import Foundation

enum CommentDraftValidationError: LocalizedError {
    case emptySubreddit
    case invalidSubreddit
    case emptyPostTitle
    case emptyParentComment
    case emptyComment
    case commentTooLong
    case invalidPostURL

    var errorDescription: String? {
        switch self {
        case .emptySubreddit:
            "Enter a subreddit to tailor the comment."
        case .invalidSubreddit:
            "Subreddit names must be 3–21 characters and use only letters, numbers, or underscores."
        case .emptyPostTitle:
            "Enter the post title so the AI understands the thread context."
        case .emptyParentComment:
            "Paste the comment you're replying to."
        case .emptyComment:
            "Write or generate a comment before copying."
        case .commentTooLong:
            "Comments must be 10,000 characters or fewer."
        case .invalidPostURL:
            "Enter a valid Reddit post URL (https://reddit.com/…)."
        }
    }
}

enum CommentDraftValidator {
    static let maxCommentLength = 10_000

    static func validateForGeneration(from draft: CommentDraft) throws {
        guard !draft.subreddit.trimmingCharacters(in: .whitespaces).isEmpty else {
            throw CommentDraftValidationError.emptySubreddit
        }
        guard PostDraftValidator.isValidSubreddit(draft.subreddit) else {
            throw CommentDraftValidationError.invalidSubreddit
        }
        guard !draft.postTitle.trimmingCharacters(in: .whitespaces).isEmpty else {
            throw CommentDraftValidationError.emptyPostTitle
        }
        if draft.commentType == .reply {
            guard !draft.parentComment.trimmingCharacters(in: .whitespaces).isEmpty else {
                throw CommentDraftValidationError.emptyParentComment
            }
        }
    }

    static func validateForExport(_ draft: CommentDraft) throws {
        try validateForGeneration(from: draft)
        let body = draft.body.trimmingCharacters(in: .whitespaces)
        guard !body.isEmpty else { throw CommentDraftValidationError.emptyComment }
        guard body.count <= maxCommentLength else { throw CommentDraftValidationError.commentTooLong }

        let url = draft.postURL.trimmingCharacters(in: .whitespaces)
        if !url.isEmpty, !isValidRedditURL(url) {
            throw CommentDraftValidationError.invalidPostURL
        }
    }

    static func canGenerate(from draft: CommentDraft, isGenerating: Bool) -> Bool {
        guard !isGenerating else { return false }
        return (try? validateForGeneration(from: draft)) != nil
    }

    static func canExport(_ draft: CommentDraft) -> Bool {
        (try? validateForExport(draft)) != nil
    }

    static func isValidRedditURL(_ string: String) -> Bool {
        let trimmed = string.trimmingCharacters(in: .whitespaces)
        guard let url = URL(string: trimmed),
              let scheme = url.scheme?.lowercased(),
              scheme == "http" || scheme == "https",
              let host = url.host?.lowercased(),
              host.contains("reddit.com")
        else { return false }
        return true
    }
}