//
//  DigestCache.swift
//  Prisme
//
//  Author: Simon-Pierre Boucher <contact@spboucher.ai>
//

import CryptoKit
import Foundation

/// Content-addressed digest cache (CLAUDE.md §4: a page is never distilled
/// twice). Keys are hashes of the extracted text, so a reloaded page with
/// identical content is a hit even if the URL differs. Digests are durable
/// artefacts: kept in memory and on disk, ready to feed history and diffs.
actor DigestCache {
    private struct Entry: Codable {
        let digest: PageDigest
        let blocks: [ContentBlock]
    }

    private var memory: [String: Entry] = [:]
    private let directory: URL

    init() {
        let base = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask)[0]
        directory = base.appendingPathComponent("Digests", isDirectory: true)
        try? FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
    }

    static func key(for content: ExtractedContent) -> String {
        var hasher = SHA256()
        hasher.update(data: Data(content.title.utf8))
        for block in content.blocks {
            hasher.update(data: Data(block.text.utf8))
        }
        return hasher.finalize().map { String(format: "%02x", $0) }.joined()
    }

    func lookup(_ key: String) -> (PageDigest, [ContentBlock])? {
        if let entry = memory[key] {
            return (entry.digest, entry.blocks)
        }
        let file = directory.appendingPathComponent("\(key).json")
        guard let data = try? Data(contentsOf: file),
              let entry = try? JSONDecoder().decode(Entry.self, from: data)
        else { return nil }
        memory[key] = entry
        return (entry.digest, entry.blocks)
    }

    func store(_ digest: PageDigest, blocks: [ContentBlock], key: String) {
        let entry = Entry(digest: digest, blocks: blocks)
        memory[key] = entry
        if memory.count > 64 {
            // Cheap pressure valve; disk keeps the long tail.
            memory.removeValue(forKey: memory.keys.first!)
        }
        let file = directory.appendingPathComponent("\(key).json")
        if let data = try? JSONEncoder().encode(entry) {
            try? data.write(to: file, options: .atomic)
        }
    }
}