//! Reverse index: for each (collection, slug) store which entries reference it.
//! Updated on create/update/delete so we can answer "where is this entry referenced?".
//! Persisted as a single JSON file in the content directory (e.g. `content/_referrers.json`).

use std::collections::HashMap;
use std::path::Path;

use serde::{Deserialize, Serialize};

/// One referrer: an entry that points to another via a reference field.
#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, Hash)]
pub struct Referrer {
    pub collection: String,
    pub slug: String,
    pub field: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub locale: Option<String>,
}

fn key(collection: &str, slug: &str) -> String {
    format!("{}:{}", collection, slug)
}

/// In-memory index: (ref_collection, ref_slug) -> list of referrers.
/// Persisted as JSON file so it survives restarts.
#[derive(Debug, Default, Clone)]
pub struct ReferrerIndex {
    /// "collection:slug" -> list of referrers
    index: HashMap<String, Vec<Referrer>>,
}

impl ReferrerIndex {
    pub fn new() -> Self {
        Self::default()
    }

    /// Load index from JSON file. Missing or invalid file => empty index.
    pub fn load(path: &Path) -> Self {
        let data = match std::fs::read_to_string(path) {
            Ok(s) => s,
            _ => return Self::new(),
        };
        let index: HashMap<String, Vec<Referrer>> = match serde_json::from_str(&data) {
            Ok(m) => m,
            _ => return Self::new(),
        };
        Self { index }
    }

    /// Persist index to JSON file.
    pub fn save(&self, path: &Path) -> std::io::Result<()> {
        if let Some(parent) = path.parent() {
            std::fs::create_dir_all(parent)?;
        }
        let data = serde_json::to_string_pretty(&self.index)?;
        std::fs::write(path, data)
    }

    /// Remove this (referrer_collection, referrer_slug) from the referrer list of (ref_collection, ref_slug).
    pub fn remove_referrer(
        &mut self,
        ref_collection: &str,
        ref_slug: &str,
        referrer: &Referrer,
    ) {
        let k = key(ref_collection, ref_slug);
        if let Some(list) = self.index.get_mut(&k) {
            list.retain(|r| {
                r.collection != referrer.collection
                    || r.slug != referrer.slug
                    || r.field != referrer.field
                    || r.locale != referrer.locale
            });
            if list.is_empty() {
                self.index.remove(&k);
            }
        }
    }

    /// Add this referrer to the list for (ref_collection, ref_slug).
    pub fn add_referrer(
        &mut self,
        ref_collection: &str,
        ref_slug: &str,
        referrer: Referrer,
    ) {
        let k = key(ref_collection, ref_slug);
        self.index
            .entry(k)
            .or_default()
            .push(referrer);
    }

    /// Remove all referrers that are (referrer_collection, referrer_slug) from every key.
    /// Used when an entry is updated: clear its old refs, then add new refs.
    pub fn remove_all_referrers_from(&mut self, referrer_collection: &str, referrer_slug: &str) {
        let keys_to_check: Vec<String> = self.index.keys().cloned().collect();
        for k in keys_to_check {
            let empty = if let Some(list) = self.index.get_mut(&k) {
                list.retain(|r| r.collection != referrer_collection || r.slug != referrer_slug);
                list.is_empty()
            } else {
                false
            };
            if empty {
                self.index.remove(&k);
            }
        }
    }

    /// Get all referrers of (collection, slug).
    pub fn get_referrers(&self, collection: &str, slug: &str) -> Vec<Referrer> {
        self.index
            .get(&key(collection, slug))
            .cloned()
            .unwrap_or_default()
    }

    /// Remove the list for (ref_collection, ref_slug). Used when that entry is deleted.
    pub fn remove_referenced_entry(&mut self, ref_collection: &str, ref_slug: &str) {
        self.index.remove(&key(ref_collection, ref_slug));
    }
}