//! Report builder: aggregates per-group metrics from signal data.
//!
//! Scans `UserSignalIndex` for each user in the experiment to compute
//! CTR, completion rate, and return rate per group. The builder is
//! read-only -- it does not write to any storage.

use std::sync::Arc;

use crate::entities::UserSignalIndex;
use crate::signals::SignalLedger;

use super::{
    ExperimentConfig, ExperimentGroup, ExperimentReport, GroupMetrics, MetricLift, assign_group,
};

// ── ReportBuilder ────────────────────────────────────────────────────────────

/// Builds an `ExperimentReport` by scanning signal data for a set of users.
pub struct ReportBuilder<'a> {
    ledger: &'a Arc<SignalLedger>,
    user_signal_index: &'a UserSignalIndex,
}

impl<'a> ReportBuilder<'a> {
    /// Create a new report builder with references to signal infrastructure.
    #[must_use]
    pub const fn new(
        ledger: &'a Arc<SignalLedger>,
        user_signal_index: &'a UserSignalIndex,
    ) -> Self {
        Self {
            ledger,
            user_signal_index,
        }
    }

    /// Build the experiment report for the given user population.
    ///
    /// Each user in `user_ids` is assigned to treatment or control via
    /// `assign_group`. Metrics are aggregated per group from signal data.
    ///
    /// # Errors
    ///
    /// Returns `TidalError::InvalidInput` if the config fails validation.
    pub fn build(
        &self,
        config: &ExperimentConfig,
        user_ids: &[u64],
    ) -> crate::Result<ExperimentReport> {
        config.validate()?;

        // Partition users into groups.
        let mut treatment_users: Vec<u64> = Vec::new();
        let mut control_users: Vec<u64> = Vec::new();

        for &uid in user_ids {
            match assign_group(uid, &config.experiment_id, config.treatment_fraction) {
                ExperimentGroup::Treatment => treatment_users.push(uid),
                ExperimentGroup::Control => control_users.push(uid),
            }
        }

        // Aggregate metrics per group.
        let treatment_metrics = self.aggregate_group_metrics(&treatment_users, config);
        let control_metrics = self.aggregate_group_metrics(&control_users, config);

        let lift = MetricLift::compute(&treatment_metrics, &control_metrics);

        Ok(ExperimentReport {
            experiment_id: config.experiment_id.clone(),
            treatment_users: treatment_users.len(),
            control_users: control_users.len(),
            treatment_metrics,
            control_metrics,
            lift,
        })
    }

    /// Aggregate engagement metrics for a group of users.
    ///
    /// Uses `UserSignalIndex::user_signal_count` to sum signal counts per user
    /// across all entities, and `user_activity_split` for return rate.
    fn aggregate_group_metrics(&self, user_ids: &[u64], config: &ExperimentConfig) -> GroupMetrics {
        let mut total_views: u64 = 0;
        let mut total_clicks: u64 = 0;
        let mut total_completions: u64 = 0;
        let mut returned_users: u64 = 0;
        let mut eligible_for_return: u64 = 0;

        // Resolve signal type IDs once.
        let view_type_id = self.ledger.resolve_signal_type("view").ok();
        let click_type_ids: Vec<_> = config
            .click_signals
            .iter()
            .filter_map(|s| self.ledger.resolve_signal_type(s).ok())
            .collect();
        let completion_type_ids: Vec<_> = config
            .completion_signals
            .iter()
            .filter_map(|s| self.ledger.resolve_signal_type(s).ok())
            .collect();

        let now_ns = crate::schema::Timestamp::now().as_nanos();

        for &user_id in user_ids {
            // Sum views for this user across all entities.
            if let Some(view_tid) = view_type_id {
                total_views += self
                    .user_signal_index
                    .user_signal_count(user_id, view_tid, now_ns);
            }

            // Sum clicks for this user.
            for &click_tid in &click_type_ids {
                total_clicks += self
                    .user_signal_index
                    .user_signal_count(user_id, click_tid, now_ns);
            }

            // Sum completions for this user.
            for &comp_tid in &completion_type_ids {
                total_completions += self
                    .user_signal_index
                    .user_signal_count(user_id, comp_tid, now_ns);
            }

            // Return rate: user has old activity AND recent activity.
            let (has_old, has_recent) = self.user_signal_index.user_activity_split(user_id, now_ns);
            if has_old {
                eligible_for_return += 1;
                if has_recent {
                    returned_users += 1;
                }
            }
        }

        #[allow(clippy::cast_precision_loss)]
        let ctr = if total_views > 0 {
            total_clicks as f64 / total_views as f64
        } else {
            0.0
        };

        #[allow(clippy::cast_precision_loss)]
        let completion_rate = if total_views > 0 {
            total_completions as f64 / total_views as f64
        } else {
            0.0
        };

        #[allow(clippy::cast_precision_loss)]
        let return_rate = if eligible_for_return > 0 {
            returned_users as f64 / eligible_for_return as f64
        } else {
            0.0
        };

        GroupMetrics {
            ctr,
            completion_rate,
            return_rate,
            total_views,
            total_clicks,
            total_completions,
        }
    }
}