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42a5f66e41
EditingHostManager is going to have exactly the same implementation as FormAssociatedElement.
82 lines
3 KiB
C++
82 lines
3 KiB
C++
/*
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* Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/IterationDecision.h>
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#include <AK/Optional.h>
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#include <LibWeb/Forward.h>
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namespace Web {
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// When we want to move the cursor from some position within a line to a visually-equivalent position in an adjacent
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// line, there are several things to consider. Let's use the following HTML as an example:
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//
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// <textarea>
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// hello 👩🏼❤️👨🏻 there
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// my 👩🏼❤️👨🏻 friends!
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// </textarea>
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//
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// And let's define the following terms:
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// * logical index = the raw code unit offset of the cursor
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// * visual index = the grapheme-aware offset of the cursor (i.e. the offset the user actually perceives)
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// * text affinity = the side (left or right) of a grapheme that the cursor is visually closest to
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//
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// If we want to move the cursor from the position just after "hello" (logical index=5, visual index=5) to the next
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// line, the user will expect the cursor to be located just after the "👩🏼❤️👨🏻" (logical index=15, visual index=4). These
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// locations do not share the same visual index, so it's not enough to simply map the visual index of 5 back to a
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// logical index on the next line. The difference becomes even more apparent when multiple fonts are used within a
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// single line.
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//
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// Instead, we must measure the text between the start of the line and the starting index. On the next line, we want
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// to find the position whose corresponding width is as close to the starting width as possible. The target width
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// might not be the same as the starting width at all, so we must further consider the text affinity. We want to
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// chose a target index whose affinity brings us closest to the starting width.
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class GraphemeEdgeTracker {
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public:
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explicit constexpr GraphemeEdgeTracker(float target_width)
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: m_target_width(target_width)
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{
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}
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constexpr IterationDecision update(size_t grapheme_length_in_code_units, float grapheme_width)
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{
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if (grapheme_width == 0)
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return IterationDecision::Continue;
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m_right_edge += grapheme_length_in_code_units;
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m_width_to_right_edge += grapheme_width;
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if (m_width_to_right_edge >= m_target_width)
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return IterationDecision::Break;
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m_left_edge = m_right_edge;
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m_width_to_left_edge = m_width_to_right_edge;
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return IterationDecision::Continue;
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}
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constexpr size_t resolve() const
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{
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if ((m_target_width - m_width_to_left_edge) < (m_width_to_right_edge - m_target_width))
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return m_left_edge;
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return m_right_edge;
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}
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private:
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float m_target_width { 0 };
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size_t m_left_edge { 0 };
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size_t m_right_edge { 0 };
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float m_width_to_left_edge { 0 };
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float m_width_to_right_edge { 0 };
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};
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Optional<size_t> compute_cursor_position_on_next_line(DOM::Text const&, size_t current_offset);
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Optional<size_t> compute_cursor_position_on_previous_line(DOM::Text const&, size_t current_offset);
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}
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