LibPDF: Create basic object structure

This commit is the start of LibPDF, and introduces some basic structure
objects. This emulates LibJS's Value structure, where Value is a simple
class that can contain a pointer to a more complex Object class with
more data. All of the basic PDF objects have a representation.

Userland/Libraries/LibPDF/Object.cpp

@@ -0,0 +1,116 @@
+/*
+ * Copyright (c) 2021, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Hex.h>
+#include <LibPDF/Object.h>
+
+namespace PDF {
+
+static void append_indent(StringBuilder& builder, int indent)
+{
+    for (int i = 0; i < indent; i++)
+        builder.append("  ");
+}
+
+String StringObject::to_string(int) const
+{
+    if (is_binary())
+        return String::formatted("<{}>", encode_hex(string().bytes()).to_uppercase());
+    return String::formatted("({})", string());
+}
+
+String NameObject::to_string(int) const
+{
+    StringBuilder builder;
+    builder.appendff("/{}", this->name());
+    return builder.to_string();
+}
+
+String ArrayObject::to_string(int indent) const
+{
+    StringBuilder builder;
+    builder.append("[\n");
+    bool first = true;
+
+    for (auto& element : elements()) {
+        if (!first)
+            builder.append(",\n");
+        first = false;
+        append_indent(builder, indent + 1);
+        builder.appendff("{}", element.to_string(indent));
+    }
+
+    builder.append('\n');
+    append_indent(builder, indent);
+    builder.append(']');
+    return builder.to_string();
+}
+
+String DictObject::to_string(int indent) const
+{
+    StringBuilder builder;
+    builder.append("<<\n");
+    bool first = true;
+
+    for (auto& [key, value] : map()) {
+        if (!first)
+            builder.append(",\n");
+        first = false;
+        append_indent(builder, indent + 1);
+        builder.appendff("/{} ", key);
+        builder.appendff("{}", value.to_string(indent + 1));
+    }
+
+    builder.append('\n');
+    append_indent(builder, indent);
+    builder.append(">>");
+    return builder.to_string();
+}
+
+String StreamObject::to_string(int indent) const
+{
+    StringBuilder builder;
+    builder.append("stream\n");
+    append_indent(builder, indent);
+    builder.appendff("{}\n", dict()->to_string(indent + 1));
+    append_indent(builder, indent + 1);
+
+    auto string = encode_hex(bytes());
+    while (true) {
+        if (string.length() > 60) {
+            builder.appendff("{}\n", string.substring(0, 60));
+            append_indent(builder, indent);
+            string = string.substring(60);
+            continue;
+        }
+
+        builder.appendff("{}\n", string);
+        break;
+    }
+
+    append_indent(builder, indent);
+    builder.append("endstream");
+    return builder.to_string();
+}
+
+String IndirectValue::to_string(int indent) const
+{
+    StringBuilder builder;
+    builder.appendff("{} {} obj\n", index(), generation_index());
+    append_indent(builder, indent + 1);
+    builder.append(value().to_string(indent + 1));
+    builder.append('\n');
+    append_indent(builder, indent);
+    builder.append("endobj");
+    return builder.to_string();
+}
+
+String IndirectValueRef::to_string(int) const
+{
+    return String::formatted("{} {} R", index(), generation_index());
+}
+
+}