pub(crate) fn user_message_positions_in_rollout(items: &[RolloutItem]) -> Vec<usize> {
    let mut user_positions = Vec::new();
    for (idx, item) in items.iter().enumerate() {
        match item {
            RolloutItem::ResponseItem(item @ ResponseItem::Message { .. })
                if matches!(
                    event_mapping::parse_turn_item(item),
                    Some(TurnItem::UserMessage(_))
                ) =>
            {
                user_positions.push(idx);
            }
            RolloutItem::EventMsg(EventMsg::ThreadRolledBack(rollback)) => {
                let num_turns = usize::try_from(rollback.num_turns).unwrap_or(usize::MAX);
                let new_len = user_positions.len().saturating_sub(num_turns);
                user_positions.truncate(new_len);
            }
            _ => {}
        }
    }
    user_positions
}

/// Return the indices of fork-turn boundaries in a rollout.
///
/// A fork-turn boundary is either:
/// - a real user message boundary, or
/// - an inter-agent communication whose `trigger_turn` is `true`, or
/// - a legacy assistant inter-agent envelope with the same flag.
///
/// Like `user_message_positions_in_rollout`, this applies `ThreadRolledBack` markers so indexing
/// reflects the effective post-rollback history. Rollback counts instruction turns, so a rollback
/// removes the stale suffix starting at the earliest rolled-back instruction-turn boundary instead
/// of simply truncating the mixed fork-boundary list.
pub(crate) fn fork_turn_positions_in_rollout(items: &[RolloutItem]) -> Vec<usize> {
    let mut rollback_turn_positions = Vec::new();
    let mut fork_turn_positions = Vec::new();
    for (idx, item) in items.iter().enumerate() {
        match item {
            RolloutItem::ResponseItem(item) => {
                if is_user_turn_boundary(item) {
                    rollback_turn_positions.push(idx);
                }
                if is_real_user_message_boundary(item) || is_trigger_turn_boundary(item) {
                    fork_turn_positions.push(idx);
                }
            }
            RolloutItem::InterAgentCommunication(communication) => {
                rollback_turn_positions.push(idx);
                if communication.trigger_turn {
                    fork_turn_positions.push(idx);
                }
            }
            RolloutItem::EventMsg(EventMsg::ThreadRolledBack(rollback)) => {
                let num_turns = usize::try_from(rollback.num_turns).unwrap_or(usize::MAX);
                if num_turns == 0 {
                    continue;
                }
                let Some(rollback_start_idx) = rollback_turn_positions
                    .len()
                    .checked_sub(num_turns)
                    .map(|rollback_start| rollback_turn_positions[rollback_start])
                    .or_else(|| rollback_turn_positions.first().copied())
                else {
                    continue;
                };
                let new_rollback_len = rollback_turn_positions.len().saturating_sub(num_turns);
                rollback_turn_positions.truncate(new_rollback_len);
                fork_turn_positions.retain(|position| *position < rollback_start_idx);
            }
            _ => {}
        }
    }
    fork_turn_positions
}

/// Return a prefix of `items` obtained by cutting strictly before the nth user message.
///
/// The boundary index is 0-based from the start of `items` (so `n_from_start = 0` returns
/// a prefix that excludes the first user message and everything after it).
///
/// If `n_from_start` is `usize::MAX`, this returns the full rollout (no truncation).
/// If fewer than or equal to `n_from_start` user messages exist, this returns the full
/// rollout unchanged.
pub(crate) fn truncate_rollout_before_nth_user_message_from_start(
    items: &[RolloutItem],
    n_from_start: usize,
) -> Vec<RolloutItem> {
    if n_from_start == usize::MAX {
        return items.to_vec();
    }

    let user_positions = user_message_positions_in_rollout(items);

    // If fewer than or equal to n user messages exist, keep the full rollout.
    if user_positions.len() <= n_from_start {
        return items.to_vec();
    }

    // Cut strictly before the nth user message (do not keep the nth itself).
    let cut_idx = user_positions[n_from_start];
    items[..cut_idx].to_vec()
}

/// Return a suffix of `items` that keeps the last `n_from_end` fork turns.
///
/// If fewer than or equal to `n_from_end` fork turns exist, this keeps from the first fork-turn
/// boundary and still drops pre-turn startup context.
pub(crate) fn truncate_rollout_to_last_n_fork_turns(
    items: &[RolloutItem],
    n_from_end: usize,
) -> Vec<RolloutItem> {
    if n_from_end == 0 {
        return Vec::new();
    }

    let fork_turn_positions = fork_turn_positions_in_rollout(items);
    let Some(keep_idx) = fork_turn_positions
        .len()
        .checked_sub(n_from_end)
        .map(|position| fork_turn_positions[position])
        .or_else(|| fork_turn_positions.first().copied())
    else {
        return Vec::new();
    };
    items[keep_idx..].to_vec()
}

fn is_real_user_message_boundary(item: &ResponseItem) -> bool {
    matches!(
        event_mapping::parse_turn_item(item),
        Some(TurnItem::UserMessage(_))
    )
}

fn is_trigger_turn_boundary(item: &ResponseItem) -> bool {
    let ResponseItem::Message { role, content, .. } = item else {
        return false;
    };

    role == "assistant"
        && InterAgentCommunication::from_message_content(content)
            .is_some_and(|communication| communication.trigger_turn)
}

#[cfg(test)]
#[path = "thread_rollout_truncation_tests.rs"]
mod tests;