/*
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* Copyright (C) 2017 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef ART_RUNTIME_SUBTYPE_CHECK_BITS_H_
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#define ART_RUNTIME_SUBTYPE_CHECK_BITS_H_
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#include "base/bit_string.h"
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#include "base/bit_struct.h"
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#include "base/bit_utils.h"
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namespace art {
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/**
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* The SubtypeCheckBits memory layout (in bits):
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*
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* 1 bit Variable
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* | |
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* v v <---- up to 23 bits ---->
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*
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* +----+-----------+--------+-------------------------+
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* | | Bitstring |
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* + +-----------+--------+-------------------------+
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* | OF | (unused) | Next | Path To Root |
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* + | |--------+----+----------+----+----+
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* | | (0....0) | | | ... | | |
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* +----+-----------+--------+----+----------+----+----+
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* MSB (most significant bit) LSB
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*
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* The bitstring takes up to 23 bits; anything exceeding that is truncated:
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* - Path To Root is a list of chars, encoded as a BitString:
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* starting at the root (in LSB), each character is a sibling index unique to the parent,
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* Paths longer than BitString::kCapacity are truncated to fit within the BitString.
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* - Next is a single BitStringChar (immediatelly following Path To Root)
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* When new children are assigned paths, they get allocated the parent's Next value.
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* The next value is subsequently incremented.
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*
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* The exact bit position of (unused) is variable-length:
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* In the cases that the "Path To Root" + "Next" does not fill up the entire
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* BitString capacity, the remaining bits are (unused) and left as 0s.
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*
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* There is also an additional "OF" (overflow) field to indicate that the
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* PathToRoot has been truncated.
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*
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* See subtype_check.h and subtype_check_info.h for more details.
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*/
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BITSTRUCT_DEFINE_START(SubtypeCheckBits, /*size=*/ BitString::BitStructSizeOf() + 1u)
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BitStructField<BitString, /*lsb=*/ 0> bitstring_;
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BitStructUint</*lsb=*/ BitString::BitStructSizeOf(), /*width=*/ 1> overflow_;
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BITSTRUCT_DEFINE_END(SubtypeCheckBits);
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} // namespace art
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#endif // ART_RUNTIME_SUBTYPE_CHECK_BITS_H_
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