libcamera: flags: Add type-safe enum-based flags

Add a Flags template class that provide type-safe bitwise operators on
enum values. This allows using enum types for bit fields, without giving
away type-safety as usually done when storing combined flags in integer
variables.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

2 files changed, 193 insertions, 0 deletions

diff --git a/src/libcamera/base/flags.cpp b/src/libcamera/base/flags.cpp
new file mode 100644
index 00000000..3e4320ac
--- /dev/null
+++ b/src/libcamera/base/flags.cpp
@@ -0,0 +1,192 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2020, Google Inc.
+ *
+ * flags.cpp - Type-safe enum-based bitfields
+ */
+
+#include <libcamera/base/flags.h>
+
+/**
+ * \file base/flags.h
+ * \brief Enum-based bit fields
+ */
+
+namespace libcamera {
+
+/**
+ * \class Flags
+ * \brief Type-safe container for enum-based bitfields
+ *
+ * The Flags template class provides type-safe bitwise operators on enum values.
+ * It allows using enum types for bitfields, while preventing unsafe casts from
+ * integer types and mixing of flags from different enum types.
+ *
+ * To use the Flags class, declare an enum containing the desired bit flags, and
+ * use the Flags<enum> class to store bitfields based on the enum. If bitwise
+ * operators on the underlying enum are also desired, they can be enabled with
+ * the LIBCAMERA_FLAGS_ENABLE_OPERATORS(enum) macro.
+ */
+
+/**
+ * \typedef Flags::Type
+ * \brief The underlying data type of the enum
+ */
+
+/**
+ * \fn Flags::Flags()
+ * \brief Construct a Flags instance with a zero value
+ */
+
+/**
+ * \fn Flags::Flags(E flag)
+ * \brief Construct a Flags instance storing the \a flag
+ * \param[in] flag The initial value
+ */
+
+/**
+ * \fn Flags &Flags::operator&=(E flag)
+ * \brief Store the bitwise AND of this Flags and the \a flag in this Flags
+ * \param[in] flag The second operand
+ * \return A reference to this Flags
+ */
+
+/**
+ * \fn Flags &Flags::operator&=(Flags other)
+ * \brief Store the bitwise AND of this Flags and the \a other Flags in this Flags
+ * \param[in] other The second operand
+ * \return A reference to this Flags
+ */
+
+/**
+ * \fn Flags &Flags::operator|=(E flag)
+ * \brief Store the bitwise OR of this Flags and the \a flag in this Flags
+ * \param[in] flag The second operand
+ * \return A reference to this Flags
+ */
+
+/**
+ * \fn Flags &Flags::operator|=(Flags other)
+ * \brief Store the bitwise OR of this Flags and the \a other Flags in this Flags
+ * \param[in] other The second operand
+ * \return A reference to this Flags
+ */
+
+/**
+ * \fn Flags &Flags::operator^=(E flag)
+ * \brief Store the bitwise XOR of this Flags and the \a flag in this Flags
+ * \param[in] flag The second operand
+ * \return A reference to this Flags
+ */
+
+/**
+ * \fn Flags &Flags::operator^=(Flags other)
+ * \brief Store the bitwise XOR of this Flags and the \a other Flags in this Flags
+ * \param[in] other The second operand
+ * \return A reference to this Flags
+ */
+
+/**
+ * \fn bool Flags::operator==(E flag)
+ * \brief Compare flags for equality
+ * \param[in] flag The second operand
+ * \return True if the Flags and \a flag are equal, false otherwise
+ */
+
+/**
+ * \fn bool Flags::operator==(Flags other)
+ * \brief Compare flags for equality
+ * \param[in] other The second operand
+ * \return True if the Flags and \a other are equal, false otherwise
+ */
+
+/**
+ * \fn bool Flags::operator!=(E flag)
+ * \brief Compare flags for non-equality
+ * \param[in] flag The second operand
+ * \return True if the Flags and \a flag are not equal, false otherwise
+ */
+
+/**
+ * \fn bool Flags::operator!=(Flags other)
+ * \brief Compare flags for non-equality
+ * \param[in] other The second operand
+ * \return True if the Flags and \a other are not equal, false otherwise
+ */
+
+/**
+ * \fn Flags::operator Type() const
+ * \brief Convert the Flags to the underlying integer type
+ * \return The Flags value as an integer
+ */
+
+/**
+ * \fn Flags::operator bool() const
+ * \brief Convert the Flags to a boolean
+ * \return True if at least one flag is set, false otherwise
+ */
+
+/**
+ * \fn Flags Flags::operator&(E flag) const
+ * \brief Compute the bitwise AND of this Flags and the \a flag
+ * \param[in] flag The second operand
+ * \return A Flags containing the result of the AND operation
+ */
+
+/**
+ * \fn Flags Flags::operator&(Flags other) const
+ * \brief Compute the bitwise AND of this Flags and the \a other Flags
+ * \param[in] other The second operand
+ * \return A Flags containing the result of the AND operation
+ */
+
+/**
+ * \fn Flags Flags::operator|(E flag) const
+ * \brief Compute the bitwise OR of this Flags and the \a flag
+ * \param[in] flag The second operand
+ * \return A Flags containing the result of the OR operation
+ */
+
+/**
+ * \fn Flags Flags::operator|(Flags other) const
+ * \brief Compute the bitwise OR of this Flags and the \a other Flags
+ * \param[in] other The second operand
+ * \return A Flags containing the result of the OR operation
+ */
+
+/**
+ * \fn Flags Flags::operator^(E flag) const
+ * \brief Compute the bitwise XOR of this Flags and the \a flag
+ * \param[in] flag The second operand
+ * \return A Flags containing the result of the XOR operation
+ */
+
+/**
+ * \fn Flags Flags::operator^(Flags other) const
+ * \brief Compute the bitwise XOR of this Flags and the \a other Flags
+ * \param[in] other The second operand
+ * \return A Flags containing the result of the XOR operation
+ */
+
+/**
+ * \fn Flags Flags::operator~() const
+ * \brief Compute the bitwise NOT of this Flags
+ * \return A Flags containing the result of the NOT operation
+ */
+
+/**
+ * \fn bool Flags::operator!() const
+ * \brief Check if flags are set
+ * \return True if no flags is set, false otherwise
+ */
+
+/**
+ * \def LIBCAMERA_FLAGS_ENABLE_OPERATORS(enum)
+ * \brief Enable bitwise operations on the \a enum enumeration
+ *
+ * This macro enables the bitwise AND, OR, XOR and NOT operators on the given
+ * \a enum. This allows the enum values to be safely used in bitwise operations
+ * with the Flags<> class.
+ */
+
+} /* namespace libcamera */
diff --git a/src/libcamera/base/meson.build b/src/libcamera/base/meson.build
index 87172157..d799c66d 100644
--- a/src/libcamera/base/meson.build
+++ b/src/libcamera/base/meson.build
@@ -7,6 +7,7 @@ libcamera_base_sources = files([
     'event_dispatcher_poll.cpp',
     'event_notifier.cpp',
     'file.cpp',
+    'flags.cpp',
     'log.cpp',
     'message.cpp',
     'object.cpp',