nimbus-eth2/beacon_chain/ssz/bitseqs.nim

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# beacon_chain
# Copyright (c) 2018-2020 Status Research & Development GmbH
# Licensed and distributed under either of
# * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
# * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.
{.push raises: [Defect].}
import
stew/[bitops2, endians2, ptrops]
type
Bytes = seq[byte]
BitSeq* = distinct Bytes
## The current design of BitSeq tries to follow precisely
## the bitwise representation of the SSZ bitlists.
## This is a relatively compact representation, but as
## evident from the code below, many of the operations
## are not trivial.
BitArray*[bits: static int] = object
bytes*: array[(bits + 7) div 8, byte]
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func bitsLen*(bytes: openArray[byte]): int =
let
bytesCount = bytes.len
lastByte = bytes[bytesCount - 1]
markerPos = log2trunc(lastByte)
bytesCount * 8 - (8 - markerPos)
template len*(s: BitSeq): int =
bitsLen(Bytes s)
template len*(a: BitArray): int =
a.bits
func add*(s: var BitSeq, value: bool) =
let
lastBytePos = s.Bytes.len - 1
lastByte = s.Bytes[lastBytePos]
if (lastByte and byte(128)) == 0:
# There is at least one leading zero, so we have enough
# room to store the new bit
let markerPos = log2trunc(lastByte)
s.Bytes[lastBytePos].changeBit markerPos, value
s.Bytes[lastBytePos].setBit markerPos + 1
else:
s.Bytes[lastBytePos].changeBit 7, value
s.Bytes.add byte(1)
func toBytesLE(x: uint): array[sizeof(x), byte] =
# stew/endians2 supports explicitly sized uints only
when sizeof(uint) == 4:
static: doAssert sizeof(uint) == sizeof(uint32)
toBytesLE(x.uint32)
elif sizeof(uint) == 8:
static: doAssert sizeof(uint) == sizeof(uint64)
toBytesLE(x.uint64)
else:
static: doAssert false, "requires a 32-bit or 64-bit platform"
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func loadLEBytes(WordType: type, bytes: openArray[byte]): WordType =
# TODO: this is a temporary proc until the endians API is improved
var shift = 0
for b in bytes:
result = result or (WordType(b) shl shift)
shift += 8
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func storeLEBytes(value: SomeUnsignedInt, dst: var openArray[byte]) =
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doAssert dst.len <= sizeof(value)
let bytesLE = toBytesLE(value)
copyMem(addr dst[0], unsafeAddr bytesLE[0], dst.len)
template loopOverWords(lhs, rhs: BitSeq,
lhsIsVar, rhsIsVar: static bool,
WordType: type,
lhsBits, rhsBits, body: untyped) =
const hasRhs = astToStr(lhs) != astToStr(rhs)
let bytesCount = len Bytes(lhs)
when hasRhs: doAssert len(Bytes(rhs)) == bytesCount
var fullWordsCount = bytesCount div sizeof(WordType)
let lastWordSize = bytesCount mod sizeof(WordType)
block:
var lhsWord: WordType
when hasRhs:
var rhsWord: WordType
var firstByteOfLastWord, lastByteOfLastWord: int
# TODO: Returning a `var` value from an iterator is always safe due to
# the way inlining works, but currently the compiler reports an error
# when a local variable escapes. We have to cheat it with this location
# obfuscation through pointers:
template lhsBits: auto = (addr(lhsWord))[]
when hasRhs:
template rhsBits: auto = (addr(rhsWord))[]
template lastWordBytes(bitseq): auto =
Bytes(bitseq).toOpenArray(firstByteOfLastWord, lastByteOfLastWord)
template initLastWords =
lhsWord = loadLEBytes(WordType, lastWordBytes(lhs))
when hasRhs: rhsWord = loadLEBytes(WordType, lastWordBytes(rhs))
if lastWordSize == 0:
firstByteOfLastWord = bytesCount - sizeof(WordType)
lastByteOfLastWord = bytesCount - 1
dec fullWordsCount
else:
firstByteOfLastWord = bytesCount - lastWordSize
lastByteOfLastWord = bytesCount - 1
initLastWords()
let markerPos = log2trunc(lhsWord)
when hasRhs: doAssert log2trunc(rhsWord) == markerPos
lhsWord.clearBit markerPos
when hasRhs: rhsWord.clearBit markerPos
body
when lhsIsVar or rhsIsVar:
let
markerBit = uint(1 shl markerPos)
mask = markerBit - 1'u
when lhsIsVar:
let lhsEndResult = (lhsWord and mask) or markerBit
storeLEBytes(lhsEndResult, lastWordBytes(lhs))
when rhsIsVar:
let rhsEndResult = (rhsWord and mask) or markerBit
storeLEBytes(rhsEndResult, lastWordBytes(rhs))
var lhsCurrAddr = cast[ptr WordType](unsafeAddr Bytes(lhs)[0])
let lhsEndAddr = offset(lhsCurrAddr, fullWordsCount)
when hasRhs:
var rhsCurrAddr = cast[ptr WordType](unsafeAddr Bytes(rhs)[0])
while lhsCurrAddr < lhsEndAddr:
template lhsBits: auto = lhsCurrAddr[]
when hasRhs:
template rhsBits: auto = rhsCurrAddr[]
body
lhsCurrAddr = offset(lhsCurrAddr, 1)
when hasRhs: rhsCurrAddr = offset(rhsCurrAddr, 1)
iterator words*(x: var BitSeq): var uint =
loopOverWords(x, x, true, false, uint, word, wordB):
yield word
iterator words*(x: BitSeq): uint =
loopOverWords(x, x, false, false, uint, word, word):
yield word
iterator words*(a, b: BitSeq): (uint, uint) =
loopOverWords(a, b, false, false, uint, wordA, wordB):
yield (wordA, wordB)
iterator words*(a: var BitSeq, b: BitSeq): (var uint, uint) =
loopOverWords(a, b, true, false, uint, wordA, wordB):
yield (wordA, wordB)
iterator words*(a, b: var BitSeq): (var uint, var uint) =
loopOverWords(a, b, true, true, uint, wordA, wordB):
yield (wordA, wordB)
func `[]`*(s: BitSeq, pos: Natural): bool {.inline.} =
doAssert pos < s.len
s.Bytes.getBit pos
func `[]=`*(s: var BitSeq, pos: Natural, value: bool) {.inline.} =
doAssert pos < s.len
s.Bytes.changeBit pos, value
func setBit*(s: var BitSeq, pos: Natural) {.inline.} =
doAssert pos < s.len
setBit s.Bytes, pos
func clearBit*(s: var BitSeq, pos: Natural) {.inline.} =
doAssert pos < s.len
clearBit s.Bytes, pos
func init*(T: type BitSeq, len: int): T =
result = BitSeq newSeq[byte](1 + len div 8)
Bytes(result).setBit len
func init*(T: type BitArray): T =
# The default zero-initializatio is fine
discard
template `[]`*(a: BitArray, pos: Natural): bool =
getBit a.bytes, pos
template `[]=`*(a: var BitArray, pos: Natural, value: bool) =
changeBit a.bytes, pos, value
template setBit*(a: var BitArray, pos: Natural) =
setBit a.bytes, pos
template clearBit*(a: var BitArray, pos: Natural) =
clearBit a.bytes, pos
# TODO: Submit this to the standard library as `cmp`
# At the moment, it doesn't work quite well because Nim selects
# the generic cmp[T] from the system module instead of choosing
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# the openArray overload
func compareArrays[T](a, b: openArray[T]): int =
result = cmp(a.len, b.len)
if result != 0: return
for i in 0 ..< a.len:
result = cmp(a[i], b[i])
if result != 0: return
template cmp*(a, b: BitSeq): int =
compareArrays(Bytes a, Bytes b)
template `==`*(a, b: BitSeq): bool =
cmp(a, b) == 0
func `$`*(a: BitSeq): string =
let length = a.len
result = newStringOfCap(2 + length)
result.add "0b"
for i in countdown(length - 1, 0):
result.add if a[i]: '1' else: '0'
func combine*(tgt: var BitSeq, src: BitSeq) =
doAssert tgt.len == src.len
for tgtWord, srcWord in words(tgt, src):
tgtWord = tgtWord or srcWord
func overlaps*(a, b: BitSeq): bool =
for wa, wb in words(a, b):
if (wa and wb) != 0:
return true
func isSubsetOf*(a, b: BitSeq): bool =
let alen = a.len
doAssert b.len == alen
for i in 0 ..< alen:
if a[i] and not b[i]:
return false
true
proc isZeros*(x: BitSeq): bool =
for w in words(x):
if w != 0: return false
return true
template bytes*(x: BitSeq): untyped =
seq[byte](x)