import Foundation
#if os(macOS)
import AppKit
#else
import UIKit
#endif

enum DocumentInputRules {
    struct Plan {
        let kind: String
        let level: Int
        let start: Int
        let paragraphLocation: Int
        let removeLength: Int
        let prefix: String
        let priorBlockType: String
        let priorAttrs: [String: String]
        let priorParagraphStyle: NSParagraphStyle
    }
    /// Matches fixtures/autoformat.mjs against at most ProseMirror's 500-character
    /// input-rule context. This describes a transform; it never changes storage.
    static func plan(in attributed: NSAttributedString, range: NSRange, replacement: String) -> Plan? {
        guard range.length == 0, range.location >= 0, range.location <= attributed.length,
              replacement.unicodeScalars.count == 1,
              let scalar = replacement.unicodeScalars.first,
              CharacterSet.whitespaces.contains(scalar),
              !CharacterSet.newlines.contains(scalar) else { return nil }
        let source = attributed.string as NSString
        var beginning = range.location
        while beginning > 0 && source.character(at: beginning - 1) != 10 {
            guard range.location - beginning < 500 else { return nil }
            beginning -= 1
        }
        let existing = source.substring(with: NSRange(location: beginning, length: range.location - beginning))
        guard !existing.contains("\u{2028}"), !existing.contains("\r") else { return nil }
        let prefix = existing + replacement
        let attributes = beginning < attributed.length ? attributed.attributes(at: beginning, effectiveRange: nil) : [:]
        let priorType = attributes[.docsBlockType] as? String ?? "paragraph"
        guard priorType != "code_block" else { return nil }
        var kind: String
        var level = 0
        if prefix.range(of: #"^\s*[-+*]\s\z"#, options: .regularExpression) != nil { kind = "bullet_list" }
        else if prefix.range(of: #"^[0-9]+\.\s\z"#, options: .regularExpression) != nil { kind = "ordered_list" }
        else if prefix.range(of: #"^\s*>\s\z"#, options: .regularExpression) != nil { kind = "blockquote" }
        else if prefix.range(of: #"^#{1,3}\s\z"#, options: .regularExpression) != nil {
            kind = "heading"
            level = existing.count
        } else { return nil }
        let priorAttrs = (attributes[.docsBlockAttributes] as? Data).flatMap {
            try? JSONDecoder().decode([String: String].self, from: $0)
        } ?? [:]
        let style = (attributes[.paragraphStyle] as? NSParagraphStyle)?.copy() as? NSParagraphStyle ?? NSParagraphStyle.default
        // The web input rule intentionally starts at 1 regardless of typed digits.
        return Plan(kind: kind, level: level, start: 1, paragraphLocation: beginning,
                    removeLength: range.location - beginning, prefix: prefix,
                    priorBlockType: priorType, priorAttrs: priorAttrs, priorParagraphStyle: style)
    }
}