layout/flow/inline/

text_run.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

use std::mem;
use std::ops::Range;
use std::sync::Arc;

use app_units::Au;
use fonts::{
    FontContext, FontRef, ShapedTextSlice, ShapedTextSlicer, ShapingFlags, ShapingOptions,
};
use icu_locid::subtags::Language;
use icu_properties::{self, LineBreak};
use log::warn;
use malloc_size_of_derive::MallocSizeOf;
use servo_arc::Arc as ServoArc;
use servo_base::text::is_bidi_control;
use style::Zero;
use style::computed_values::font_kerning::T as FontKerning;
use style::computed_values::text_rendering::T as TextRendering;
use style::computed_values::white_space_collapse::T as WhiteSpaceCollapse;
use style::computed_values::word_break::T as WordBreak;
use style::properties::ComputedValues;
use style::str::char_is_whitespace;
use style::values::computed::OverflowWrap;
use unicode_bidi::{BidiInfo, Level};
use unicode_script::Script;

use super::line_breaker::LineBreaker;
use super::{InlineFormattingContextLayout, SharedInlineStyles};
use crate::ArcRefCell;
use crate::context::LayoutContext;
use crate::dom::WeakLayoutBox;
use crate::flow::inline::line::TextRunOffsets;
use crate::flow::inline::{LineBlockSizes, LineItem, SegmentContentFlags};
use crate::fragment_tree::BaseFragmentInfo;

// There are two reasons why we might want to break at the start:
//
//  1. The line breaker told us that a break was necessary between two separate
//     instances of sending text to it.
//  2. We are following replaced content ie `have_deferred_soft_wrap_opportunity`.
//
// In both cases, we don't want to do this if the first character prevents a
// soft wrap opportunity.
#[derive(PartialEq)]
enum SegmentStartSoftWrapPolicy {
    Force,
    FollowLinebreaker,
}

/// A data structure which contains information used when shaping a [`TextRunSegment`].
#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
pub(crate) struct FontAndScriptInfo {
    /// The font used when shaping a [`TextRunSegment`].
    pub font: FontRef,
    /// The script used when shaping a [`TextRunSegment`].
    pub script: Script,
    /// The BiDi [`Level`] used when shaping a [`TextRunSegment`].
    pub bidi_level: Level,
    /// The [`Language`] used when shaping a [`TextRunSegment`].
    pub language: Language,
    /// Spacing to add between each letter. Corresponds to the CSS 2.1 `letter-spacing` property.
    /// NB: You will probably want to set the `IGNORE_LIGATURES_SHAPING_FLAG` if this is non-null.
    ///
    /// Letter spacing is not applied to all characters. Use [Self::letter_spacing_for_character] to
    /// determine the amount of spacing to apply.
    pub letter_spacing: Option<Au>,
    /// Spacing to add between each word. Corresponds to the CSS 2.1 `word-spacing` property.
    pub word_spacing: Option<Au>,
    /// The [`TextRendering`] value from the original style.
    pub text_rendering: TextRendering,
    /// The value of the `font-kerning` property from the original style.
    pub kerning: FontKerning,
}

impl FontAndScriptInfo {
    /// Creates a minimal [`FontAndScriptInfo`] for a single font, with generic language settings
    /// and the default shaping configuration. This is only used to generate placeholders for
    /// text carets on otherwise empty lines.
    pub(crate) fn simple_for_font(font: FontRef) -> Self {
        Self {
            font,
            script: Script::Common,
            bidi_level: Level::ltr(),
            language: Language::UND,
            letter_spacing: None,
            word_spacing: None,
            text_rendering: TextRendering::Auto,
            kerning: FontKerning::Auto,
        }
    }
}

impl From<&FontAndScriptInfo> for ShapingOptions {
    fn from(info: &FontAndScriptInfo) -> Self {
        let mut flags = ShapingFlags::empty();
        if info.bidi_level.is_rtl() {
            flags.insert(ShapingFlags::RTL_FLAG);
        }

        // From https://www.w3.org/TR/css-text-3/#cursive-script:
        // Cursive scripts do not admit gaps between their letters for either
        // justification or letter-spacing.
        let letter_spacing = info
            .letter_spacing
            .filter(|_| !is_cursive_script(info.script));
        if letter_spacing.is_some() {
            flags.insert(ShapingFlags::IGNORE_LIGATURES_SHAPING_FLAG);
        };
        if info.text_rendering == TextRendering::Optimizespeed {
            flags.insert(ShapingFlags::IGNORE_LIGATURES_SHAPING_FLAG);
            flags.insert(ShapingFlags::DISABLE_KERNING_SHAPING_FLAG)
        }

        // We currently always leave kerning enabled for "font-kerning: auto".
        if info.kerning == FontKerning::None {
            flags.insert(ShapingFlags::DISABLE_KERNING_SHAPING_FLAG);
        }

        Self {
            letter_spacing,
            word_spacing: info.word_spacing,
            script: info.script,
            language: info.language,
            flags,
        }
    }
}

#[derive(Clone, Debug, MallocSizeOf)]
pub(crate) struct TextRunSegment {
    /// Information about the font and language used in this text run. This is produced by
    /// segmenting the inline formatting context's text content by font, script, and bidi level.
    #[conditional_malloc_size_of]
    pub info: Arc<FontAndScriptInfo>,

    /// The range of bytes in the parent [`super::InlineFormattingContext`]'s text content.
    pub byte_range: Range<usize>,

    /// The range of characters in the parent [`super::InlineFormattingContext`]'s text content.
    pub character_range: Range<usize>,

    /// Whether or not the linebreaker said that we should allow a line break at the start of this
    /// segment.
    pub break_at_start: bool,

    /// The shaped runs within this segment.
    #[conditional_malloc_size_of]
    pub runs: Vec<Arc<ShapedTextSlice>>,
}

impl TextRunSegment {
    fn new(
        info: Arc<FontAndScriptInfo>,
        byte_range: Range<usize>,
        character_range: Range<usize>,
    ) -> Self {
        Self {
            info,
            byte_range,
            character_range,
            runs: Vec::new(),
            break_at_start: false,
        }
    }

    /// Returns true if the new `Font`, `Script` and BiDi `Level` are compatible with this segment
    /// or false otherwise.
    fn is_compatible(
        &self,
        new_font: &Option<FontRef>,
        new_script: Script,
        new_bidi_level: Level,
    ) -> bool {
        if self.info.bidi_level != new_bidi_level {
            return false;
        }
        if new_font
            .as_ref()
            .is_some_and(|new_font| !Arc::ptr_eq(&self.info.font, new_font))
        {
            return false;
        }

        !script_is_specific(self.info.script) ||
            !script_is_specific(new_script) ||
            self.info.script == new_script
    }

    /// Update this segment to end at the given byte and character index. The update will only ever
    /// make the Script specific and will not change it otherwise.
    fn update(&mut self, next_byte_index: usize, next_character_index: usize, new_script: Script) {
        if !script_is_specific(self.info.script) && script_is_specific(new_script) {
            self.info = Arc::new(FontAndScriptInfo {
                script: new_script,
                ..(*self.info).clone()
            });
        }
        self.character_range.end = next_character_index;
        self.byte_range.end = next_byte_index;
    }

    fn layout_into_line_items(
        &self,
        text_run: &TextRun,
        mut soft_wrap_policy: SegmentStartSoftWrapPolicy,
        ifc: &mut InlineFormattingContextLayout,
    ) {
        if self.break_at_start && soft_wrap_policy == SegmentStartSoftWrapPolicy::FollowLinebreaker
        {
            soft_wrap_policy = SegmentStartSoftWrapPolicy::Force;
        }

        let mut character_range_start = self.character_range.start;
        for (run_index, run) in self.runs.iter().enumerate() {
            let new_character_range_end = character_range_start + run.character_count();
            let offsets = ifc
                .ifc
                .shared_selection
                .clone()
                .map(|shared_selection| TextRunOffsets {
                    shared_selection,
                    character_range: character_range_start..new_character_range_end,
                });

            // Break before each unbreakable run in this TextRun, except the first unless the
            // linebreaker was set to break before the first run.
            if run_index != 0 || soft_wrap_policy == SegmentStartSoftWrapPolicy::Force {
                ifc.process_soft_wrap_opportunity();
            }

            ifc.push_glyph_store_to_unbreakable_segment(run.clone(), text_run, &self.info, offsets);
            character_range_start = new_character_range_end;
        }
    }

    /// Shape the text of this [`TextRunSegment`], first finding "words" for the shaper by processing
    /// the linebreaks found in the owning [`super::InlineFormattingContext`]. Linebreaks are filtered,
    /// based on the style of the parent inline box.
    fn shape_text(
        &mut self,
        parent_style: &ComputedValues,
        formatting_context_text: &str,
        linebreaker: &mut LineBreaker,
        old_text_run_item: Option<TextRunItem>,
    ) {
        // Gather the linebreaks that apply to this segment from the inline formatting context's collection
        // of line breaks. Also add a simulated break at the end of the segment in order to ensure the final
        // piece of text is processed.
        let range = self.byte_range.clone();
        let linebreaks = linebreaker.advance_to_linebreaks_in_range(self.byte_range.clone());
        let linebreak_iter = linebreaks.iter().chain(std::iter::once(&range.end));

        let options: ShapingOptions = (&*self.info).into();
        let shaped_text = old_text_run_item
            .and_then(|old_text_run_item| {
                let TextRunItem::TextSegment(old_text_segment) = old_text_run_item else {
                    return None;
                };
                if !self.is_compatible_with_old_shaping_result(&old_text_segment) {
                    return None;
                }
                Some(old_text_segment.runs.first()?.shaped_text())
            })
            .unwrap_or_else(|| {
                self.info
                    .font
                    .shape_text(&formatting_context_text[range.clone()], &options)
            });

        let mut shaped_text_slicer = ShapedTextSlicer::new(shaped_text);

        self.runs.clear();
        self.runs.reserve(linebreaks.len());
        self.break_at_start = false;

        let text_style = parent_style.get_inherited_text().clone();
        let can_break_anywhere = text_style.word_break == WordBreak::BreakAll ||
            text_style.overflow_wrap == OverflowWrap::Anywhere ||
            text_style.overflow_wrap == OverflowWrap::BreakWord;

        let mut last_slice = self.byte_range.start..self.byte_range.start;
        for break_index in linebreak_iter {
            if *break_index == self.byte_range.start {
                self.break_at_start = true;
                continue;
            }

            // Extend the slice to the next UAX#14 line break opportunity.
            let mut slice = last_slice.end..*break_index;
            let word = &formatting_context_text[slice.clone()];

            // Split off any trailing whitespace into a separate glyph run.
            let mut whitespace = slice.end..slice.end;
            let rev_char_indices = word.char_indices().rev().peekable();

            let mut non_whitespace_slice_ends_with_whitespace = false;
            let mut ends_with_whitespace = false;
            if let Some((first_white_space_index, first_white_space_character)) = rev_char_indices
                .take_while(|&(_, character)| char_is_whitespace(character))
                .last()
            {
                ends_with_whitespace = true;
                whitespace.start = slice.start + first_white_space_index;

                // If line breaking for a piece of text that has `white-space-collapse:
                // break-spaces` there is a line break opportunity *after* every preserved space,
                // but not before. This means that we should not split off the first whitespace.
                //
                // An exception to this is if the style tells us that we can break in the middle of words.
                if text_style.white_space_collapse == WhiteSpaceCollapse::BreakSpaces &&
                    !can_break_anywhere
                {
                    whitespace.start += first_white_space_character.len_utf8();
                    non_whitespace_slice_ends_with_whitespace = true;
                }

                slice.end = whitespace.start;
            }

            // If there's no whitespace and `word-break` is set to `keep-all`, try increasing the slice.
            // TODO: This should only happen for CJK text.
            if !ends_with_whitespace &&
                *break_index != self.byte_range.end &&
                text_style.word_break == WordBreak::KeepAll &&
                !can_break_anywhere
            {
                continue;
            }

            // Only advance the last slice if we are not going to try to expand the slice.
            last_slice = slice.start..*break_index;

            // Push the non-whitespace part of the range.
            if !slice.is_empty() {
                let character_count = formatting_context_text[slice].chars().count();
                self.runs.push(shaped_text_slicer.slice_for_character_count(
                    character_count,
                    false, /* is_whitespace */
                    non_whitespace_slice_ends_with_whitespace,
                ));
            }

            if whitespace.is_empty() {
                continue;
            }

            // If `white-space-collapse: break-spaces` is active, insert a line breaking opportunity
            // between each white space character in the white space that we trimmed off.
            if text_style.white_space_collapse == WhiteSpaceCollapse::BreakSpaces {
                for _ in formatting_context_text[whitespace].chars() {
                    self.runs.push(shaped_text_slicer.slice_for_character_count(
                        1, true, /* is_whitespace */
                        true, /* ends_with_whitespace */
                    ));
                }
                continue;
            }

            let character_count = formatting_context_text[whitespace].chars().count();
            self.runs.push(shaped_text_slicer.slice_for_character_count(
                character_count,
                true, /* is_whitespace */
                true, /* ends_with_whitespace */
            ));
        }
    }

    fn is_compatible_with_old_shaping_result(&self, old_segment: &Self) -> bool {
        *old_segment.info == *self.info && self.byte_range == old_segment.byte_range
    }
}

/// A single item in a [`TextRun`].
#[derive(Debug, MallocSizeOf)]
pub(crate) enum TextRunItem {
    /// A hard line break i.e. a "\n" as other types line breaks are normalized to "\n".
    LineBreak { character_index: usize },
    /// A preserved tab character that should advance the line to a tab stop.
    Tab { bidi_level: Level },
    /// Any other text for which a font can be matched. We store a `Box` here as [`TextRunSegment`]
    /// is quite a bit larger than the other enum variants.
    TextSegment(Box<TextRunSegment>),
}

/// A single [`TextRun`] for the box tree. These are all descendants of
/// [`super::InlineBox`] or the root of the [`super::InlineFormattingContext`].  During
/// box tree construction, text is split into [`TextRun`]s based on their font, script,
/// etc. When these are created text is already shaped.
///
/// <https://www.w3.org/TR/css-display-3/#css-text-run>
#[derive(Debug, MallocSizeOf)]
pub(crate) struct TextRun {
    /// The [`BaseFragmentInfo`] for this [`TextRun`]. Usually this comes from the
    /// original text node in the DOM for the text.
    pub base_fragment_info: BaseFragmentInfo,

    /// A weak reference to the parent of this layout box. This becomes valid as soon
    /// as the *parent* of this box is added to the tree.
    pub parent_box: Option<WeakLayoutBox>,

    /// The [`crate::SharedStyle`] from this [`TextRun`]s parent element. This is
    /// shared so that incremental layout can simply update the parent element and
    /// this [`TextRun`] will be updated automatically.
    pub inline_styles: SharedInlineStyles,

    /// The range of text in [`super::InlineFormattingContext::text_content`] of the
    /// [`super::InlineFormattingContext`] that owns this [`TextRun`]. These are UTF-8 offsets.
    pub text_range: Range<usize>,

    /// The range of characters in this text in [`super::InlineFormattingContext::text_content`]
    /// of the [`super::InlineFormattingContext`] that owns this [`TextRun`]. These are *not*
    /// UTF-8 offsets.
    pub character_range: Range<usize>,

    /// The [`TextRunItem`]s of this text run. This is produced by segmenting the incoming text
    /// by things such as font and script as well as separating out hard line breaks.
    /// segments, and shaped.
    pub items: Vec<TextRunItem>,
}

impl TextRun {
    pub(crate) fn new(
        base_fragment_info: BaseFragmentInfo,
        inline_styles: SharedInlineStyles,
        text_range: Range<usize>,
        character_range: Range<usize>,
        old_text_run: Option<ArcRefCell<TextRun>>,
    ) -> Self {
        // If there was a previous box tree layout of this text run, try to preserve the old shaped text.
        let items = old_text_run
            .map(|old_text_run| std::mem::take(&mut old_text_run.borrow_mut().items))
            .unwrap_or_default();
        Self {
            base_fragment_info,
            parent_box: None,
            inline_styles,
            text_range,
            character_range,
            items,
        }
    }

    pub(super) fn segment_and_shape(
        &mut self,
        formatting_context_text: &str,
        layout_context: &LayoutContext,
        linebreaker: &mut LineBreaker,
        bidi_info: &BidiInfo,
    ) {
        let parent_style = self.inline_styles.style.borrow().clone();
        let items = self.segment_text_by_font(
            layout_context,
            formatting_context_text,
            bidi_info,
            &parent_style,
        );

        // If a previous box tree layout seeded this [`TextRun`] with old shaping results, use those
        // to try to prevent re-shaping.
        let mut old_text_run_items = std::mem::replace(&mut self.items, items).into_iter();
        for item in self.items.iter_mut() {
            let old_text_run_item = old_text_run_items.next();
            if let TextRunItem::TextSegment(text_segment) = item {
                text_segment.shape_text(
                    &parent_style,
                    formatting_context_text,
                    linebreaker,
                    old_text_run_item,
                );
            }
        }
    }

    /// Take the [`TextRun`]'s text and turn it into [`TextRunSegment`]s. Each segment has a matched
    /// font and script. Fonts may differ when glyphs are found in fallback fonts.
    /// [`super::InlineFormattingContext`].
    fn segment_text_by_font(
        &mut self,
        layout_context: &LayoutContext,
        formatting_context_text: &str,
        bidi_info: &BidiInfo,
        parent_style: &ServoArc<ComputedValues>,
    ) -> Vec<TextRunItem> {
        let font_style = parent_style.clone_font();
        let language = font_style._x_lang.0.parse().unwrap_or(Language::UND);
        let font_size = font_style.font_size.computed_size().into();
        let kerning = font_style.font_kerning;
        let font_group = layout_context.font_context.font_group(font_style);
        let inherited_text_style = parent_style.get_inherited_text();
        let word_spacing = Some(inherited_text_style.word_spacing.to_used_value(font_size));
        let letter_spacing = inherited_text_style
            .letter_spacing
            .0
            .to_used_value(font_size);
        let letter_spacing = if !letter_spacing.is_zero() {
            Some(letter_spacing)
        } else {
            None
        };
        let text_rendering = inherited_text_style.text_rendering;

        let mut current: Option<TextRunSegment> = None;
        let mut results = Vec::new();
        let finish_current_segment =
            |current: &mut Option<TextRunSegment>, results: &mut Vec<TextRunItem>| {
                if let Some(current) = current.take() {
                    results.push(TextRunItem::TextSegment(Box::new(current)));
                }
            };

        let text_run_text = &formatting_context_text[self.text_range.clone()];
        let char_iterator = TwoCharsAtATimeIterator::new(text_run_text.chars());
        // The next bytes index of the character within the entire inline formatting context's text.
        let mut next_byte_index = self.text_range.start;
        for (relative_character_index, (character, next_character)) in char_iterator.enumerate() {
            // The current character index within the entire inline formatting context's text.
            let current_character_index = self.character_range.start + relative_character_index;

            let current_byte_index = next_byte_index;
            next_byte_index += character.len_utf8();

            if character == '\n' {
                finish_current_segment(&mut current, &mut results);
                results.push(TextRunItem::LineBreak {
                    character_index: current_character_index,
                });
                continue;
            }

            if character == '\t' {
                finish_current_segment(&mut current, &mut results);
                results.push(TextRunItem::Tab {
                    bidi_level: bidi_info.levels[current_byte_index],
                });
                continue;
            }

            let (font, script, bidi_level) = if character_cannot_change_font(character) {
                (None, Script::Common, bidi_info.levels[current_byte_index])
            } else {
                (
                    font_group.find_by_codepoint(
                        &layout_context.font_context,
                        character,
                        next_character,
                        language,
                    ),
                    Script::from(character),
                    bidi_info.levels[current_byte_index],
                )
            };

            // If the existing segment is compatible with the character, just merge the character into it.
            if let Some(current) = current.as_mut() {
                if current.is_compatible(&font, script, bidi_level) {
                    current.update(next_byte_index, current_character_index + 1, script);
                    continue;
                }
            }

            let Some(font) = font.or_else(|| font_group.first(&layout_context.font_context)) else {
                continue;
            };
            let info = FontAndScriptInfo {
                font,
                script,
                bidi_level,
                language,
                word_spacing,
                letter_spacing,
                text_rendering,
                kerning,
            };

            finish_current_segment(&mut current, &mut results);
            assert!(current.is_none());

            current = Some(TextRunSegment::new(
                Arc::new(info),
                current_byte_index..next_byte_index,
                current_character_index..current_character_index + 1,
            ));
        }

        finish_current_segment(&mut current, &mut results);
        results
    }

    pub(super) fn layout_into_line_items(&self, ifc: &mut InlineFormattingContextLayout) {
        if self.text_range.is_empty() {
            return;
        }

        // If we are following replaced content, we should have a soft wrap opportunity, unless the
        // first character of this `TextRun` prevents that soft wrap opportunity. If we see such a
        // character it should also override the LineBreaker's indication to break at the start.
        let have_deferred_soft_wrap_opportunity =
            mem::replace(&mut ifc.have_deferred_soft_wrap_opportunity, false);
        let mut soft_wrap_policy = match have_deferred_soft_wrap_opportunity {
            true => SegmentStartSoftWrapPolicy::Force,
            false => SegmentStartSoftWrapPolicy::FollowLinebreaker,
        };

        for item in self.items.iter() {
            ifc.possibly_flush_deferred_forced_line_break();

            match item {
                // If this whitespace forces a line break, queue up a hard line break the next time we
                // see any content. We don't line break immediately, because we'd like to finish processing
                // any ongoing inline boxes before ending the line.
                TextRunItem::LineBreak { character_index } => {
                    ifc.defer_forced_line_break_at_character_offset(*character_index);
                },
                TextRunItem::Tab { bidi_level } => self.process_preserved_tab(ifc, *bidi_level),
                TextRunItem::TextSegment(segment) => {
                    segment.layout_into_line_items(self, soft_wrap_policy, ifc)
                },
            }
            soft_wrap_policy = SegmentStartSoftWrapPolicy::FollowLinebreaker;
        }
    }

    fn process_preserved_tab(
        &self,
        ifc_layout: &mut InlineFormattingContextLayout,
        bidi_level: Level,
    ) {
        let advance = ifc_layout
            .ifc
            .next_tab_stop_after_inline_advance(ifc_layout.potential_line_size().inline);
        if advance.is_zero() {
            return;
        }

        ifc_layout.update_unbreakable_segment_for_new_content(
            &LineBlockSizes::zero(),
            advance,
            SegmentContentFlags::empty(),
        );
        ifc_layout.push_line_item_to_unbreakable_segment(LineItem::Tab {
            inline_box_identifier: ifc_layout.current_inline_box_identifier(),
            advance,
            bidi_level,
        });

        if ifc_layout
            .current_inline_container_state()
            .style
            .get_inherited_text()
            .white_space_collapse ==
            WhiteSpaceCollapse::BreakSpaces
        {
            ifc_layout.process_soft_wrap_opportunity();
        }
    }
}

/// From <https://www.w3.org/TR/css-text-3/#cursive-script>:
/// Cursive scripts do not admit gaps between their letters for either justification
/// or letter-spacing. The following Unicode scripts are included: Arabic, Hanifi
/// Rohingya, Mandaic, Mongolian, N’Ko, Phags Pa, Syriac
fn is_cursive_script(script: Script) -> bool {
    matches!(
        script,
        Script::Arabic |
            Script::Hanifi_Rohingya |
            Script::Mandaic |
            Script::Mongolian |
            Script::Nko |
            Script::Phags_Pa |
            Script::Syriac
    )
}

/// Whether or not this character should be able to change the font during segmentation.  Certain
/// character are not rendered at all, so it doesn't matter what font we use to render them. They
/// should just be added to the current segment.
fn character_cannot_change_font(character: char) -> bool {
    if character.is_control() {
        return true;
    }
    if character == '\u{00A0}' {
        return true;
    }
    if is_bidi_control(character) {
        return false;
    }

    matches!(
        icu_properties::maps::line_break().get(character),
        LineBreak::CombiningMark |
            LineBreak::Glue |
            LineBreak::ZWSpace |
            LineBreak::WordJoiner |
            LineBreak::ZWJ
    )
}

pub(super) fn get_font_for_first_font_for_style(
    style: &ComputedValues,
    font_context: &FontContext,
) -> Option<FontRef> {
    let font = font_context
        .font_group(style.clone_font())
        .first(font_context);
    if font.is_none() {
        warn!("Could not find font for style: {:?}", style.clone_font());
    }
    font
}
pub(crate) struct TwoCharsAtATimeIterator<InputIterator> {
    /// The input character iterator.
    iterator: InputIterator,
    /// The first character to produce in the next run of the iterator.
    next_character: Option<char>,
}

impl<InputIterator> TwoCharsAtATimeIterator<InputIterator> {
    fn new(iterator: InputIterator) -> Self {
        Self {
            iterator,
            next_character: None,
        }
    }
}

impl<InputIterator> Iterator for TwoCharsAtATimeIterator<InputIterator>
where
    InputIterator: Iterator<Item = char>,
{
    type Item = (char, Option<char>);

    fn next(&mut self) -> Option<Self::Item> {
        // If the iterator isn't initialized do that now.
        if self.next_character.is_none() {
            self.next_character = self.iterator.next();
        }
        let character = self.next_character?;
        self.next_character = self.iterator.next();
        Some((character, self.next_character))
    }
}

fn script_is_specific(script: Script) -> bool {
    script != Script::Common && script != Script::Inherited
}