Tusker/Tusker/Screens/Compose/ComposeContentWarningTextField.swift

//
//  ComposeContentWarningTextField.swift
//  Tusker
//
//  Created by Shadowfacts on 10/12/20.
//  Copyright © 2020 Shadowfacts. All rights reserved.
//

import SwiftUI

struct ComposeContentWarningTextField: UIViewRepresentable {
    typealias UIViewType = UITextField
    
    @Binding var text: String
    
    @EnvironmentObject private var uiState: ComposeUIState
    
    func makeUIView(context: Context) -> UITextField {
        let view = UITextField()
        
        view.placeholder = "Write your warning here"
        view.borderStyle = .roundedRect
        
        view.delegate = context.coordinator
        view.addTarget(context.coordinator, action: #selector(Coordinator.didChange(_:)), for: .editingChanged)
        
        context.coordinator.textField = view
        context.coordinator.uiState = uiState
        context.coordinator.text = $text
        
        return view
    }
    
    func updateUIView(_ uiView: UITextField, context: Context) {
        uiView.text = text
    }
    
    func makeCoordinator() -> Coordinator {
        return Coordinator()
    }
    
    class Coordinator: NSObject, UITextFieldDelegate, ComposeAutocompleteHandler {
        weak var textField: UITextField?
        var text: Binding<String>!
        var uiState: ComposeUIState!
        
        @objc func didChange(_ textField: UITextField) {
            text.wrappedValue = textField.text ?? ""
        }
        
        func textFieldDidBeginEditing(_ textField: UITextField) {
            uiState.autocompleteHandler = self
            updateAutocompleteState(textField: textField)
        }
        
        func textFieldDidEndEditing(_ textField: UITextField) {
            updateAutocompleteState(textField: textField)
        }
        
        func textFieldDidChangeSelection(_ textField: UITextField) {
            // Update text binding before potentially triggering SwiftUI view update.
            // See comment in MainComposeTextView.Coordinator.textViewDidChangeSelection
            text.wrappedValue = textField.text ?? ""
            
            updateAutocompleteState(textField: textField)
        }
        
        func autocomplete(with string: String) {
            guard let textField = textField,
                  let text = textField.text,
                  let selectedRange = textField.selectedTextRange,
                  let lastWordStartIndex = findAutocompleteLastWord(textField: textField) else {
                return
            }
            
            let distanceToEnd = textField.offset(from: selectedRange.start, to: textField.endOfDocument)
            
            let selectedRangeStartUTF16 = textField.offset(from: textField.beginningOfDocument, to: selectedRange.start)
            let characterBeforeCursorIndex = text.utf16.index(text.startIndex, offsetBy: selectedRangeStartUTF16)
            
            let insertSpace: Bool
            if distanceToEnd > 0 {
                let charAfterCursor = text[characterBeforeCursorIndex]
                insertSpace = charAfterCursor != " " && charAfterCursor != "\n"
            } else {
                insertSpace = true
            }
            let string = insertSpace ? string + " " : string
            
            textField.text!.replaceSubrange(lastWordStartIndex..<characterBeforeCursorIndex, with: string)
            self.didChange(textField)
            self.updateAutocompleteState(textField: textField)
            
            // keep the cursor at the same position in the text, immediately after what was inserted
            // if we inserted a space, move the cursor 1 farther so it's immediately after the pre-existing space
            let insertSpaceOffset = insertSpace ? 0 : 1
            let newCursorPosition = textField.position(from: textField.endOfDocument, offset: -distanceToEnd + insertSpaceOffset)!
            textField.selectedTextRange = textField.textRange(from: newCursorPosition, to: newCursorPosition)
        }
        
        private func updateAutocompleteState(textField: UITextField) {
            guard let selectedRange = textField.selectedTextRange,
                  let text = textField.text,
                  let lastWordStartIndex = findAutocompleteLastWord(textField: textField) else {
                uiState.autocompleteState = nil
                return
            }
            
            if lastWordStartIndex > text.startIndex {
                let c = text[text.index(before: lastWordStartIndex)]
                if isPermittedForAutocomplete(c) || c == ":" {
                    uiState.autocompleteState = nil
                    return
                }
            }
            
            let selectedRangeStartUTF16 = textField.offset(from: textField.beginningOfDocument, to: selectedRange.start)
            let cursorIndex = text.utf16.index(text.startIndex, offsetBy: selectedRangeStartUTF16)

            if lastWordStartIndex >= text.startIndex {
                let lastWord = text[lastWordStartIndex..<cursorIndex]
                let exceptFirst = lastWord[lastWord.index(after: lastWord.startIndex)...]
                
                if lastWord.first == ":" {
                    uiState.autocompleteState = .emoji(String(exceptFirst))
                } else {
                    uiState.autocompleteState = nil
                }
            } else {
                uiState.autocompleteState = nil
            }
        }
        
        private func isPermittedForAutocomplete(_ c: Character) -> Bool {
            return (c >= "a" && c <= "z") || (c >= "A" && c <= "Z") || (c >= "0" && c <= "9") || c == "_"
        }
        
        private func findAutocompleteLastWord(textField: UITextField) -> String.Index? {
            guard textField.isFirstResponder,
                  let selectedRange = textField.selectedTextRange,
                  selectedRange.isEmpty,
                  let text = textField.text,
                  !text.isEmpty else {
                return nil
            }
            
            let selectedRangeStartUTF16 = textField.offset(from: textField.beginningOfDocument, to: selectedRange.start)
            let cursorIndex = text.utf16.index(text.startIndex, offsetBy: selectedRangeStartUTF16)
            
            var lastWordStartIndex = text.index(before: cursorIndex)
            while true {
                let c = text[lastWordStartIndex]
                
                if !isPermittedForAutocomplete(c) {
                    break
                }
                
                if lastWordStartIndex > text.startIndex {
                    lastWordStartIndex = text.index(before: lastWordStartIndex)
                } else {
                    break
                }
            }
            
            return lastWordStartIndex
        }
    }
    
}