import Foundation

/// Modifies the header section of a `social.org` file.
///
/// All methods are pure string transformations — they return an updated copy
/// of the feed content without performing any network operations.
///
/// ```swift
/// var content = try await FeedFetcher().fetch(from: myFeedURL)
/// content = ProfileWriter().addFollow(url: friendURL, nick: "friend", to: content)
/// try await VFileUploader(tokenURL: tokenURL).upload(content: content)
/// ```
public struct ProfileWriter: Sendable {

    public init() {}

    // MARK: - Single-value keyword

    /// Sets or replaces a single-value header keyword (e.g. `NICK`, `DESCRIPTION`, `AVATAR`).
    ///
    /// - If the keyword already exists, its line is replaced in-place.
    /// - If it does not exist, a new line is inserted before `* Posts`.
    public func setKeyword(_ keyword: String, value: String, in content: String) -> String {
        var lines = content.components(separatedBy: "\n")
        let kwUpper = keyword.uppercased()
        let prefix = "#+\(kwUpper):"

        // Replace existing line
        var found = false
        for i in lines.indices {
            if lines[i].trimmingCharacters(in: .whitespaces).uppercased().hasPrefix(prefix) {
                lines[i] = "#+\(kwUpper): \(value)"
                found = true
                break
            }
        }
        if found { return lines.joined(separator: "\n") }

        // Insert before * Posts
        if let postsIdx = postsHeadingIndex(in: lines) {
            lines.insert("#+\(kwUpper): \(value)", at: postsIdx)
            return lines.joined(separator: "\n")
        }

        return content + "\n#+\(kwUpper): \(value)"
    }

    /// Removes a single-value header keyword line if present.
    public func removeKeyword(_ keyword: String, from content: String) -> String {
        let kwUpper = keyword.uppercased()
        let prefix = "#+\(kwUpper):"
        return content.components(separatedBy: "\n")
            .filter { !$0.trimmingCharacters(in: .whitespaces).uppercased().hasPrefix(prefix) }
            .joined(separator: "\n")
    }

    // MARK: - Multi-value keywords (LINK, CONTACT)

    /// Replaces all occurrences of a repeatable keyword with a new set of values.
    /// Pass an empty array to remove all lines for that keyword.
    public func setMultiKeyword(_ keyword: String, values: [String], in content: String) -> String {
        let kwUpper = keyword.uppercased()
        let prefix = "#+\(kwUpper):"
        var lines = content.components(separatedBy: "\n")

        // Remove all existing lines for this keyword
        let firstIdx = lines.firstIndex { $0.trimmingCharacters(in: .whitespaces).uppercased().hasPrefix(prefix) }
        lines.removeAll { $0.trimmingCharacters(in: .whitespaces).uppercased().hasPrefix(prefix) }

        let newLines = values.map { "#+\(kwUpper): \($0)" }
        if newLines.isEmpty { return lines.joined(separator: "\n") }

        let insertIdx = firstIdx ?? postsHeadingIndex(in: lines) ?? lines.count
        lines.insert(contentsOf: newLines, at: min(insertIdx, lines.count))
        return lines.joined(separator: "\n")
    }

    // MARK: - Follow management

    /// Adds a `#+FOLLOW:` entry for `url` with an optional nick.
    ///
    /// The new entry is inserted after the last existing `#+FOLLOW:` line,
    /// or before `* Posts` if none exist. Does nothing if already following.
    @discardableResult
    public func addFollow(url: URL, nick: String? = nil, to content: String) -> String {
        guard !isFollowing(url: url, in: content) else { return content }

        let entry: String
        if let nick, !nick.isEmpty {
            entry = "#+FOLLOW: \(nick) \(url.absoluteString)"
        } else {
            entry = "#+FOLLOW: \(url.absoluteString)"
        }

        var lines = content.components(separatedBy: "\n")

        if let lastFollowIdx = lines.lastIndex(where: {
            $0.trimmingCharacters(in: .whitespaces).uppercased().hasPrefix("#+FOLLOW:")
        }) {
            lines.insert(entry, at: lastFollowIdx + 1)
            return lines.joined(separator: "\n")
        }

        if let postsIdx = postsHeadingIndex(in: lines) {
            lines.insert(entry, at: postsIdx)
            return lines.joined(separator: "\n")
        }

        return content + "\n" + entry
    }

    /// Removes all `#+FOLLOW:` lines containing `url`.
    @discardableResult
    public func removeFollow(url: URL, from content: String) -> String {
        let urlStr = url.absoluteString
        return content.components(separatedBy: "\n")
            .filter { line in
                let t = line.trimmingCharacters(in: .whitespaces)
                guard t.uppercased().hasPrefix("#+FOLLOW:") else { return true }
                return !t.contains(urlStr)
            }
            .joined(separator: "\n")
    }

    /// Returns `true` if the content has a `#+FOLLOW:` line containing `url`.
    public func isFollowing(url: URL, in content: String) -> Bool {
        let urlStr = url.absoluteString
        return content.components(separatedBy: "\n").contains { line in
            let t = line.trimmingCharacters(in: .whitespaces)
            return t.uppercased().hasPrefix("#+FOLLOW:") && t.contains(urlStr)
        }
    }

    // MARK: - Helpers

    private func postsHeadingIndex(in lines: [String]) -> Int? {
        lines.firstIndex { $0.trimmingCharacters(in: .whitespaces) == "* Posts" }
    }
}