incular-text 0.1.0

//! Renderer-independent editing and selection state.

use crate::{TextLayout, TextStyle};
use icu_segmenter::GraphemeClusterSegmenter;
use incular_core::{RestorationKey, RestorationScope};
use std::{cell::RefCell, fmt, ops::Range, rc::Rc, time::Instant};

/// Whether a caret is associated with the leading or trailing edge of a
/// bidirectional run.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
pub enum TextAffinity {
    #[default]
    Downstream,
    Upstream,
}

/// A half-open UTF-8 byte range. Text engines use byte offsets so ranges can
/// be passed directly to shaping and grapheme segmentation without lossy
/// conversions. Call [`Self::clamp_to`] before slicing arbitrary user input.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
pub struct TextRange {
    pub start: usize,
    pub end: usize,
}

impl TextRange {
    pub const EMPTY: Self = Self { start: 0, end: 0 };

    #[must_use]
    pub const fn new(start: usize, end: usize) -> Self {
        Self { start, end }
    }

    #[must_use]
    pub const fn collapsed(offset: usize) -> Self {
        Self::new(offset, offset)
    }

    #[must_use]
    pub const fn is_collapsed(self) -> bool {
        self.start == self.end
    }

    #[must_use]
    pub const fn is_valid(self) -> bool {
        self.start <= self.end
    }

    #[must_use]
    pub const fn len(self) -> usize {
        self.end.saturating_sub(self.start)
    }

    #[must_use]
    pub const fn is_empty(self) -> bool {
        self.start == self.end
    }

    #[must_use]
    pub const fn as_range(self) -> Range<usize> {
        self.start..self.end
    }

    /// Clamps the range to a string and moves each endpoint to a valid UTF-8
    /// boundary. This keeps editing operations panic-free for IME byte offsets.
    #[must_use]
    pub fn clamp_to(self, text: &str) -> Self {
        let start = nearest_char_boundary(text, self.start.min(text.len()));
        let end = nearest_char_boundary(text, self.end.min(text.len())).max(start);
        Self::new(start, end)
    }

    #[must_use]
    pub fn normalized(self) -> Self {
        if self.start <= self.end {
            self
        } else {
            Self::new(self.end, self.start)
        }
    }
}

/// A selection preserves directional base/extent while exposing a normalized
/// range for text replacement.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
pub struct TextSelection {
    pub base: usize,
    pub extent: usize,
}

impl TextSelection {
    #[must_use]
    pub const fn new(base: usize, extent: usize) -> Self {
        Self { base, extent }
    }

    #[must_use]
    pub const fn collapsed(offset: usize) -> Self {
        Self::new(offset, offset)
    }

    #[must_use]
    pub const fn start(self) -> usize {
        if self.base < self.extent {
            self.base
        } else {
            self.extent
        }
    }

    #[must_use]
    pub const fn end(self) -> usize {
        if self.base > self.extent {
            self.base
        } else {
            self.extent
        }
    }

    #[must_use]
    pub const fn is_collapsed(self) -> bool {
        self.base == self.extent
    }

    #[must_use]
    pub const fn is_forward(self) -> bool {
        self.base <= self.extent
    }

    #[must_use]
    pub const fn range(self) -> TextRange {
        TextRange::new(self.start(), self.end())
    }

    /// Clamps both edges into the text at UTF-8 char boundaries,
    /// snapping down. Char boundaries are deliberately not grapheme
    /// boundaries: a mid-grapheme edge survives clamping and the caret
    /// mapping below resolves it to the shaping-cluster edge instead.
    #[must_use]
    pub fn clamp_to(self, text: &str) -> Self {
        let base = nearest_char_boundary(text, self.base.min(text.len()));
        let extent = nearest_char_boundary(text, self.extent.min(text.len()));
        Self::new(base, extent)
    }
}

/// A composing/marked range supplied by an IME.
pub type ComposingRange = TextRange;

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct TextEditingValue {
    pub text: String,
    pub selection: TextSelection,
    pub composing: Option<ComposingRange>,
}

impl TextEditingValue {
    #[must_use]
    pub fn new(text: impl Into<String>) -> Self {
        let text = text.into();
        let caret = text.len();
        Self {
            text,
            selection: TextSelection::collapsed(caret),
            composing: None,
        }
    }

    #[must_use]
    pub const fn empty() -> Self {
        Self {
            text: String::new(),
            selection: TextSelection::collapsed(0),
            composing: None,
        }
    }

    #[must_use]
    pub fn with_selection(mut self, selection: TextSelection) -> Self {
        self.selection = selection.clamp_to(&self.text);
        self
    }

    #[must_use]
    pub fn with_composing(mut self, composing: Option<ComposingRange>) -> Self {
        self.composing = composing.map(|range| range.clamp_to(&self.text));
        self
    }

    #[must_use]
    pub fn replace(mut self, range: TextRange, replacement: &str) -> Self {
        let range = range.normalized().clamp_to(&self.text);
        self.text.replace_range(range.as_range(), replacement);
        let caret = range.start + replacement.len();
        self.selection = TextSelection::collapsed(caret);
        self.composing = None;
        self
    }
}

impl Default for TextEditingValue {
    fn default() -> Self {
        Self::empty()
    }
}

/// An atomic editing operation, useful for integrating native IMEs and undo
/// stacks without coupling those systems to a widget.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct TextEditingDelta {
    pub range: TextRange,
    pub replacement: String,
    pub selection: Option<TextSelection>,
    pub composing: Option<ComposingRange>,
}

impl TextEditingDelta {
    #[must_use]
    pub fn replace(range: TextRange, replacement: impl Into<String>) -> Self {
        Self {
            range,
            replacement: replacement.into(),
            selection: None,
            composing: None,
        }
    }

    #[must_use]
    pub fn apply(&self, value: &TextEditingValue) -> TextEditingValue {
        let mut next = value.clone().replace(self.range, &self.replacement);
        if let Some(selection) = self.selection {
            next.selection = selection.clamp_to(&next.text);
        }
        next.composing = self.composing.map(|range| range.clamp_to(&next.text));
        next
    }
}

#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
pub enum SelectionChangedCause {
    #[default]
    Unknown,
    Keyboard,
    Tap,
    DoubleTap,
    LongPress,
    Drag,
    Accessibility,
    Programmatic,
}

type Listener = Rc<dyn Fn(&TextEditingValue)>;
type EditTransform = Rc<dyn Fn(&TextEditingValue, &TextEditingValue) -> TextEditingValue>;

#[derive(Default)]
struct ControllerState {
    value: TextEditingValue,
    listeners: Vec<(usize, Listener)>,
    next_listener: usize,
    content_revision: u64,
    visual_revision: u64,
    caret_reset: Option<Instant>,
    preferred_caret_x: Option<f32>,
    caret_affinity: TextAffinity,
    preedit: Option<String>,
    preedit_selection: Option<TextRange>,
    restoration: Option<TextRestoration>,
    restoration_generation: u64,
}

#[derive(Clone)]
struct TextRestoration {
    scope: RestorationScope,
    key: RestorationKey,
}

/// Cloneable editing controller. It owns no platform resources; platform and
/// widget layers can subscribe to value changes and decide when to repaint.
#[derive(Clone, Default)]
pub struct TextEditingController {
    changes: incular_core::reactivity::DependencySource,
    transforms: incular_core::reactivity::DependencySource,
    inner: Rc<RefCell<ControllerState>>,
}

/// Owns a text-controller listener until dropped.
#[must_use = "keep the subscription alive to observe changes"]
pub struct TextEditingSubscription {
    _subscription: incular_core::reactivity::Subscription,
}

impl PartialEq for TextEditingController {
    fn eq(&self, other: &Self) -> bool {
        Rc::ptr_eq(&self.inner, &other.inner)
    }
}

impl fmt::Debug for TextEditingController {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        formatter
            .debug_struct("TextEditingController")
            .field("value", &self.value())
            .finish()
    }
}

impl TextEditingController {
    #[must_use]
    pub fn new() -> Self {
        Self::from_value(TextEditingValue::default())
    }

    /// Creates a controller initialized with committed text.
    #[must_use]
    pub fn with_text(text: impl Into<String>) -> Self {
        Self::from_value(TextEditingValue::new(text))
    }

    /// Creates an empty editor and restores its committed state, if present.
    #[must_use]
    pub fn restored(scope: RestorationScope, key: RestorationKey) -> Self {
        let controller = Self::new();
        controller.bind_restoration(scope, key);
        controller
    }

    /// Binds committed text and selection to a stable restoration value.
    pub fn bind_restoration(&self, scope: RestorationScope, key: RestorationKey) {
        let restored = scope.get_json(&key).and_then(value_from_json);
        let changed = restored
            .as_ref()
            .is_some_and(|value| *value != self.inner.borrow().value);
        let mut state = self.inner.borrow_mut();
        state.restoration = Some(TextRestoration { scope, key });
        // Restoration is a baseline, not an edit: bump the generation so
        // history observers rebaseline instead of recording an undo step.
        state.restoration_generation = state
            .restoration_generation
            .checked_add(1)
            .expect("text restoration generation exhausted");
        if let Some(value) = restored {
            state.value = value;
            state.content_revision = state
                .content_revision
                .checked_add(1)
                .expect("content revision exhausted");
            state.visual_revision = state
                .visual_revision
                .checked_add(1)
                .expect("visual revision exhausted");
            state.preedit = None;
            state.preedit_selection = None;
            state.caret_reset = None;
            state.preferred_caret_x = None;
            state.caret_affinity = TextAffinity::Downstream;
        }
        drop(state);
        if changed {
            self.notify_listeners();
        }
    }

    /// Stops persisting subsequent editor mutations without deleting the saved value.
    pub fn unbind_restoration(&self) {
        self.inner.borrow_mut().restoration = None;
    }

    #[must_use]
    pub fn from_value(value: TextEditingValue) -> Self {
        Self {
            changes: incular_core::reactivity::DependencySource::default(),
            transforms: incular_core::reactivity::DependencySource::default(),
            inner: Rc::new(RefCell::new(ControllerState {
                value,
                ..ControllerState::default()
            })),
        }
    }

    #[must_use]
    pub fn value(&self) -> TextEditingValue {
        self.changes.track();
        self.inner.borrow().value.clone()
    }

    #[must_use]
    pub fn text(&self) -> String {
        self.changes.track();
        self.inner.borrow().value.text.clone()
    }

    #[must_use]
    pub fn selection(&self) -> TextSelection {
        self.changes.track();
        self.inner.borrow().value.selection
    }

    #[must_use]
    pub fn composing(&self) -> Option<ComposingRange> {
        self.changes.track();
        self.inner.borrow().value.composing
    }

    pub fn set_value(&self, value: TextEditingValue) {
        self.update(value, true);
        self.inner.borrow_mut().caret_affinity = TextAffinity::Downstream;
    }

    pub fn set_text(&self, text: impl Into<String>) {
        self.update(TextEditingValue::new(text), true);
        self.inner.borrow_mut().caret_affinity = TextAffinity::Downstream;
    }

    pub fn clear(&self) {
        self.set_text("");
    }

    pub fn set_selection(&self, selection: TextSelection) {
        let mut value = self.value();
        value.selection = selection.clamp_to(&value.text);
        self.update(value, false);
        self.inner.borrow_mut().caret_affinity = TextAffinity::Downstream;
    }

    /// Sets a selection while preserving which visual edge of a bidi run owns
    /// a collapsed caret. Native hit testing should use this overload.
    pub fn set_selection_with_affinity(&self, selection: TextSelection, affinity: TextAffinity) {
        let mut value = self.value();
        value.selection = selection.clamp_to(&value.text);
        self.update(value, false);
        self.inner.borrow_mut().caret_affinity = affinity;
    }

    pub fn set_composing(&self, composing: Option<ComposingRange>) {
        let mut value = self.value();
        value.composing = composing.map(|range| range.clamp_to(&value.text));
        self.update(value, false);
    }

    /// Observes committed editing changes with automatic cleanup on drop.
    pub fn observe(
        &self,
        listener: impl Fn(&TextEditingValue) + 'static,
    ) -> TextEditingSubscription {
        let state = Rc::downgrade(&self.inner);
        TextEditingSubscription {
            _subscription: self.changes.subscribe((), move || {
                if let Some(state) = state.upgrade() {
                    let value = state.borrow().value.clone();
                    listener(&value);
                }
            }),
        }
    }

    #[doc(hidden)]
    pub fn register_edit_transform(
        &self,
        transform: impl Fn(&TextEditingValue, &TextEditingValue) -> TextEditingValue + 'static,
    ) -> incular_core::reactivity::Subscription {
        let transform: EditTransform = Rc::new(transform);
        self.transforms.subscribe(transform, || {})
    }

    /// Low-level manual registration for backend integrations. Prefer `observe`.
    /// Returns an opaque token accepted by
    /// [`Self::remove_listener`].
    pub fn add_listener(&self, listener: impl Fn(&TextEditingValue) + 'static) -> usize {
        let mut state = self.inner.borrow_mut();
        let token = state.next_listener;
        state.next_listener = state
            .next_listener
            .checked_add(1)
            .expect("text editing listener identity space exhausted");
        state.listeners.push((token, Rc::new(listener)));
        token
    }

    pub fn remove_listener(&self, token: usize) -> bool {
        let mut state = self.inner.borrow_mut();
        let before = state.listeners.len();
        state.listeners.retain(|(id, _)| *id != token);
        before != state.listeners.len()
    }

    pub fn apply_delta(&self, delta: &TextEditingDelta) {
        let next = delta.apply(&self.value());
        self.update(next, true);
    }

    pub fn replace_selection(&self, replacement: &str) {
        let value = self.value();
        let range = value.selection.range();
        self.update(value.replace(range, replacement), true);
    }

    pub fn insert_text(&self, text: &str) {
        self.replace_selection(text);
    }

    pub fn delete_backward(&self) {
        self.backspace();
    }

    pub fn delete_forward(&self) {
        let value = self.value();
        let range = if value.selection.is_collapsed() {
            next_grapheme_boundary(&value.text, value.selection.extent)
                .map_or(TextRange::collapsed(value.selection.extent), |end| {
                    TextRange::new(value.selection.extent, end)
                })
        } else {
            value.selection.range()
        };
        self.update(value.replace(range, ""), true);
    }

    /// Alias used by the Widgets editor API.
    pub fn insert(&self, text: &str) {
        self.insert_text(text);
    }

    /// Deletes one grapheme cluster behind the caret, or the active selection.
    pub fn backspace(&self) {
        let value = self.value();
        let range = if value.selection.is_collapsed() {
            previous_grapheme_boundary(&value.text, value.selection.extent)
                .map_or(TextRange::collapsed(value.selection.extent), |start| {
                    TextRange::new(start, value.selection.extent)
                })
        } else {
            value.selection.range()
        };
        self.update(value.replace(range, ""), true);
    }

    /// Deletes one grapheme cluster ahead of the caret, or the active selection.
    pub fn delete(&self) {
        self.delete_forward();
    }

    pub fn move_left(&self, extend: bool) {
        self.move_cursor(
            previous_grapheme_boundary(&self.value().text, self.value().selection.extent)
                .unwrap_or(0),
            extend,
        );
    }

    pub fn move_right(&self, extend: bool) {
        let value = self.value();
        self.move_cursor(
            next_grapheme_boundary(&value.text, value.selection.extent).unwrap_or(value.text.len()),
            extend,
        );
    }

    /// Moves one grapheme in screen/visual order using the bidi caret stops
    /// emitted by [`TextLayout`]. The logical byte offset may move backwards
    /// while the caret moves right, which is required for right-to-left text.
    pub fn move_left_visual(&self, layout: &TextLayout, extend: bool) {
        self.move_visual(layout, false, extend);
    }

    /// Moves one grapheme in screen/visual order using the bidi caret stops
    /// emitted by [`TextLayout`].
    pub fn move_right_visual(&self, layout: &TextLayout, extend: bool) {
        self.move_visual(layout, true, extend);
    }

    pub fn move_home(&self, extend: bool) {
        self.move_cursor(0, extend);
    }

    pub fn move_end(&self, extend: bool) {
        self.move_cursor(self.text().len(), extend);
    }

    pub fn select_all(&self) {
        self.set_selection(TextSelection::new(0, self.text().len()));
    }

    #[must_use]
    pub fn selected_text(&self) -> String {
        let value = self.value();
        let range = value.selection.range();
        value.text[range.as_range()].to_owned()
    }

    /// Sets transient IME preedit text without changing committed content.
    pub fn set_preedit(&self, text: impl Into<String>, selection: Option<TextRange>) {
        incular_core::reactivity::assert_mutation_allowed();
        let text = text.into();
        let mut state = self.inner.borrow_mut();
        state.preedit_selection = selection.map(|range| range.clamp_to(&text));
        state.preedit = (!text.is_empty()).then_some(text);
        state.visual_revision = state
            .visual_revision
            .checked_add(1)
            .expect("visual revision exhausted");
        drop(state);
        self.notify_listeners();
    }

    pub fn commit_preedit(&self, text: &str) {
        let value = self.value();
        let range = value.selection.range();
        self.update_committed(value.replace(range, text), true, true);
    }

    pub fn clear_preedit(&self) {
        incular_core::reactivity::assert_mutation_allowed();
        let mut state = self.inner.borrow_mut();
        if state.preedit.take().is_some() {
            state.preedit_selection = None;
            state.visual_revision = state
                .visual_revision
                .checked_add(1)
                .expect("visual revision exhausted");
            drop(state);
            self.notify_listeners();
        }
    }

    #[must_use]
    pub fn preedit(&self) -> Option<String> {
        self.inner.borrow().preedit.clone()
    }

    #[must_use]
    pub fn preedit_selection(&self) -> Option<TextRange> {
        self.inner.borrow().preedit_selection
    }

    /// Returns committed and visual revisions used by retained repaint scheduling.
    #[doc(hidden)]
    #[must_use]
    pub fn revisions(&self) -> (u64, u64) {
        let state = self.inner.borrow();
        (state.content_revision, state.visual_revision)
    }

    /// Restoration baseline generation; history rebaselines when observed.
    #[doc(hidden)]
    #[must_use]
    pub fn restoration_generation(&self) -> u64 {
        self.inner.borrow().restoration_generation
    }

    /// Restarts the caret blink after an editing interaction.
    #[doc(hidden)]
    pub fn reset_caret(&self, now: Instant) {
        let mut state = self.inner.borrow_mut();
        state.caret_reset = Some(now);
        state.visual_revision = state
            .visual_revision
            .checked_add(1)
            .expect("visual revision exhausted");
    }

    /// Returns whether the caret should be painted at `now`.
    #[doc(hidden)]
    #[must_use]
    pub fn caret_visible(&self, now: Instant) -> bool {
        self.inner.borrow().caret_reset.is_some_and(|start| {
            (now.checked_duration_since(start)
                .unwrap_or_default()
                .as_millis()
                / 500)
                .is_multiple_of(2)
        })
    }

    #[doc(hidden)]
    #[must_use]
    pub fn preferred_caret_x(&self) -> Option<f32> {
        self.inner.borrow().preferred_caret_x
    }

    /// Retains the horizontal column used by vertical movement without
    /// changing the editing value.
    #[doc(hidden)]
    pub fn set_preferred_caret_x(&self, x: f32) {
        self.inner.borrow_mut().preferred_caret_x = x.is_finite().then_some(x);
    }

    /// Returns the visual affinity of the collapsed caret.
    #[must_use]
    pub fn caret_affinity(&self) -> TextAffinity {
        self.inner.borrow().caret_affinity
    }

    #[doc(hidden)]
    pub fn move_cursor(&self, target: usize, extend: bool) {
        self.move_cursor_impl(target, extend, None, TextAffinity::Downstream);
    }

    #[doc(hidden)]
    pub fn move_cursor_with_affinity(&self, target: usize, affinity: TextAffinity, extend: bool) {
        self.move_cursor_impl(target, extend, None, affinity);
    }

    fn move_cursor_impl(
        &self,
        target: usize,
        extend: bool,
        preferred_x: Option<f32>,
        affinity: TextAffinity,
    ) {
        let mut value = self.value();
        let target = nearest_char_boundary(&value.text, target.min(value.text.len()));
        value.selection = if extend {
            TextSelection::new(value.selection.base, target)
        } else {
            TextSelection::collapsed(target)
        };
        self.update(value, false);
        let mut state = self.inner.borrow_mut();
        state.preferred_caret_x = preferred_x;
        state.caret_affinity = affinity;
    }

    fn move_visual(&self, layout: &TextLayout, right: bool, extend: bool) {
        let value = self.value();
        let extent = value.selection.extent;
        // An unextended arrow first collapses an active selection, matching
        // native editors and avoiding a surprising extra visual step.
        if !extend && !value.selection.is_collapsed() {
            let target = if right {
                value.selection.end()
            } else {
                value.selection.start()
            };
            self.move_cursor_with_affinity(target, TextAffinity::Downstream, false);
            return;
        }
        let line = layout
            .lines
            .iter()
            .position(|line| extent >= line.start && extent <= line.end)
            .unwrap_or_else(|| layout.lines.len().saturating_sub(1));
        let stops = layout.line_caret_positions(line);
        if stops.is_empty() {
            return;
        }
        let affinity = self.caret_affinity();
        let current = stops
            .iter()
            .position(|stop| stop.offset == extent && stop.affinity == affinity)
            .or_else(|| stops.iter().position(|stop| stop.offset == extent))
            .unwrap_or_else(|| {
                if right {
                    0
                } else {
                    stops.len().saturating_sub(1)
                }
            });
        let target = if right {
            stops.get(current + 1).copied().or_else(|| {
                (line + 1..layout.lines.len())
                    .find_map(|next_line| layout.line_caret_positions(next_line).first().copied())
            })
        } else {
            current
                .checked_sub(1)
                .and_then(|index| stops.get(index).copied())
                .or_else(|| {
                    (0..line).rev().find_map(|previous_line| {
                        layout.line_caret_positions(previous_line).last().copied()
                    })
                })
        };
        let Some(target) = target else {
            return;
        };
        self.move_cursor_with_affinity(target.offset, target.affinity, extend);
    }

    fn update(&self, value: TextEditingValue, content_changed: bool) {
        self.update_committed(value, content_changed, false);
    }

    fn update_committed(&self, value: TextEditingValue, content_changed: bool, end_preedit: bool) {
        incular_core::reactivity::assert_mutation_allowed();
        let (old, revision) = {
            let state = self.inner.borrow();
            (state.value.clone(), state.visual_revision)
        };
        let value = {
            let transforms = self.transforms.subscribers::<EditTransform>();
            let mut next = value;
            for transform in transforms {
                let _guard = incular_core::reactivity::ReadOnlyGuard::enter();
                next = transform(&old, &next);
                if self.inner.borrow().visual_revision != revision {
                    return;
                }
            }
            next.selection = next.selection.clamp_to(&next.text);
            next.composing = next.composing.map(|range| range.clamp_to(&next.text));
            next
        };
        let listeners = {
            let mut state = self.inner.borrow_mut();
            if state.value == value && !(end_preedit && state.preedit.is_some()) {
                return;
            }
            state.value = value;
            if content_changed {
                state.content_revision = state
                    .content_revision
                    .checked_add(1)
                    .expect("content revision exhausted");
            }
            state.visual_revision = state
                .visual_revision
                .checked_add(1)
                .expect("visual revision exhausted");
            state.preedit = None;
            state.preedit_selection = None;
            state.preferred_caret_x = None;
            state
                .listeners
                .iter()
                .map(|(_, listener)| listener.clone())
                .collect::<Vec<_>>()
        };
        let value = self.inner.borrow().value.clone();
        for listener in listeners {
            listener(&value);
        }
        self.persist_restoration();
        self.changes.notify();
    }

    fn notify_listeners(&self) {
        let (value, listeners) = {
            let state = self.inner.borrow();
            (
                state.value.clone(),
                state
                    .listeners
                    .iter()
                    .map(|(_, listener)| listener.clone())
                    .collect::<Vec<_>>(),
            )
        };
        for listener in listeners {
            listener(&value);
        }
        self.changes.notify();
    }

    fn persist_restoration(&self) {
        let (restoration, value) = {
            let state = self.inner.borrow();
            (state.restoration.clone(), value_to_json(&state.value))
        };
        if let Some(restoration) = restoration {
            restoration.scope.set_json(&restoration.key, value);
        }
    }
}

fn value_to_json(value: &TextEditingValue) -> serde_json::Value {
    serde_json::json!({
        "text": value.text,
        "selection": {
            "base": value.selection.base,
            "extent": value.selection.extent,
        },
    })
}

fn value_from_json(value: serde_json::Value) -> Option<TextEditingValue> {
    let object = value.as_object()?;
    let text = object.get("text")?.as_str()?.to_owned();
    let selection = object
        .get("selection")
        .and_then(serde_json::Value::as_object)
        .and_then(|selection| {
            let base = usize::try_from(selection.get("base")?.as_u64()?).ok()?;
            let extent = usize::try_from(selection.get("extent")?.as_u64()?).ok()?;
            Some(TextSelection::new(base, extent))
        })
        .unwrap_or_else(|| TextSelection::collapsed(text.len()));
    Some(TextEditingValue::new(text.clone()).with_selection(selection.clamp_to(&text)))
}

/// A renderer-neutral editable text description.
#[derive(Clone, Debug)]
pub struct EditableText {
    pub controller: TextEditingController,
    pub style: TextStyle,
    pub read_only: bool,
    pub obscure_text: bool,
    pub max_lines: Option<usize>,
}

impl EditableText {
    #[must_use]
    pub fn new(controller: TextEditingController) -> Self {
        Self {
            controller,
            style: TextStyle::default(),
            read_only: false,
            obscure_text: false,
            max_lines: Some(1),
        }
    }

    #[must_use]
    pub fn style(mut self, style: TextStyle) -> Self {
        self.style = style;
        self
    }

    #[must_use]
    pub fn read_only(mut self, read_only: bool) -> Self {
        self.read_only = read_only;
        self
    }

    #[must_use]
    pub fn obscure_text(mut self, obscure_text: bool) -> Self {
        self.obscure_text = obscure_text;
        self
    }

    #[must_use]
    pub fn max_lines(mut self, max_lines: Option<usize>) -> Self {
        self.max_lines = max_lines;
        self
    }
}

/// Convenience text-field configuration built on [`EditableText`].
#[derive(Clone, Debug)]
pub struct TextField {
    pub editable: EditableText,
    pub placeholder: String,
}

impl TextField {
    #[must_use]
    pub fn new(controller: TextEditingController) -> Self {
        Self {
            editable: EditableText::new(controller),
            placeholder: String::new(),
        }
    }

    #[must_use]
    pub fn style(mut self, style: TextStyle) -> Self {
        self.editable = self.editable.style(style);
        self
    }

    #[must_use]
    pub fn placeholder(mut self, placeholder: impl Into<String>) -> Self {
        self.placeholder = placeholder.into();
        self
    }

    #[must_use]
    pub fn read_only(mut self, read_only: bool) -> Self {
        self.editable = self.editable.read_only(read_only);
        self
    }
}

fn nearest_char_boundary(text: &str, offset: usize) -> usize {
    if text.is_char_boundary(offset) {
        return offset;
    }
    let mut boundary = offset;
    while boundary > 0 && !text.is_char_boundary(boundary) {
        boundary -= 1;
    }
    boundary
}

fn previous_grapheme_boundary(text: &str, offset: usize) -> Option<usize> {
    let offset = nearest_char_boundary(text, offset.min(text.len()));
    (offset > 0).then(|| {
        GraphemeClusterSegmenter::new()
            .segment_str(text)
            .take_while(|index| *index < offset)
            .last()
            .unwrap_or(0)
    })
}

fn next_grapheme_boundary(text: &str, offset: usize) -> Option<usize> {
    let offset = nearest_char_boundary(text, offset.min(text.len()));
    (offset < text.len()).then(|| {
        GraphemeClusterSegmenter::new()
            .segment_str(text)
            .find(|index| *index > offset)
            .unwrap_or(text.len())
    })
}