third_party/WebKit/Source/core/css/FontStyleMatcher.cpp

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#include "core/css/FontStyleMatcher.h"

#include "core/css/CSSSegmentedFontFace.h"
#include "wtf/Assertions.h"

#include <stdlib.h>

namespace blink {

static inline unsigned stretchDistanceToDesired(FontTraits desired, FontTraits candidate)
{
    return abs(static_cast<int>(desired.stretch() - candidate.stretch()));
}

static inline unsigned styleScore(FontTraits desired, FontTraits candidate)
{
    static_assert(FontStyleNormal == 0 && FontStyleItalic == 2,
        "Enumeration values need to match lookup table.");
    unsigned styleScoreLookupTable[][FontStyleItalic + 1] = {
        // "If the value is normal, normal faces are checked first, then oblique faces, then italic faces."
        // i.e. normal has the highest score, then oblique, then italic.
        { 2, 1, 0 },
        // "If the value is oblique, oblique faces are checked first, then italic faces and then normal faces."
        // i.e. normal gets the lowest score, oblique gets the highest, italic second best.
        { 0, 2, 1 },
        // "If the value of font-style is italic, italic faces are checked first, then oblique, then normal faces"
        // i.e. normal gets the lowest score, oblique is second best, italic highest.
        { 0, 1, 2 }
    };

    ASSERT_WITH_SECURITY_IMPLICATION(desired.style() < FontStyleItalic + 1);
    ASSERT_WITH_SECURITY_IMPLICATION(candidate.style() < FontStyleItalic + 1);

    return styleScoreLookupTable[desired.style()][candidate.style()];
}

static inline unsigned weightScore(FontTraits desired, FontTraits candidate)
{
    static_assert(FontWeight100 == 0 && FontWeight900 - FontWeight100 == 8,
        "Enumeration values need to match lookup table.");
    static const unsigned scoreLookupSize = FontWeight900 + 1;
    // https://drafts.csswg.org/css-fonts/#font-style-matching
    // "..if the desired weight is available that face matches. "
    static const unsigned weightScoreLookup[scoreLookupSize][scoreLookupSize] = {
        // "If the desired weight is less than 400, weights below the desired
        // weight are checked in descending order followed by weights above the
        // desired weight in ascending order until a match is found."
        { 9, 8, 7, 6, 5, 4, 3, 2, 1 }, // FontWeight100 desired
        { 8, 9, 7, 6, 5, 4, 3, 2, 1 }, // FontWeight200 desired
        { 7, 8, 9, 6, 5, 4, 3, 2, 1 }, // FontWeight300 desired

        // "If the desired weight is 400, 500 is checked first and then the rule
        // for desired weights less than 400 is used."
        { 5, 6, 7, 9, 8, 4, 3, 2, 1 }, // FontWeight400 desired

        // "If the desired weight is 500, 400 is checked first and then the rule
        // for desired weights less than 400 is used."
        { 5, 6, 7, 8, 9, 4, 3, 2, 1 }, // FontWeight500 desired

        // "If the desired weight is greater than 500, weights above the desired
        // weight are checked in ascending order followed by weights below the
        // desired weight in descending order until a match is found."
        { 1, 2, 3, 4, 5, 9, 8, 7, 6 }, // FontWeight600 desired
        { 1, 2, 3, 4, 5, 6, 9, 8, 7 }, // FontWeight700 desired
        { 1, 2, 3, 4, 5, 6, 7, 9, 8 }, // FontWeight800 desired
        { 1, 2, 3, 4, 5, 6, 7, 8, 9 } // FontWeight900 desired
    };

    unsigned desiredScoresLookup = static_cast<unsigned>(desired.weight());
    unsigned candidateScoreLookup = static_cast<unsigned>(candidate.weight());
    ASSERT_WITH_SECURITY_IMPLICATION(desiredScoresLookup < scoreLookupSize);
    ASSERT_WITH_SECURITY_IMPLICATION(candidateScoreLookup < scoreLookupSize);

    return weightScoreLookup[desiredScoresLookup][candidateScoreLookup];
}

bool FontStyleMatcher::isCandidateBetter(CSSSegmentedFontFace *candidate, CSSSegmentedFontFace *current)
{
    const FontTraits& candidateTraits = candidate->traits();
    const FontTraits& currentTraits = current->traits();

    // According to CSS3 Fonts Font Style matching, there is a precedence for matching:
    // A better stretch match wins over a better style match, a better style match
    // wins over a better weight match, where "better" means closer to the desired
    // traits.
    int stretchComparison = 0, styleComparison = 0, weightComparison = 0;

    stretchComparison = stretchDistanceToDesired(m_fontTraits, candidateTraits) -
        stretchDistanceToDesired(m_fontTraits, currentTraits);

    if (stretchComparison > 0)
        return false;
    if (stretchComparison < 0)
        return true;

    styleComparison = styleScore(m_fontTraits, candidateTraits) -
        styleScore(m_fontTraits, currentTraits);

    if (styleComparison > 0)
        return true;
    if (styleComparison < 0)
        return false;

    weightComparison = weightScore(m_fontTraits, candidateTraits) -
        weightScore(m_fontTraits, currentTraits);

    if (weightComparison > 0)
        return true;

    return false;
}

}