// Copyright (c) 2026 Storm Dragon
// Authors: Storm Dragon, Angus Kola
// SPDX-License-Identifier: GPL-3.0-or-later

// Package interaction implements focus-free controls. It owns no keyboard,
// accessibility objects, output services or callbacks; the reader owns those.
package interaction

import (
	"errors"
	"fmt"
	"strings"
	"unicode"
	"unicode/utf8"
)

type Kind uint8

const (
	List Kind = iota + 1
	Entry
	CheckBox
	Button
)

type Item struct {
	ID       string `json:"id"`
	Label    string `json:"label"`
	Disabled bool   `json:"disabled,omitempty"`
}
type Element struct {
	ID        string `json:"id"`
	Label     string `json:"label"`
	Kind      Kind   `json:"kind"`
	Items     []Item `json:"items,omitempty"`
	Text      string `json:"text,omitempty"`
	Checked   bool   `json:"checked,omitempty"`
	Disabled  bool   `json:"disabled,omitempty"`
	Required  bool   `json:"required,omitempty"`
	MaxLength int    `json:"max_length,omitempty"`
}
type Form struct {
	Title    string    `json:"title"`
	Elements []Element `json:"elements"`
}
type Key uint8

const (
	Text Key = iota
	Up
	Down
	Left
	Right
	Home
	End
	PageUp
	PageDown
	Tab
	Enter
	Escape
	Backspace
	Delete
)

type Event struct {
	Key                 Key
	Text                string
	Shift, Control, Alt bool
}
type Presentation struct {
	Speech, Braille string
	Cursor          int
}

// Effect describes a host operation. Activation does not close the form: the
// caller can validate, replace it with another form, or finish the interaction.
type Effect struct {
	Presentation
	Activated        string
	Cancelled, Paste bool
	Copy             *string
}
type Value struct {
	Text     string `json:"text,omitempty"`
	Selected string `json:"selected,omitempty"`
	Checked  bool   `json:"checked,omitempty"`
}
type elementState struct {
	element                 Element
	selected, caret, anchor int
	prefix                  string
}
type Session struct {
	title    string
	elements []elementState
	focus    int
	closed   bool
}

const maxTextLength = 8192

func NewSession(form Form) (*Session, error) {
	if form.Title == "" || len(form.Title) > 1024 || len(form.Elements) == 0 || len(form.Elements) > 64 {
		return nil, errors.New("a titled form with 1 to 64 controls is required")
	}
	session := &Session{title: form.Title, focus: -1}
	ids := map[string]bool{}
	for _, element := range form.Elements {
		if element.ID == "" || len(element.ID) > 1024 || element.Label == "" || len(element.Label) > 1024 || ids[element.ID] || element.Kind < List || element.Kind > Button {
			return nil, errors.New("controls need unique IDs, labels and supported kinds")
		}
		ids[element.ID] = true
		if element.MaxLength == 0 {
			element.MaxLength = maxTextLength
		}
		if element.MaxLength < 1 || element.MaxLength > maxTextLength || len([]rune(element.Text)) > element.MaxLength {
			return nil, errors.New("invalid entry length")
		}
		if !utf8.ValidString(element.Text) || strings.ContainsAny(element.Text, "\r\n\x00") {
			return nil, errors.New("entries must contain single-line text")
		}
		for _, character := range element.Text {
			if unicode.IsControl(character) {
				return nil, errors.New("entries must contain single-line text")
			}
		}
		element.Items = append([]Item(nil), element.Items...)
		state := elementState{element: element, selected: -1, caret: len([]rune(element.Text)), anchor: len([]rune(element.Text))}
		itemIDs := map[string]bool{}
		if len(element.Items) > 256 {
			return nil, errors.New("too many list items")
		}
		for index, item := range element.Items {
			if item.ID == "" || len(item.ID) > 1024 || item.Label == "" || len(item.Label) > 1024 || itemIDs[item.ID] {
				return nil, errors.New("list items need unique IDs and labels")
			}
			itemIDs[item.ID] = true
			if state.selected < 0 && !item.Disabled {
				state.selected = index
			}
		}
		if element.Kind == List && state.selected < 0 {
			state.element.Disabled = true
		}
		session.elements = append(session.elements, state)
		if session.focus < 0 && !state.element.Disabled {
			session.focus = len(session.elements) - 1
		}
	}
	if session.focus < 0 {
		return nil, errors.New("form has no enabled controls")
	}
	return session, nil
}

func (session *Session) Presentation() Presentation {
	if session.closed {
		return Presentation{}
	}
	state := &session.elements[session.focus]
	element := state.element
	text := element.Label
	cursor := -1
	switch element.Kind {
	case List:
		text += fmt.Sprintf(": %s, %d of %d", element.Items[state.selected].Label, state.selected+1, len(element.Items))
	case Entry:
		text += ", edit: " + element.Text
		cursor = len([]rune(element.Label+", edit: ")) + state.caret
		if state.caret != state.anchor {
			text += fmt.Sprintf(" (selected %d characters)", abs(state.caret-state.anchor))
		}
	case CheckBox:
		if element.Checked {
			text += ", checked"
		} else {
			text += ", unchecked"
		}
	case Button:
		text += ", button"
	}
	speech := text
	if element.Kind == List {
		speech = element.Items[state.selected].Label
	}
	return Presentation{Speech: speech, Braille: text, Cursor: cursor}
}

func (session *Session) Opening() Presentation {
	output := session.Presentation()
	if !session.closed && len(session.elements) > 1 && session.elements[session.focus].element.Kind == List {
		output.Speech = session.elements[session.focus].element.Label + ". " + output.Speech
	}
	output.Speech = session.title + ". " + output.Speech
	return output
}

func (session *Session) Values() map[string]Value {
	values := make(map[string]Value, len(session.elements))
	for _, state := range session.elements {
		value := Value{Text: state.element.Text, Checked: state.element.Checked}
		if state.element.Kind == List && state.selected >= 0 {
			value.Selected = state.element.Items[state.selected].ID
		}
		values[state.element.ID] = value
	}
	return values
}

func (session *Session) Handle(event Event) Effect {
	if session.closed {
		return Effect{}
	}
	if event.Key == Escape {
		session.closed = true
		return Effect{Cancelled: true}
	}
	if event.Key == Tab {
		step := 1
		if event.Shift {
			step = -1
		}
		for range len(session.elements) {
			session.focus = (session.focus + step + len(session.elements)) % len(session.elements)
			if !session.elements[session.focus].element.Disabled {
				break
			}
		}
		output := session.Presentation()
		// A list in a multi-control form needs its field label when Tab enters
		// it. A menu's only control uses its opening title instead.
		state := &session.elements[session.focus]
		if len(session.elements) > 1 && state.element.Kind == List {
			output.Speech = state.element.Label + ". " + output.Speech
		}
		return Effect{Presentation: output}
	}
	state := &session.elements[session.focus]
	if event.Alt {
		return Effect{}
	}
	if event.Key == Enter || state.element.Kind == Button && event.Key == Text && event.Text == " " && !event.Control {
		for _, candidate := range session.elements {
			if candidate.element.Kind == Entry && candidate.element.Required && !candidate.element.Disabled && strings.TrimSpace(candidate.element.Text) == "" {
				return Effect{Presentation: Presentation{Speech: candidate.element.Label + " is required", Braille: candidate.element.Label + " is required", Cursor: -1}}
			}
		}
		id := state.element.ID
		if state.element.Kind == List {
			id = state.element.Items[state.selected].ID
		}
		return Effect{Activated: id}
	}
	switch state.element.Kind {
	case List:
		step := 0
		switch event.Key {
		case Up:
			step = -1
		case Down:
			step = 1
		case PageUp:
			step = -10
		case PageDown:
			step = 10
		case Home:
			state.selected = 0
			step = 1
		case End:
			state.selected = len(state.element.Items) - 1
			step = -1
		case Text:
			if event.Control || event.Text == "" {
				return Effect{}
			}
			prefix := state.prefix + strings.ToLower(event.Text)
			if !state.find_prefix(prefix) {
				prefix = strings.ToLower(event.Text)
				state.find_prefix(prefix)
			}
			state.prefix = prefix
			return Effect{Presentation: session.Presentation()}
		default:
			return Effect{}
		}
		state.prefix = ""
		if event.Key == Home || event.Key == End {
			for state.element.Items[state.selected].Disabled {
				state.selected += step
			}
		} else {
			for count := 0; count < abs(step); count++ {
				previous := state.selected
				for next := previous + sign(step); next >= 0 && next < len(state.element.Items); next += sign(step) {
					if !state.element.Items[next].Disabled {
						state.selected = next
						break
					}
				}
			}
		}
	case CheckBox:
		if event.Key != Text || event.Text != " " || event.Control {
			return Effect{}
		}
		state.element.Checked = !state.element.Checked
	case Button:
		return Effect{}
	case Entry:
		return session.edit(event)
	}
	return Effect{Presentation: session.Presentation()}
}

func (state *elementState) find_prefix(prefix string) bool {
	for offset := 1; offset <= len(state.element.Items); offset++ {
		index := (state.selected + offset) % len(state.element.Items)
		item := state.element.Items[index]
		if !item.Disabled && strings.HasPrefix(strings.ToLower(item.Label), prefix) {
			state.selected = index
			return true
		}
	}
	return false
}
func sign(value int) int {
	if value < 0 {
		return -1
	}
	return 1
}
func abs(value int) int {
	if value < 0 {
		return -value
	}
	return value
}

func (session *Session) edit(event Event) Effect {
	state := &session.elements[session.focus]
	characters := []rune(state.element.Text)
	start, end := min(state.caret, state.anchor), max(state.caret, state.anchor)
	if event.Control && event.Key == Text {
		switch strings.ToLower(event.Text) {
		case "a":
			state.anchor, state.caret = 0, len(characters)
			output := session.Presentation()
			output.Speech = "Selected all"
			return Effect{Presentation: output}
		case "v":
			return Effect{Paste: true}
		case "c", "x":
			if start == end {
				return Effect{}
			}
			text := string(characters[start:end])
			// Cutting is completed by the host only after clipboard write succeeds.
			return Effect{Copy: &text}
		default:
			return Effect{}
		}
	}
	if event.Key == Left || event.Key == Right || event.Key == Home || event.Key == End {
		next := state.caret
		switch event.Key {
		case Left:
			if !event.Shift && start != end {
				next = start
			} else {
				next = max(0, next-1)
				if event.Control {
					for next > 0 && unicode.IsSpace(characters[next]) {
						next--
					}
					for next > 0 && !unicode.IsSpace(characters[next-1]) {
						next--
					}
				}
			}
		case Right:
			if !event.Shift && start != end {
				next = end
			} else {
				next = min(len(characters), next+1)
				if event.Control {
					for next < len(characters) && !unicode.IsSpace(characters[next]) {
						next++
					}
					for next < len(characters) && unicode.IsSpace(characters[next]) {
						next++
					}
				}
			}
		case Home:
			next = 0
		case End:
			next = len(characters)
		}
		state.caret = next
		if !event.Shift {
			state.anchor = next
		}
		output := session.Presentation()
		if next == len(characters) {
			output.Speech = "End of text"
		} else {
			output.Speech = string(characters[next])
		}
		if event.Shift {
			output.Speech += ", selected"
		}
		return Effect{Presentation: output}
	}
	if event.Key == Backspace || event.Key == Delete {
		if start == end {
			if event.Key == Backspace {
				start = max(0, start-1)
			} else {
				end = min(len(characters), end+1)
			}
		}
		if start == end {
			return Effect{}
		}
		removed := string(characters[start:end])
		state.element.Text = string(append(characters[:start], characters[end:]...))
		state.caret, state.anchor = start, start
		output := session.Presentation()
		output.Speech = "Deleted " + removed
		return Effect{Presentation: output}
	}
	if event.Key == Text && event.Text != "" && !event.Control {
		return session.Insert(event.Text)
	}
	return Effect{}
}

// Insert handles typing and paste atomically; invalid/oversized input leaves
// the field and selection untouched.
func (session *Session) Insert(text string) Effect {
	if session.closed {
		return Effect{}
	}
	state := &session.elements[session.focus]
	if state.element.Kind != Entry {
		return Effect{}
	}
	if !utf8.ValidString(text) {
		return Effect{Presentation: Presentation{Speech: "Invalid text", Braille: "Invalid text", Cursor: -1}}
	}
	characters := []rune(state.element.Text)
	start, end := min(state.caret, state.anchor), max(state.caret, state.anchor)
	inserted := []rune(text)
	for _, character := range inserted {
		if unicode.IsControl(character) {
			return Effect{Presentation: Presentation{Speech: "Only single-line text is accepted", Braille: "Only single-line text is accepted", Cursor: -1}}
		}
	}
	if len(characters)-(end-start)+len(inserted) > state.element.MaxLength {
		return Effect{Presentation: Presentation{Speech: "Text limit reached", Braille: "Text limit reached", Cursor: -1}}
	}
	state.element.Text = string(characters[:start]) + text + string(characters[end:])
	state.caret, state.anchor = start+len(inserted), start+len(inserted)
	output := session.Presentation()
	output.Speech = text
	return Effect{Presentation: output}
}