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// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/metrics/ukm_source_id.h"
#include <cmath>
#include "base/atomic_sequence_num.h"
#include "base/check_op.h"
#include "base/rand_util.h"
namespace base {
namespace {
const int64_t kLowBitsMask = (INT64_C(1) << 32) - 1;
int64_t GetNumTypeBits() {
return std::ceil(
std::log2(static_cast<int64_t>(UkmSourceId::Type::kMaxValue) + 1));
}
} // namespace
// static
UkmSourceId UkmSourceId::New() {
// Generate some bits which are unique to this process, so we can generate
// IDs independently in different processes. IDs generated by this method may
// collide, but it should be sufficiently rare enough to not impact data
// quality.
const static int64_t process_id_bits =
static_cast<int64_t>(RandUint64()) & ~kLowBitsMask;
// Generate some bits which are unique within the process, using a counter.
static AtomicSequenceNumber seq;
UkmSourceId local_id =
FromOtherId(seq.GetNext() + 1, UkmSourceId::Type::DEFAULT);
// Combine the local and process bits to generate a unique ID.
return UkmSourceId((local_id.value_ & kLowBitsMask) | process_id_bits);
}
// static
UkmSourceId UkmSourceId::FromOtherId(int64_t other_id, UkmSourceId::Type type) {
// Note on syntax: std::ceil and std::log2 are not constexpr functions thus
// these variables cannot be initialized statically in the global scope above.
// Function static initialization here is thread safe; so they are initialized
// at most once.
static const int64_t kNumTypeBits = GetNumTypeBits();
static const int64_t kTypeMask = (INT64_C(1) << kNumTypeBits) - 1;
const int64_t type_bits = static_cast<int64_t>(type);
DCHECK_EQ(type_bits, type_bits & kTypeMask);
// Stores the type of the source ID in its lower bits, and shift the rest of
// the ID to make room. This could cause the original ID to overflow, but
// that should be rare enough that it won't matter for UKM's purposes.
return UkmSourceId((other_id << kNumTypeBits) | type_bits);
}
UkmSourceId::Type UkmSourceId::GetType() const {
static const int64_t kNumTypeBits = GetNumTypeBits();
static const int64_t kTypeMask = (INT64_C(1) << kNumTypeBits) - 1;
return static_cast<UkmSourceId::Type>(value_ & kTypeMask);
}
} // namespace base
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