text_overflow.hpp

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// SPDX-License-Identifier: MIT
//
// pulp #1407 — CSS `text-overflow: ellipsis` truncation helper.
//
// Translates a string to its ellipsised form when its measured width
// exceeds the available content-box. Operates on UTF-8 codepoints so
// truncation never splits a multibyte sequence, and uses U+2026 (…)
// rather than three ASCII dots, matching CSS behavior.
 
#pragma once
 
#include <pulp/canvas/canvas.hpp>
#include <string>
#include <string_view>
 
namespace pulp::view {

inline constexpr const char* kEllipsis = "\xe2\x80\xa6";

inline std::size_t utf8_advance(std::string_view text, std::size_t count) {
    std::size_t i = 0;
    while (count > 0 && i < text.size()) {
        unsigned char c = static_cast<unsigned char>(text[i]);
        if (c < 0x80)        i += 1;
        else if (c < 0xC0) {
            i += 1;  // stray continuation byte -> skip
            continue;
        }
        else if (c < 0xE0)   i += 2;
        else if (c < 0xF0)   i += 3;
        else                 i += 4;
        if (i > text.size()) i = text.size();
        --count;
    }
    return i;
}

inline std::size_t utf8_codepoint_count(std::string_view text) {
    std::size_t n = 0;
    for (std::size_t i = 0; i < text.size();) {
        unsigned char c = static_cast<unsigned char>(text[i]);
        if (c < 0x80)        i += 1;
        else if (c < 0xC0) {
            i += 1;
            continue;
        }
        else if (c < 0xE0)   i += 2;
        else if (c < 0xF0)   i += 3;
        else                 i += 4;
        if (i > text.size()) i = text.size();
        ++n;
    }
    return n;
}

inline std::string truncate_to_width(canvas::Canvas& canvas,
                                     std::string_view text,
                                     float available_width,
                                     bool force_ellipsis = false) {
    if (available_width <= 0.0f) return std::string(kEllipsis);
    if (!force_ellipsis &&
        canvas.measure_text(std::string(text)) <= available_width)
        return std::string(text);
 
    const std::size_t total = utf8_codepoint_count(text);
    if (total == 0) return std::string(text);
 
    // Reserve at least the ellipsis itself.
    if (canvas.measure_text(kEllipsis) > available_width) {
        // Even "…" doesn't fit — return "…" anyway so callers can still
        // signal truncation. The widget will visually clip via its
        // bounding box.
        return std::string(kEllipsis);
    }
 
    // Binary search for the largest codepoint prefix that fits with "…".
    std::size_t lo = 0;          // always fits (with ellipsis) — invariant: 0-prefix + "…"
    std::size_t hi = total;      // never fits at hi (we know total doesn't fit)
    while (lo + 1 < hi) {
        std::size_t mid = (lo + hi) / 2;
        std::string candidate(text.substr(0, utf8_advance(text, mid)));
        candidate.append(kEllipsis);
        if (canvas.measure_text(candidate) <= available_width) lo = mid;
        else                                                    hi = mid;
    }
 
    std::string out(text.substr(0, utf8_advance(text, lo)));
    out.append(kEllipsis);
    return out;
}
 
} // namespace pulp::view