AK: Move the LEB128 logic to AK and make it usable with InputStream

AK/LEB128.h

@@ -0,0 +1,85 @@
+/*
+ * Copyright (c) 2020-2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Stream.h>
+#include <AK/Types.h>
+
+namespace AK {
+
+struct LEB128 {
+    template<typename StreamT>
+    static bool read_unsigned(StreamT& stream, size_t& result)
+    {
+        [[maybe_unused]] size_t backup_offset = 0;
+        if constexpr (requires { stream.offset(); })
+            backup_offset = stream.offset();
+        InputStream& input_stream { stream };
+
+        result = 0;
+        size_t num_bytes = 0;
+        while (true) {
+            if (input_stream.unreliable_eof()) {
+                if constexpr (requires { stream.seek(backup_offset); })
+                    stream.seek(backup_offset);
+                input_stream.set_fatal_error();
+                return false;
+            }
+
+            u8 byte = 0;
+            input_stream >> byte;
+            if (input_stream.has_any_error())
+                return false;
+
+            result = (result) | (static_cast<size_t>(byte & ~(1 << 7)) << (num_bytes * 7));
+            if (!(byte & (1 << 7)))
+                break;
+            ++num_bytes;
+        }
+
+        return true;
+    }
+
+    template<typename StreamT>
+    static bool read_signed(StreamT& stream, ssize_t& result)
+    {
+        [[maybe_unused]] size_t backup_offset = 0;
+        if constexpr (requires { stream.offset(); })
+            backup_offset = stream.offset();
+        InputStream& input_stream { stream };
+
+        result = 0;
+        size_t num_bytes = 0;
+        u8 byte = 0;
+
+        do {
+            if (input_stream.unreliable_eof()) {
+                if constexpr (requires { stream.seek(backup_offset); })
+                    stream.seek(backup_offset);
+                input_stream.set_fatal_error();
+                return false;
+            }
+
+            input_stream >> byte;
+            if (input_stream.has_any_error())
+                return false;
+            result = (result) | (static_cast<size_t>(byte & ~(1 << 7)) << (num_bytes * 7));
+            ++num_bytes;
+        } while (byte & (1 << 7));
+
+        if (num_bytes * 7 < sizeof(size_t) * 4 && (byte & 0x40)) {
+            // sign extend
+            result |= ((size_t)(-1) << (num_bytes * 7));
+        }
+
+        return true;
+    }
+};
+
+}
+
+using AK::LEB128;