use std::collections::HashSet;
use std::sync::Arc;

use crate::Prompt;
use crate::compact::CompactionAnalyticsAttempt;
use crate::compact::InitialContextInjection;
use crate::compact::compaction_status_from_result;
use crate::compact::insert_initial_context_before_last_real_user_or_summary;
use crate::context_manager::ContextManager;
use crate::context_manager::TotalTokenUsageBreakdown;
use crate::context_manager::estimate_response_item_model_visible_bytes;
use crate::context_manager::is_codex_generated_item;
use crate::session::session::Session;
use crate::session::turn::built_tools;
use crate::session::turn_context::TurnContext;
use codex_analytics::CompactionImplementation;
use codex_analytics::CompactionPhase;
use codex_analytics::CompactionReason;
use codex_analytics::CompactionTrigger;
use codex_protocol::error::CodexErr;
use codex_protocol::error::Result as CodexResult;
use codex_protocol::items::ContextCompactionItem;
use codex_protocol::items::TurnItem;
use codex_protocol::models::BaseInstructions;
use codex_protocol::models::ResponseItem;
use codex_protocol::protocol::CompactedItem;
use codex_protocol::protocol::EventMsg;
use codex_protocol::protocol::TurnStartedEvent;
use futures::TryFutureExt;
use tokio_util::sync::CancellationToken;
use tracing::error;
use tracing::info;

pub(crate) async fn run_inline_remote_auto_compact_task(
    sess: Arc<Session>,
    turn_context: Arc<TurnContext>,
    initial_context_injection: InitialContextInjection,
    reason: CompactionReason,
    phase: CompactionPhase,
) -> CodexResult<()> {
    run_remote_compact_task_inner(
        &sess,
        &turn_context,
        initial_context_injection,
        CompactionTrigger::Auto,
        reason,
        phase,
    )
    .await?;
    Ok(())
}

pub(crate) async fn run_remote_compact_task(
    sess: Arc<Session>,
    turn_context: Arc<TurnContext>,
) -> CodexResult<()> {
    let start_event = EventMsg::TurnStarted(TurnStartedEvent {
        turn_id: turn_context.sub_id.clone(),
        started_at: turn_context.turn_timing_state.started_at_unix_secs().await,
        model_context_window: turn_context.model_context_window(),
        collaboration_mode_kind: turn_context.collaboration_mode.mode,
    });
    sess.send_event(&turn_context, start_event).await;

    run_remote_compact_task_inner(
        &sess,
        &turn_context,
        InitialContextInjection::DoNotInject,
        CompactionTrigger::Manual,
        CompactionReason::UserRequested,
        CompactionPhase::StandaloneTurn,
    )
    .await
}

async fn run_remote_compact_task_inner(
    sess: &Arc<Session>,
    turn_context: &Arc<TurnContext>,
    initial_context_injection: InitialContextInjection,
    trigger: CompactionTrigger,
    reason: CompactionReason,
    phase: CompactionPhase,
) -> CodexResult<()> {
    let attempt = CompactionAnalyticsAttempt::begin(
        sess.as_ref(),
        turn_context.as_ref(),
        trigger,
        reason,
        CompactionImplementation::ResponsesCompact,
        phase,
    )
    .await;
    let result =
        run_remote_compact_task_inner_impl(sess, turn_context, initial_context_injection).await;
    attempt
        .track(
            sess.as_ref(),
            compaction_status_from_result(&result),
            result.as_ref().err().map(ToString::to_string),
        )
        .await;
    if let Err(err) = result {
        let event = EventMsg::Error(
            err.to_error_event(Some("Error running remote compact task".to_string())),
        );
        sess.send_event(turn_context, event).await;
        return Err(err);
    }
    Ok(())
}

async fn run_remote_compact_task_inner_impl(
    sess: &Arc<Session>,
    turn_context: &Arc<TurnContext>,
    initial_context_injection: InitialContextInjection,
) -> CodexResult<()> {
    let compaction_item = TurnItem::ContextCompaction(ContextCompactionItem::new());
    sess.emit_turn_item_started(turn_context, &compaction_item)
        .await;
    let mut history = sess.clone_history().await;
    let base_instructions = sess.get_base_instructions().await;
    let deleted_items = trim_function_call_history_to_fit_context_window(
        &mut history,
        turn_context.as_ref(),
        &base_instructions,
    );
    if deleted_items > 0 {
        info!(
            turn_id = %turn_context.sub_id,
            deleted_items,
            "trimmed history items before remote compaction"
        );
    }
    // Required to keep `/undo` available after compaction
    let ghost_snapshots: Vec<ResponseItem> = history
        .raw_items()
        .iter()
        .filter(|item| matches!(item, ResponseItem::GhostSnapshot { .. }))
        .cloned()
        .collect();

    let prompt_input = history.for_prompt(&turn_context.model_info.input_modalities);
    let tool_router = built_tools(
        sess.as_ref(),
        turn_context.as_ref(),
        &prompt_input,
        &HashSet::new(),
        /*skills_outcome*/ None,
        &CancellationToken::new(),
    )
    .await?;
    let prompt = Prompt {
        input: prompt_input,
        tools: tool_router.model_visible_specs(),
        parallel_tool_calls: turn_context.model_info.supports_parallel_tool_calls,
        base_instructions,
        personality: turn_context.personality,
        output_schema: None,
    };

    let mut new_history = sess
        .services
        .model_client
        .compact_conversation_history(
            &prompt,
            &turn_context.model_info,
            turn_context.reasoning_effort,
            turn_context.reasoning_summary,
            &turn_context.session_telemetry,
        )
        .or_else(|err| async {
            let total_usage_breakdown = sess.get_total_token_usage_breakdown().await;
            let compact_request_log_data =
                build_compact_request_log_data(&prompt.input, &prompt.base_instructions.text);
            log_remote_compact_failure(
                turn_context,
                &compact_request_log_data,
                total_usage_breakdown,
                &err,
            );
            Err(err)
        })
        .await?;
    new_history = process_compacted_history(
        sess.as_ref(),
        turn_context.as_ref(),
        new_history,
        initial_context_injection,
    )
    .await;

    if !ghost_snapshots.is_empty() {
        new_history.extend(ghost_snapshots);
    }
    let reference_context_item = match initial_context_injection {
        InitialContextInjection::DoNotInject => None,
        InitialContextInjection::BeforeLastUserMessage => Some(turn_context.to_turn_context_item()),
    };
    let compacted_item = CompactedItem {
        message: String::new(),
        replacement_history: Some(new_history.clone()),
    };
    sess.replace_compacted_history(new_history, reference_context_item, compacted_item)
        .await;
    sess.recompute_token_usage(turn_context).await;

    sess.emit_turn_item_completed(turn_context, compaction_item)
        .await;
    Ok(())
}

pub(crate) async fn process_compacted_history(
    sess: &Session,
    turn_context: &TurnContext,
    mut compacted_history: Vec<ResponseItem>,
    initial_context_injection: InitialContextInjection,
) -> Vec<ResponseItem> {
    // Mid-turn compaction is the only path that must inject initial context above the last user
    // message in the replacement history. Pre-turn compaction instead injects context after the
    // compaction item, but mid-turn compaction keeps the compaction item last for model training.
    let initial_context = if matches!(
        initial_context_injection,
        InitialContextInjection::BeforeLastUserMessage
    ) {
        sess.build_initial_context(turn_context).await
    } else {
        Vec::new()
    };

    compacted_history.retain(should_keep_compacted_history_item);
    insert_initial_context_before_last_real_user_or_summary(compacted_history, initial_context)
}

/// Returns whether an item from remote compaction output should be preserved.
///
/// Called while processing the model-provided compacted transcript, before we
/// append fresh canonical context from the current session.
///
/// We drop:
/// - `developer` messages because remote output can include stale/duplicated
///   instruction content.
/// - non-user-content `user` messages (session prefix/instruction wrappers),
///   while preserving real user messages and persisted hook prompts.
///
/// This intentionally keeps:
/// - `assistant` messages (future remote compaction models may emit them)
/// - `user`-role warnings and compaction-generated summary messages because
///   they parse as `TurnItem::UserMessage`.
fn should_keep_compacted_history_item(item: &ResponseItem) -> bool {
    match item {
        ResponseItem::Message { role, .. } if role == "developer" => false,
        ResponseItem::Message { role, .. } if role == "user" => {
            matches!(
                crate::event_mapping::parse_turn_item(item),
                Some(TurnItem::UserMessage(_) | TurnItem::HookPrompt(_))
            )
        }
        ResponseItem::Message { role, .. } if role == "assistant" => true,
        ResponseItem::Message { .. } => false,
        ResponseItem::Compaction { .. } => true,
        ResponseItem::Reasoning { .. }
        | ResponseItem::LocalShellCall { .. }
        | ResponseItem::FunctionCall { .. }
        | ResponseItem::ToolSearchCall { .. }
        | ResponseItem::FunctionCallOutput { .. }
        | ResponseItem::ToolSearchOutput { .. }
        | ResponseItem::CustomToolCall { .. }
        | ResponseItem::CustomToolCallOutput { .. }
        | ResponseItem::WebSearchCall { .. }
        | ResponseItem::ImageGenerationCall { .. }
        | ResponseItem::GhostSnapshot { .. }
        | ResponseItem::Other => false,
    }
}

#[derive(Debug)]
struct CompactRequestLogData {
    failing_compaction_request_model_visible_bytes: i64,
}

fn build_compact_request_log_data(
    input: &[ResponseItem],
    instructions: &str,
) -> CompactRequestLogData {
    let failing_compaction_request_model_visible_bytes = input
        .iter()
        .map(estimate_response_item_model_visible_bytes)
        .fold(
            i64::try_from(instructions.len()).unwrap_or(i64::MAX),
            i64::saturating_add,
        );

    CompactRequestLogData {
        failing_compaction_request_model_visible_bytes,
    }
}

fn log_remote_compact_failure(
    turn_context: &TurnContext,
    log_data: &CompactRequestLogData,
    total_usage_breakdown: TotalTokenUsageBreakdown,
    err: &CodexErr,
) {
    error!(
        turn_id = %turn_context.sub_id,
        last_api_response_total_tokens = total_usage_breakdown.last_api_response_total_tokens,
        all_history_items_model_visible_bytes = total_usage_breakdown.all_history_items_model_visible_bytes,
        estimated_tokens_of_items_added_since_last_successful_api_response = total_usage_breakdown.estimated_tokens_of_items_added_since_last_successful_api_response,
        estimated_bytes_of_items_added_since_last_successful_api_response = total_usage_breakdown.estimated_bytes_of_items_added_since_last_successful_api_response,
        model_context_window_tokens = ?turn_context.model_context_window(),
        failing_compaction_request_model_visible_bytes = log_data.failing_compaction_request_model_visible_bytes,
        compact_error = %err,
        "remote compaction failed"
    );
}

fn trim_function_call_history_to_fit_context_window(
    history: &mut ContextManager,
    turn_context: &TurnContext,
    base_instructions: &BaseInstructions,
) -> usize {
    let mut deleted_items = 0usize;
    let Some(context_window) = turn_context.model_context_window() else {
        return deleted_items;
    };

    while history
        .estimate_token_count_with_base_instructions(base_instructions)
        .is_some_and(|estimated_tokens| estimated_tokens > context_window)
    {
        let Some(last_item) = history.raw_items().last() else {
            break;
        };
        if !is_codex_generated_item(last_item) {
            break;
        }
        if !history.remove_last_item() {
            break;
        }
        deleted_items += 1;
    }

    deleted_items
}