LibWeb: Add a class to represent the "source set" concept from HTML

Also comes with a little extra CSS parser helper for parsing "sizes"
attributes in images.

Userland/Libraries/LibWeb/HTML/SourceSet.cpp

@@ -0,0 +1,395 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Function.h>
+#include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/HTML/SourceSet.h>
+#include <LibWeb/Infra/CharacterTypes.h>
+
+namespace Web::HTML {
+
+SourceSet::SourceSet()
+    : m_source_size(CSS::Length::make_auto())
+{
+}
+
+bool SourceSet::is_empty() const
+{
+    return m_sources.is_empty();
+}
+
+// https://html.spec.whatwg.org/multipage/images.html#select-an-image-source-from-a-source-set
+ImageSourceAndPixelDensity SourceSet::select_an_image_source()
+{
+    // FIXME: 1. If an entry b in sourceSet has the same associated pixel density descriptor as an earlier entry a in sourceSet,
+    //           then remove entry b.
+    //           Repeat this step until none of the entries in sourceSet have the same associated pixel density descriptor
+    //           as an earlier entry.
+
+    // FIXME: 2. In an implementation-defined manner, choose one image source from sourceSet. Let this be selectedSource.
+
+    // FIXME: 3. Return selectedSource and its associated pixel density.
+
+    return { m_sources.first(), 1.0f };
+}
+
+static StringView collect_a_sequence_of_code_points(Function<bool(u32 code_point)> condition, StringView input, size_t& position)
+{
+    // 1. Let result be the empty string.
+    // 2. While position doesn’t point past the end of input and the code point at position within input meets the condition condition:
+    //    1. Append that code point to the end of result.
+    //    2. Advance position by 1.
+    // 3. Return result.
+
+    size_t start = position;
+    while (position < input.length() && condition(input[position]))
+        ++position;
+    return input.substring_view(start, position - start);
+}
+
+// https://html.spec.whatwg.org/multipage/images.html#parse-a-srcset-attribute
+SourceSet parse_a_srcset_attribute(StringView input)
+{
+    // 1. Let input be the value passed to this algorithm.
+
+    // 2. Let position be a pointer into input, initially pointing at the start of the string.
+    size_t position = 0;
+
+    // 3. Let candidates be an initially empty source set.
+    SourceSet candidates;
+
+splitting_loop:
+    // 4. Splitting loop: Collect a sequence of code points that are ASCII whitespace or U+002C COMMA characters from input given position.
+    //    If any U+002C COMMA characters were collected, that is a parse error.
+    collect_a_sequence_of_code_points(
+        [](u32 code_point) {
+            if (code_point == ',') {
+                // FIXME: Report a parse error somehow.
+                return true;
+            }
+            return Infra::is_ascii_whitespace(code_point);
+        },
+        input, position);
+
+    // 5. If position is past the end of input, return candidates.
+    if (position >= input.length()) {
+        return candidates;
+    }
+
+    // 6. Collect a sequence of code points that are not ASCII whitespace from input given position, and let that be url.
+    auto url = collect_a_sequence_of_code_points(
+        [](u32 code_point) { return !Infra::is_ascii_whitespace(code_point); },
+        input, position);
+
+    // 7. Let descriptors be a new empty list.
+    Vector<String> descriptors;
+
+    // 8. If url ends with U+002C (,), then:
+    if (url.ends_with(',')) {
+        // 1. Remove all trailing U+002C COMMA characters from url. If this removed more than one character, that is a parse error.
+        while (url.ends_with(','))
+            url = url.substring_view(0, url.length() - 1);
+    }
+    // Otherwise:
+    else {
+        // 1. Descriptor tokenizer: Skip ASCII whitespace within input given position.
+        collect_a_sequence_of_code_points(
+            [](u32 code_point) { return Infra::is_ascii_whitespace(code_point); },
+            input, position);
+
+        // 2. Let current descriptor be the empty string.
+        StringBuilder current_descriptor;
+
+        enum class State {
+            InDescriptor,
+            InParens,
+            AfterDescriptor,
+        };
+        // 3. Let state be in descriptor.
+        auto state = State::InDescriptor;
+
+        // 4. Let c be the character at position. Do the following depending on the value of state.
+        //    For the purpose of this step, "EOF" is a special character representing that position is past the end of input.
+        for (;;) {
+            Optional<u32> c;
+            if (position < input.length()) {
+                c = input[position];
+            }
+
+            switch (state) {
+            // - In descriptor
+            case State::InDescriptor:
+                // Do the following, depending on the value of c:
+
+                // - ASCII whitespace
+                if (c.has_value() && Infra::is_ascii_whitespace(c.value())) {
+                    // If current descriptor is not empty, append current descriptor to descriptors and let current descriptor be the empty string.
+                    if (!current_descriptor.is_empty()) {
+                        descriptors.append(current_descriptor.to_string().release_value_but_fixme_should_propagate_errors());
+                    }
+                    // Set state to after descriptor.
+                    state = State::AfterDescriptor;
+                }
+                // U+002C COMMA (,)
+                else if (c.has_value() && c.value() == ',') {
+                    // Advance position to the next character in input.
+                    position += 1;
+
+                    // If current descriptor is not empty, append current descriptor to descriptors.
+                    if (!current_descriptor.is_empty()) {
+                        descriptors.append(current_descriptor.to_string().release_value_but_fixme_should_propagate_errors());
+                    }
+
+                    // Jump to the step labeled descriptor parser.
+                    goto descriptor_parser;
+                }
+
+                // U+0028 LEFT PARENTHESIS (()
+                else if (c.has_value() && c.value() == '(') {
+                    // Append c to current descriptor.
+                    current_descriptor.try_append_code_point(c.value()).release_value_but_fixme_should_propagate_errors();
+
+                    // Set state to in parens.
+                    state = State::InParens;
+                }
+                // EOF
+                else if (!c.has_value()) {
+                    // If current descriptor is not empty, append current descriptor to descriptors.
+                    if (!current_descriptor.is_empty()) {
+                        descriptors.append(current_descriptor.to_string().release_value_but_fixme_should_propagate_errors());
+                    }
+
+                    // Jump to the step labeled descriptor parser.
+                    goto descriptor_parser;
+                }
+                // Anything else
+                else {
+                    // Append c to current descriptor.
+                    current_descriptor.try_append_code_point(c.value()).release_value_but_fixme_should_propagate_errors();
+                }
+                break;
+
+                // - In parens
+            case State::InParens:
+                // Do the following, depending on the value of c:
+                // U+0029 RIGHT PARENTHESIS ())
+                if (c.has_value() && c.value() == ')') {
+                    // Append c to current descriptor.
+                    current_descriptor.try_append_code_point(c.value()).release_value_but_fixme_should_propagate_errors();
+                    // Set state to in descriptor.
+                    state = State::InDescriptor;
+                }
+                // EOF
+                else if (!c.has_value()) {
+                    // Append current descriptor to descriptors.
+                    descriptors.append(current_descriptor.to_string().release_value_but_fixme_should_propagate_errors());
+
+                    // Jump to the step labeled descriptor parser.
+                    goto descriptor_parser;
+                }
+                // Anything else
+                else {
+                    // Append c to current descriptor.
+                    current_descriptor.try_append_code_point(c.value()).release_value_but_fixme_should_propagate_errors();
+                }
+                break;
+
+                // - After descriptor
+            case State::AfterDescriptor:
+                // Do the following, depending on the value of c:
+                // ASCII whitespace
+                if (c.has_value() && Infra::is_ascii_whitespace(c.value())) {
+                    // Stay in this state.
+                }
+                // EOF
+                else if (!c.has_value()) {
+                    // Jump to the step labeled descriptor parser.
+                    goto descriptor_parser;
+                }
+                // Anything else
+                else {
+                    // Set state to in descriptor.
+                    state = State::InDescriptor;
+                    // Set position to the previous character in input.
+                    position -= 1;
+                }
+                break;
+            }
+            // Advance position to the next character in input. Repeat this step.
+            position += 1;
+        }
+    }
+descriptor_parser:
+    // 9. Descriptor parser: Let error be no.
+    bool error = false;
+
+    // 10. Let width be absent.
+    Optional<int> width;
+
+    // 11. Let density be absent.
+    Optional<float> density;
+
+    // 12. Let future-compat-h be absent.
+    Optional<int> future_compat_h;
+
+    // 13. For each descriptor in descriptors, run the appropriate set of steps from the following list:
+    for (auto& descriptor : descriptors) {
+        auto last_character = descriptor.bytes_as_string_view().bytes().last();
+        auto descriptor_without_last_character = descriptor.bytes_as_string_view().substring_view(0, descriptor.bytes_as_string_view().length() - 1);
+
+        auto as_int = descriptor_without_last_character.to_int<i32>();
+        auto as_float = descriptor_without_last_character.to_float();
+
+        // - If the descriptor consists of a valid non-negative integer followed by a U+0077 LATIN SMALL LETTER W character
+        if (last_character == 'w' && as_int.has_value()) {
+            // NOOP: 1. If the user agent does not support the sizes attribute, let error be yes.
+
+            // 2. If width and density are not both absent, then let error be yes.
+
+            if (width.has_value() || density.has_value()) {
+                error = true;
+            }
+
+            // FIXME: 3. Apply the rules for parsing non-negative integers to the descriptor.
+            //           If the result is zero, let error be yes. Otherwise, let width be the result.
+            width = as_int.value();
+        }
+
+        // - If the descriptor consists of a valid floating-point number followed by a U+0078 LATIN SMALL LETTER X character
+        else if (last_character == 'x' && as_float.has_value()) {
+            // 1. If width, density and future-compat-h are not all absent, then let error be yes.
+            if (width.has_value() || density.has_value() || future_compat_h.has_value()) {
+                error = true;
+            }
+
+            // FIXME: 2. Apply the rules for parsing floating-point number values to the descriptor.
+            //           If the result is less than zero, let error be yes. Otherwise, let density be the result.
+            density = as_float.value();
+        }
+        // - If the descriptor consists of a valid non-negative integer followed by a U+0068 LATIN SMALL LETTER H character
+        else if (last_character == 'h' && as_int.has_value()) {
+            // This is a parse error.
+            // 1. If future-compat-h and density are not both absent, then let error be yes.
+            if (future_compat_h.has_value() || density.has_value()) {
+                error = true;
+            }
+            // FIXME: 2. Apply the rules for parsing non-negative integers to the descriptor.
+            //           If the result is zero, let error be yes. Otherwise, let future-compat-h be the result.
+            future_compat_h = as_int.value();
+        }
+        // - Anything else
+        else {
+            // Let error be yes.
+            error = true;
+        }
+    }
+
+    // 14. If future-compat-h is not absent and width is absent, let error be yes.
+    if (future_compat_h.has_value() && !width.has_value()) {
+        error = true;
+    }
+
+    // 15. If error is still no, then append a new image source to candidates whose URL is url,
+    //     associated with a width width if not absent and a pixel density density if not absent.
+    //     Otherwise, there is a parse error.
+    if (!error) {
+        ImageSource source;
+        source.url = String::from_utf8(url).release_value_but_fixme_should_propagate_errors();
+        if (width.has_value())
+            source.descriptor = ImageSource::WidthDescriptorValue { width.value() };
+        else if (density.has_value())
+            source.descriptor = ImageSource::PixelDensityDescriptorValue { density.value() };
+        candidates.m_sources.append(move(source));
+    }
+
+    // 16. Return to the step labeled splitting loop.
+    goto splitting_loop;
+}
+
+// https://html.spec.whatwg.org/multipage/images.html#parse-a-sizes-attribute
+CSS::Length parse_a_sizes_attribute(DOM::Document const& document, StringView sizes)
+{
+    auto css_parser = CSS::Parser::Parser::create(CSS::Parser::ParsingContext { document }, sizes).release_value_but_fixme_should_propagate_errors();
+    return css_parser.parse_as_sizes_attribute();
+}
+
+// https://html.spec.whatwg.org/multipage/images.html#create-a-source-set
+SourceSet SourceSet::create(DOM::Document const& document, String default_source, String srcset, String sizes)
+{
+    // 1. Let source set be an empty source set.
+    SourceSet source_set;
+
+    // 2. If srcset is not an empty string, then set source set to the result of parsing srcset.
+    if (!srcset.is_empty())
+        source_set = parse_a_srcset_attribute(srcset);
+
+    // 3. Let source size be the result of parsing sizes.
+    auto source_size = parse_a_sizes_attribute(document, sizes);
+
+    // 4. If default source is not the empty string and source set does not contain an image source
+    //    with a pixel density descriptor value of 1, and no image source with a width descriptor,
+    //    append default source to source set.
+    if (!default_source.is_empty()) {
+        bool contains_image_source_with_pixel_density_descriptor_value_of_1 = false;
+        bool contains_image_source_with_width_descriptor = false;
+        for (auto& source : source_set.m_sources) {
+            if (source.descriptor.has<ImageSource::PixelDensityDescriptorValue>()) {
+                if (source.descriptor.get<ImageSource::PixelDensityDescriptorValue>().value == 1.0f)
+                    contains_image_source_with_pixel_density_descriptor_value_of_1 = true;
+            }
+            if (source.descriptor.has<ImageSource::WidthDescriptorValue>())
+                contains_image_source_with_width_descriptor = true;
+        }
+        if (!contains_image_source_with_pixel_density_descriptor_value_of_1 && !contains_image_source_with_width_descriptor)
+            source_set.m_sources.append({ .url = default_source, .descriptor = {} });
+    }
+
+    // 5. Normalize the source densities of source set.
+    source_set.normalize_source_densities();
+
+    // 6. Return source set.
+    return source_set;
+}
+
+// https://html.spec.whatwg.org/multipage/images.html#normalise-the-source-densities
+void SourceSet::normalize_source_densities()
+{
+    // 1. Let source size be source set's source size.
+    auto source_size = m_source_size;
+
+    // 2. For each image source in source set:
+    for (auto& image_source : m_sources) {
+        // 1. If the image source has a pixel density descriptor, continue to the next image source.
+        if (image_source.descriptor.has<ImageSource::PixelDensityDescriptorValue>())
+            continue;
+
+        // 2. Otherwise, if the image source has a width descriptor,
+        //    replace the width descriptor with a pixel density descriptor
+        //    with a value of the width descriptor value divided by the source size and a unit of x.
+        if (image_source.descriptor.has<ImageSource::WidthDescriptorValue>()) {
+            auto& width_descriptor = image_source.descriptor.get<ImageSource::WidthDescriptorValue>();
+            if (source_size.is_absolute()) {
+                image_source.descriptor = ImageSource::PixelDensityDescriptorValue {
+                    .value = (width_descriptor.value / source_size.absolute_length_to_px()).value()
+                };
+            } else {
+                dbgln("FIXME: Handle relative sizes: {}", source_size);
+                image_source.descriptor = ImageSource::PixelDensityDescriptorValue {
+                    .value = 1,
+                };
+            }
+        }
+
+        // 3. Otherwise, give the image source a pixel density descriptor of 1x.
+        else {
+            image_source.descriptor = ImageSource::PixelDensityDescriptorValue {
+                .value = 1.0f
+            };
+        }
+    }
+}
+}