LibPDF: Add a basic parser and Document structure

This commit adds a parser as well as the Reader class, which serves
as a utility to aid in reading the PDF both forwards and in reverse.
The parser currently is capable of reading xref tables, as well as
all values. We don't really do anything with any of this information,
however.

1 files changed, 96 insertions, 0 deletions

diff --git a/Userland/Libraries/LibPDF/XRefTable.h b/Userland/Libraries/LibPDF/XRefTable.h
new file mode 100644
index 0000000000..fe084218a3
--- /dev/null
+++ b/Userland/Libraries/LibPDF/XRefTable.h
@@ -0,0 +1,96 @@
+/*
+ * Copyright (c) 2021, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Format.h>
+
+namespace PDF {
+
+struct XRefEntry {
+    long byte_offset { -1L };
+    u16 generation_number { 0 };
+    bool in_use { false };
+};
+
+struct XRefSection {
+    int starting_index;
+    int count;
+    Vector<XRefEntry> entries;
+};
+
+class XRefTable {
+public:
+    void add_section(const XRefSection& section)
+    {
+        m_entries.ensure_capacity(section.starting_index + section.count);
+
+        for (int i = static_cast<int>(m_entries.size()); i < section.starting_index; i++)
+            m_entries.append(XRefEntry {});
+
+        for (auto& entry : section.entries)
+            m_entries.append(entry);
+    }
+
+    [[nodiscard]] ALWAYS_INLINE bool has_object(size_t index) const
+    {
+        return index < m_entries.size() && m_entries[index].byte_offset != -1;
+    }
+
+    [[nodiscard]] ALWAYS_INLINE long byte_offset_for_object(size_t index) const
+    {
+        VERIFY(has_object(index));
+        return m_entries[index].byte_offset;
+    }
+
+    [[nodiscard]] ALWAYS_INLINE u16 generation_number_for_object(size_t index) const
+    {
+        VERIFY(has_object(index));
+        return m_entries[index].generation_number;
+    }
+
+    [[nodiscard]] ALWAYS_INLINE bool is_object_in_use(size_t index) const
+    {
+        VERIFY(has_object(index));
+        return m_entries[index].in_use;
+    }
+
+private:
+    friend struct AK::Formatter<PDF::XRefTable>;
+
+    Vector<XRefEntry> m_entries;
+};
+
+}
+
+namespace AK {
+
+template<>
+struct Formatter<PDF::XRefEntry> : Formatter<StringView> {
+    void format(FormatBuilder& builder, const PDF::XRefEntry& entry)
+    {
+        Formatter<StringView>::format(builder,
+            String::formatted("XRefEntry {{ offset={} generation={} used={} }}",
+                entry.byte_offset,
+                entry.generation_number,
+                entry.in_use));
+    }
+};
+
+template<>
+struct Formatter<PDF::XRefTable> : Formatter<StringView> {
+    void format(FormatBuilder& format_builder, const PDF::XRefTable& table)
+    {
+        StringBuilder builder;
+        builder.append("XRefTable {");
+        for (auto& entry : table.m_entries)
+            builder.appendff("\n  {}", entry);
+        builder.append("\n}");
+        Formatter<StringView>::format(format_builder, builder.to_string());
+    }
+};
+
+}