css.cpp

Go to the documentation of this file.

/*******************************************************************************
Copyright (c) 2021, Jan Koester jan.koester@gmx.net
All rights reserved.
 
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.
    * Neither the name of the <organization> nor the
      names of its contributors may be used to endorse or promote products
      derived from this software without specific prior written permission.
 
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*******************************************************************************/
 
#include "css.h"
#include "exception.h"
 
#include <algorithm>
#include <sstream>
 
namespace {
 
    bool isWhitespace(char c) {
        return c == ' ' || c == '\t' || c == '\n' || c == '\r';
    }
 
    std::string trim(const std::string &s) {
        size_t start = 0;
        while (start < s.size() && isWhitespace(s[start])) ++start;
        size_t end = s.size();
        while (end > start && isWhitespace(s[end - 1])) --end;
        return s.substr(start, end - start);
    }
 
    // Strips a trailing "!important" (case-insensitive, any whitespace
    // before the "!") from a CSS declaration value, returning whether it was
    // present. CSSDeclaration's parser captures "!important" as ordinary
    // trailing text with no special handling, so a caller comparing/storing
    // plain property values needs this to see past it.
    bool stripImportant(std::string &value) {
        size_t bang = value.rfind('!');
        if (bang == std::string::npos) return false;
 
        size_t start = value.find_first_not_of(" \t\n\r", bang + 1);
        size_t end = value.find_last_not_of(" \t\n\r");
        if (start == std::string::npos || end == std::string::npos || start > end) return false;
 
        std::string tail = value.substr(start, end - start + 1);
        std::transform(tail.begin(), tail.end(), tail.begin(),
            [](unsigned char c) { return std::tolower(c); });
        if (tail != "important") return false;
 
        size_t valEnd = bang;
        while (valEnd > 0 && isWhitespace(value[valEnd - 1])) --valEnd;
        value.resize(valEnd);
        return true;
    }
 
    // A combinator-stripped compound selector like "div.card.featured#hero"
    // broken into its optional bare tag (lowercased), every ".class" token,
    // and an optional "#id" token.
    struct CompoundParts {
        std::string tag;
        std::vector<std::string> classes;
        std::string id;
    };
 
    CompoundParts parseCompoundSelector(const std::string &matchSel) {
        CompoundParts result;
        size_t pos = 0;
        while (pos < matchSel.size()) {
            char c = matchSel[pos];
            if (c == '.' || c == '#') {
                size_t next = matchSel.find_first_of(".#", pos + 1);
                size_t tokenEnd = (next == std::string::npos) ? matchSel.size() : next;
                std::string token = matchSel.substr(pos + 1, tokenEnd - (pos + 1));
                if (c == '.') result.classes.push_back(token);
                else result.id = token;
                pos = tokenEnd;
            } else {
                size_t next = matchSel.find_first_of(".#", pos);
                size_t tokenEnd = (next == std::string::npos) ? matchSel.size() : next;
                result.tag = matchSel.substr(pos, tokenEnd - pos);
                pos = tokenEnd;
            }
        }
        std::transform(result.tag.begin(), result.tag.end(), result.tag.begin(),
            [](unsigned char c) { return std::tolower(c); });
        return result;
    }
 
    // A selector matches only if EVERY part it specifies is present on this
    // element -- in particular every class listed, not just one. A bare tag
    // left over after stripping a combinator's ancestor part (e.g.
    // "[data-atom=header]>div" reduces to bare "div") can't be verified --
    // this matcher only ever sees the CURRENT element's own tag/class/id,
    // never its ancestor chain -- so treating it as a match would apply the
    // rule to literally every element of that tag in the whole document
    // (confirmed against a real scraped page where exactly this pattern
    // matched ~24,000 times and blew up an HTML-import's output to 36MB). A
    // class/id qualifier alongside the tag (e.g. "div.foo") is still
    // specific enough to check.
    bool compoundMatches(const CompoundParts &compound, bool hadCombinator,
                          const std::string &tag, const std::vector<std::string> &classes,
                          const std::string &id)
    {
        bool bareTagAfterCombinator = hadCombinator &&
            compound.classes.empty() && compound.id.empty() && !compound.tag.empty();
        bool specifiedSomething = !compound.tag.empty() || !compound.classes.empty() || !compound.id.empty();
        if (bareTagAfterCombinator || !specifiedSomething) return false;
 
        bool tagOk = compound.tag.empty() || compound.tag == tag;
        bool idOk = compound.id.empty() || compound.id == id;
        bool classesOk = true;
        for (const auto &cls : compound.classes) {
            if (std::find(classes.begin(), classes.end(), cls) == classes.end()) {
                classesOk = false;
                break;
            }
        }
        return tagOk && idOk && classesOk;
    }
 
    // Strips a trailing combinator (space/">"/"+"/"~") from `singleSel`,
    // returning the trailing compound selector text and whether a
    // combinator was actually present. Only the trailing compound is ever
    // checked -- this matcher never verifies ancestor/parent context.
    std::string stripCombinator(const std::string &singleSel, bool &hadCombinator) {
        std::string matchSel = singleSel;
        hadCombinator = false;
        size_t lastSep = matchSel.find_last_of(" >+~");
        if (lastSep != std::string::npos) {
            size_t mstart = matchSel.find_first_not_of(" >+~", lastSep);
            if (mstart != std::string::npos) {
                matchSel = matchSel.substr(mstart);
                hadCombinator = true;
            }
        }
        return matchSel;
    }
 
    // Selector syntax this matcher can't safely evaluate -- attribute
    // selectors and pseudo-classes/elements describe context (an ancestor's
    // attribute) or state (":hover") this matcher has no way to check, and
    // "*" would blanket-match every element. Guessing wrong here is worse
    // than not matching at all.
    bool hasUnsupportedSelectorSyntax(const std::string &matchSel) {
        return matchSel.find('[') != std::string::npos ||
               matchSel.find(':') != std::string::npos ||
               matchSel == "*";
    }
 
    // Finds the index of the ')' matching the '(' at `openPos` (which must
    // point at '(' itself), respecting nested parens. Returns npos if
    // unterminated.
    size_t findMatchingParen(const std::string &s, size_t openPos) {
        int depth = 0;
        for (size_t i = openPos; i < s.size(); ++i) {
            if (s[i] == '(') ++depth;
            else if (s[i] == ')') {
                --depth;
                if (depth == 0) return i;
            }
        }
        return std::string::npos;
    }
 
    // Splits `inner` (the text between "var(" and its matching ")") at the
    // first top-level comma (depth 0, not inside a nested "(...)") into a
    // trimmed custom-property name and a trimmed fallback ("" if no comma
    // is present, i.e. no fallback was given).
    void splitVarArgs(const std::string &inner, std::string &name, std::string &fallback) {
        int depth = 0;
        size_t commaPos = std::string::npos;
        for (size_t i = 0; i < inner.size(); ++i) {
            if (inner[i] == '(') ++depth;
            else if (inner[i] == ')') --depth;
            else if (inner[i] == ',' && depth == 0) { commaPos = i; break; }
        }
        std::string rawName = (commaPos == std::string::npos) ? inner : inner.substr(0, commaPos);
        std::string rawFallback = (commaPos == std::string::npos) ? "" : inner.substr(commaPos + 1);
        name = trim(rawName);
        fallback = trim(rawFallback);
    }
 
    // Substitutes every var(...) occurrence in `value`, recursively --
    // `resolving` (the set of custom-property names currently being
    // expanded on this call stack) provides cycle detection: a name already
    // in it resolves to "" instead of recursing forever.
    std::string substituteVars(const std::string &value,
                                const std::map<std::string,std::string> &customProperties,
                                std::set<std::string> &resolving)
    {
        if (value.find("var(") == std::string::npos) return value;
 
        std::string out;
        out.reserve(value.size());
        size_t pos = 0;
        while (pos < value.size()) {
            if (value.compare(pos, 4, "var(") == 0) {
                size_t openParen = pos + 3;
                size_t closeParen = findMatchingParen(value, openParen);
                if (closeParen == std::string::npos) {
                    // Malformed/unterminated var( -- leave the rest as-is
                    // rather than risk corrupting it.
                    out += value.substr(pos);
                    break;
                }
                std::string inner = value.substr(openParen + 1, closeParen - openParen - 1);
                std::string name, fallback;
                splitVarArgs(inner, name, fallback);
 
                std::string substitution;
                if (resolving.count(name)) {
                    substitution = ""; // cyclic reference -- invalid per spec
                } else {
                    auto it = customProperties.find(name);
                    if (it != customProperties.end()) {
                        resolving.insert(name);
                        substitution = substituteVars(it->second, customProperties, resolving);
                        resolving.erase(name);
                    } else if (!fallback.empty()) {
                        substitution = substituteVars(fallback, customProperties, resolving);
                    } else {
                        substitution = ""; // unresolvable, no fallback -- invalid per spec
                    }
                }
                out += substitution;
                pos = closeParen + 1;
            } else {
                out += value[pos];
                ++pos;
            }
        }
        return out;
    }
 
}
 
// --- CSSProperty ---
 
libhtmlpp::CSSProperty::CSSProperty() {}
 
libhtmlpp::CSSProperty::CSSProperty(const std::string &name, const std::string &value)
    : _Name(name), _Value(value) {}
 
libhtmlpp::CSSProperty::CSSProperty(const CSSProperty &prop)
    : _Name(prop._Name), _Value(prop._Value) {}
 
libhtmlpp::CSSProperty::~CSSProperty() {}
 
libhtmlpp::CSSProperty& libhtmlpp::CSSProperty::operator=(const CSSProperty &prop) {
    if (this != &prop) {
        _Name  = prop._Name;
        _Value = prop._Value;
    }
    return *this;
}
 
const std::string& libhtmlpp::CSSProperty::getName() const { return _Name; }
void libhtmlpp::CSSProperty::setName(const std::string &name) { _Name = name; }
 
const std::string& libhtmlpp::CSSProperty::getValue() const { return _Value; }
void libhtmlpp::CSSProperty::setValue(const std::string &value) { _Value = value; }
 
// --- CSSDeclaration ---
 
libhtmlpp::CSSDeclaration::CSSDeclaration() {}
 
libhtmlpp::CSSDeclaration::CSSDeclaration(const CSSDeclaration &decl)
    : _Properties(decl._Properties) {}
 
libhtmlpp::CSSDeclaration::~CSSDeclaration() {}
 
libhtmlpp::CSSDeclaration& libhtmlpp::CSSDeclaration::operator=(const CSSDeclaration &decl) {
    if (this != &decl) {
        _Properties = decl._Properties;
    }
    return *this;
}
 
void libhtmlpp::CSSDeclaration::addProperty(const std::string &name, const std::string &value) {
    std::string tname  = trim(name);
    std::string tvalue = trim(value);
 
    if (tname.empty()) return;
 
    for (auto &prop : _Properties) {
        if (prop.getName() == tname) {
            prop.setValue(tvalue);
            return;
        }
    }
    _Properties.emplace_back(tname, tvalue);
}
 
void libhtmlpp::CSSDeclaration::removeProperty(const std::string &name) {
    std::string tname = trim(name);
    _Properties.erase(
        std::remove_if(_Properties.begin(), _Properties.end(),
            [&tname](const CSSProperty &p) { return p.getName() == tname; }),
        _Properties.end()
    );
}
 
const libhtmlpp::CSSProperty* libhtmlpp::CSSDeclaration::getProperty(const std::string &name) const {
    std::string tname = trim(name);
    for (const auto &prop : _Properties) {
        if (prop.getName() == tname) return &prop;
    }
    return nullptr;
}
 
const std::vector<libhtmlpp::CSSProperty>& libhtmlpp::CSSDeclaration::getProperties() const {
    return _Properties;
}
 
std::string libhtmlpp::CSSDeclaration::serialize() const {
    std::string result;
    for (size_t i = 0; i < _Properties.size(); ++i) {
        result += _Properties[i].getName();
        result += ": ";
        result += _Properties[i].getValue();
        result += ";";
        if (i + 1 < _Properties.size()) result += " ";
    }
    return result;
}
 
void libhtmlpp::CSSDeclaration::parse(const std::string &input) {
    _Properties.clear();
 
    size_t pos = 0;
    size_t len = input.size();
 
    while (pos < len) {
        // Skip whitespace
        while (pos < len && isWhitespace(input[pos])) ++pos;
        if (pos >= len) break;
 
        // Find the colon separating property name from value
        size_t colon = input.find(':', pos);
        if (colon == std::string::npos) break;
 
        std::string propName = trim(input.substr(pos, colon - pos));
 
        pos = colon + 1;
 
        // Find the semicolon ending this declaration
        // Handle parentheses for functions like rgb(), url()
        size_t valueStart = pos;
        int parenDepth = 0;
        bool inSingleQuote = false;
        bool inDoubleQuote = false;
 
        while (pos < len) {
            char c = input[pos];
 
            if (inSingleQuote) {
                if (c == '\'') inSingleQuote = false;
                ++pos;
                continue;
            }
            if (inDoubleQuote) {
                if (c == '"') inDoubleQuote = false;
                ++pos;
                continue;
            }
 
            if (c == '\'') { inSingleQuote = true; ++pos; continue; }
            if (c == '"')  { inDoubleQuote = true; ++pos; continue; }
            if (c == '(')  { ++parenDepth; ++pos; continue; }
            if (c == ')')  { if (parenDepth > 0) --parenDepth; ++pos; continue; }
 
            if (c == ';' && parenDepth == 0) {
                break;
            }
            ++pos;
        }
 
        std::string propValue = trim(input.substr(valueStart, pos - valueStart));
 
        if (!propName.empty()) {
            addProperty(propName, propValue);
        }
 
        if (pos < len && input[pos] == ';') ++pos;
    }
}
 
void libhtmlpp::CSSDeclaration::clear() {
    _Properties.clear();
}
 
// --- CSSRule ---
 
libhtmlpp::CSSRule::CSSRule() {}
 
libhtmlpp::CSSRule::CSSRule(const std::string &selector)
    : _Selector(trim(selector)) {}
 
libhtmlpp::CSSRule::CSSRule(const CSSRule &rule)
    : _Selector(rule._Selector), _Declaration(rule._Declaration) {}
 
libhtmlpp::CSSRule::~CSSRule() {}
 
libhtmlpp::CSSRule& libhtmlpp::CSSRule::operator=(const CSSRule &rule) {
    if (this != &rule) {
        _Selector    = rule._Selector;
        _Declaration = rule._Declaration;
    }
    return *this;
}
 
const std::string& libhtmlpp::CSSRule::getSelector() const { return _Selector; }
void libhtmlpp::CSSRule::setSelector(const std::string &selector) { _Selector = trim(selector); }
 
libhtmlpp::CSSDeclaration& libhtmlpp::CSSRule::getDeclaration() { return _Declaration; }
const libhtmlpp::CSSDeclaration& libhtmlpp::CSSRule::getDeclaration() const { return _Declaration; }
 
std::string libhtmlpp::CSSRule::serialize(bool formatted) const {
    std::string result;
    if (formatted) {
        result += _Selector + " {\n";
        for (const auto &prop : _Declaration.getProperties()) {
            result += "    " + prop.getName() + ": " + prop.getValue() + ";\n";
        }
        result += "}";
    } else {
        result += _Selector + "{";
        result += _Declaration.serialize();
        result += "}";
    }
    return result;
}
 
// --- CSSStyleSheet ---
 
libhtmlpp::CSSStyleSheet::CSSStyleSheet() {}
 
libhtmlpp::CSSStyleSheet::CSSStyleSheet(const CSSStyleSheet &sheet)
    : _Rules(sheet._Rules) {}
 
libhtmlpp::CSSStyleSheet::~CSSStyleSheet() {}
 
libhtmlpp::CSSStyleSheet& libhtmlpp::CSSStyleSheet::operator=(const CSSStyleSheet &sheet) {
    if (this != &sheet) {
        _Rules = sheet._Rules;
    }
    return *this;
}
 
void libhtmlpp::CSSStyleSheet::_skipWhitespace(const std::string &input, size_t &pos) const {
    while (pos < input.size() && isWhitespace(input[pos])) ++pos;
}
 
void libhtmlpp::CSSStyleSheet::_skipComment(const std::string &input, size_t &pos) const {
    if (pos + 1 < input.size() && input[pos] == '/' && input[pos + 1] == '*') {
        pos += 2;
        while (pos + 1 < input.size()) {
            if (input[pos] == '*' && input[pos + 1] == '/') {
                pos += 2;
                return;
            }
            ++pos;
        }
        pos = input.size();
    }
}
 
void libhtmlpp::CSSStyleSheet::parse(const std::string &input) {
    _Rules.clear();
 
    size_t pos = 0;
    size_t len = input.size();
 
    while (pos < len) {
        _skipWhitespace(input, pos);
        if (pos >= len) break;
 
        // Skip comments
        if (pos + 1 < len && input[pos] == '/' && input[pos + 1] == '*') {
            _skipComment(input, pos);
            continue;
        }
 
        // Handle @-rules
        if (input[pos] == '@') {
            size_t atStart = pos;
 
            // Check for @-rules with blocks like @media, @keyframes, @supports, @font-face
            // and simple @-rules like @import, @charset
            size_t semiPos = input.find(';', pos);
            size_t bracePos = input.find('{', pos);
 
            if (bracePos != std::string::npos && (semiPos == std::string::npos || bracePos < semiPos)) {
                // Block @-rule: find matching closing brace
                std::string atSelector = trim(input.substr(atStart, bracePos - atStart));
                pos = bracePos + 1;
 
                int depth = 1;
                size_t blockStart = pos;
                while (pos < len && depth > 0) {
                    if (input[pos] == '{') ++depth;
                    else if (input[pos] == '}') --depth;
                    if (depth > 0) ++pos;
                }
 
                std::string blockContent = input.substr(blockStart, pos - blockStart);
                pos = (pos < len) ? pos + 1 : pos;
 
                // Parse nested rules inside the @-block
                CSSStyleSheet nested;
                nested.parse(blockContent);
 
                // Wrap each nested rule with the @-selector
                for (const auto &nestedRule : nested.getRules()) {
                    CSSRule rule;
                    rule.setSelector(atSelector + " " + nestedRule.getSelector());
                    for (const auto &prop : nestedRule.getDeclaration().getProperties()) {
                        rule.getDeclaration().addProperty(prop.getName(), prop.getValue());
                    }
                    _Rules.push_back(rule);
                }
 
                // If no nested rules (e.g. @font-face), store as single rule
                if (nested.getRuleCount() == 0 && !blockContent.empty()) {
                    CSSRule rule(atSelector);
                    rule.getDeclaration().parse(blockContent);
                    _Rules.push_back(rule);
                }
            } else if (semiPos != std::string::npos) {
                // Simple @-rule ending with semicolon (e.g. @import, @charset)
                std::string atRule = trim(input.substr(atStart, semiPos - atStart));
                CSSRule rule(atRule);
                _Rules.push_back(rule);
                pos = semiPos + 1;
            } else {
                // Malformed @-rule, skip
                ++pos;
            }
            continue;
        }
 
        // Standard rule: find selector then { declarations }
        size_t braceOpen = input.find('{', pos);
        if (braceOpen == std::string::npos) break;
 
        std::string selector = trim(input.substr(pos, braceOpen - pos));
        pos = braceOpen + 1;
 
        // Find matching closing brace
        int depth = 1;
        size_t declStart = pos;
        while (pos < len && depth > 0) {
            if (pos + 1 < len && input[pos] == '/' && input[pos + 1] == '*') {
                _skipComment(input, pos);
                continue;
            }
            if (input[pos] == '{') ++depth;
            else if (input[pos] == '}') --depth;
            if (depth > 0) ++pos;
        }
 
        std::string declarations = input.substr(declStart, pos - declStart);
        pos = (pos < len) ? pos + 1 : pos;
 
        if (!selector.empty()) {
            CSSRule rule(selector);
            rule.getDeclaration().parse(declarations);
            _Rules.push_back(rule);
        }
    }
}
 
void libhtmlpp::CSSStyleSheet::addRule(const CSSRule &rule) {
    _Rules.push_back(rule);
}
 
void libhtmlpp::CSSStyleSheet::removeRule(size_t index) {
    if (index < _Rules.size()) {
        _Rules.erase(_Rules.begin() + static_cast<std::ptrdiff_t>(index));
    }
}
 
const libhtmlpp::CSSRule* libhtmlpp::CSSStyleSheet::getRule(size_t index) const {
    if (index < _Rules.size()) return &_Rules[index];
    return nullptr;
}
 
size_t libhtmlpp::CSSStyleSheet::getRuleCount() const {
    return _Rules.size();
}
 
const std::vector<libhtmlpp::CSSRule>& libhtmlpp::CSSStyleSheet::getRules() const {
    return _Rules;
}
 
std::string libhtmlpp::CSSStyleSheet::serialize(bool formatted) const {
    std::string result;
    for (size_t i = 0; i < _Rules.size(); ++i) {
        result += _Rules[i].serialize(formatted);
        if (formatted) result += "\n";
        if (i + 1 < _Rules.size() && formatted) result += "\n";
    }
    return result;
}
 
void libhtmlpp::CSSStyleSheet::clear() {
    _Rules.clear();
}
 
libhtmlpp::CSSDeclaration libhtmlpp::CSSStyleSheet::parseInlineStyle(const std::string &style) {
    CSSDeclaration decl;
    decl.parse(style);
    return decl;
}
 
bool libhtmlpp::CSSStyleSheet::approximateSelectorMatch(const std::string &selector,
                                                          const std::string &tag,
                                                          const std::vector<std::string> &classes,
                                                          const std::string &id)
{
    std::string tagLower = tag;
    std::transform(tagLower.begin(), tagLower.end(), tagLower.begin(),
        [](unsigned char c) { return std::tolower(c); });
 
    std::istringstream selStream(selector);
    std::string singleSel;
    while (std::getline(selStream, singleSel, ',')) {
        size_t start = singleSel.find_first_not_of(" \t\n\r");
        if (start == std::string::npos) continue;
        size_t end = singleSel.find_last_not_of(" \t\n\r");
        singleSel = singleSel.substr(start, end - start + 1);
 
        bool hadCombinator = false;
        std::string matchSel = stripCombinator(singleSel, hadCombinator);
        if (hasUnsupportedSelectorSyntax(matchSel)) continue;
 
        CompoundParts compound = parseCompoundSelector(matchSel);
        if (compoundMatches(compound, hadCombinator, tagLower, classes, id)) return true;
    }
    return false;
}
 
void libhtmlpp::CSSStyleSheet::collectApproximateMatches(
    const std::string &tag,
    const std::string &cssClass,
    const std::string &id,
    std::map<std::string,std::string> &props,
    std::string &mediaRules,
    std::set<std::string> &seenMediaBlocks) const
{
    std::vector<std::string> classes;
    if (!cssClass.empty()) {
        std::istringstream iss(cssClass);
        std::string cls;
        while (iss >> cls) classes.push_back(cls);
    }
 
    // Property names already present in @p props when we're called (e.g.
    // the element's own inline style, set by the caller before calling this)
    // outrank a plain stylesheet rule, unless that rule is "!important" --
    // matching real cascade precedence.
    std::set<std::string> inlineKeys;
    for (const auto &kv : props) inlineKeys.insert(kv.first);
    std::set<std::string> importantKeys;
 
    for (const auto &rule : _Rules) {
        const std::string &sel = rule.getSelector();
        if (sel.empty()) continue;
 
        bool isAtRule = sel[0] == '@';
        std::string atWrapper;
        std::string innerSel;
 
        if (isAtRule) {
            size_t parenDepth = 0;
            size_t splitPos = std::string::npos;
            for (size_t i = 0; i < sel.size(); ++i) {
                if (sel[i] == '(') ++parenDepth;
                else if (sel[i] == ')') {
                    if (parenDepth > 0) --parenDepth;
                    if (parenDepth == 0) { splitPos = i + 1; break; }
                }
            }
            if (splitPos != std::string::npos && splitPos < sel.size()) {
                atWrapper = sel.substr(0, splitPos);
                size_t innerStart = sel.find_first_not_of(" \t\n\r", splitPos);
                if (innerStart != std::string::npos) innerSel = sel.substr(innerStart);
            }
            if (innerSel.empty()) continue;
        }
 
        const std::string &matchTarget = isAtRule ? innerSel : sel;
 
        std::istringstream selStream(matchTarget);
        std::string singleSel;
        while (std::getline(selStream, singleSel, ',')) {
            size_t start = singleSel.find_first_not_of(" \t\n\r");
            if (start == std::string::npos) continue;
            size_t end = singleSel.find_last_not_of(" \t\n\r");
            singleSel = singleSel.substr(start, end - start + 1);
 
            if (!approximateSelectorMatch(singleSel, tag, classes, id)) continue;
 
            if (isAtRule) {
                // The same @media block, once present anywhere in the final
                // output, applies document-wide regardless of which element
                // it's attached to -- so including it more than once across
                // many matching elements is pure bloat, not a correctness
                // requirement.
                std::string block = atWrapper + " { " + singleSel + " { ";
                for (const auto &prop : rule.getDeclaration().getProperties()) {
                    block += prop.getName() + ": " + prop.getValue() + "; ";
                }
                block += "} } ";
                if (seenMediaBlocks.insert(block).second) {
                    mediaRules += block;
                }
            } else {
                for (const auto &prop : rule.getDeclaration().getProperties()) {
                    std::string value = prop.getValue();
                    bool isImportant = stripImportant(value);
                    const std::string &key = prop.getName();
 
                    // Inline style and an already-!important value both
                    // outrank a later plain rule; a later rule of equal
                    // priority (both plain, or both !important) wins --
                    // approximating the real cascade's "last rule of
                    // equal-or-higher precedence wins" without computing
                    // full selector specificity.
                    if (inlineKeys.count(key) && !isImportant) continue;
                    if (importantKeys.count(key) && !isImportant) continue;
 
                    props[key] = value;
                    if (isImportant) importantKeys.insert(key);
                }
            }
        }
    }
}
 
std::string libhtmlpp::resolveCSSVariables(const std::string &value,
                                             const std::map<std::string,std::string> &customProperties)
{
    std::set<std::string> resolving;
    return substituteVars(value, customProperties, resolving);
}