我在 VB.Net (.NET Framework 4.8) 中编写了一个函数，该函数从给定位置开始从字节数组中读取特定位数并对结果求和。示例：
NextBits({255, 255, 255}, 0, 24)如果读取了一些位并且这些位跨越了字节边界，则返回一个新字节，并将指定位作为最高有效位。示例：
NextBits({0, 15, 200}, 12, 5)我最初从用 C 编写的 Stackoverflow answer 复制了该函数。在该版本中，结果数组是通过引用函数传递的 - 但是，我需要结果字节的总和。该功能似乎有几个bug，所以我做了两点改进：
tmp |= tmp2 >> (8 - offset);现在的问题是我昨天发现了一个bug：在测试用例中，
NextBits({48, 2, 250}, 0, 24)编辑 10 月 8 日：我刚刚确定问题实际上在
If CUInt(tmp) << CInt(8UI - CUInt(offset)) > 255UI ThenElseIf我做了一个测试项目：
Option Strict On
Module Module1
Sub Main()
' 1111 1111 1111 1111 1111 1111
'--------------------------------
' 255 255 255
Dim nextBitsResult1 As UInteger = NextBits({255, 255, 255}, 0, 23)
Debug.WriteLine("NextBits 764 = " & nextBitsResult1.ToString())
' 0000 0000 0000 0001 0000 0010
'--------------------------------
' 0 1 2
Dim nextBitsResult2 As UInteger = NextBits({0, 1, 2}, 0, 24)
Debug.WriteLine("NextBits 3 = " & nextBitsResult2.ToString())
' 0000 0000 0000 1111 1100 1000
' ____ _
'--------------------------------
' 0 15 200
Dim nextBitsResult3 As UInteger = NextBits({0, 15, 200}, 12, 5)
Debug.WriteLine("NextBits 248 = " & nextBitsResult3.ToString())
' 0000 0000 0001 1111 0000 0000
' _ ____
'--------------------------------
' 0 31 0
'Dim nextBitsResult4 As UInteger = NextBits({0, 31, 0}, 11, 5)
'Debug.WriteLine("NextBits 31 = " & nextBitsResult4.ToString())
' 0011 0000 0000 0010 1111 1010
'--------------------------------
' 48 2 250
Dim nextBitsResult5 As UInteger = NextBits({48, 2, 250}, 0, 24)
Debug.WriteLine("NextBits 300 = " & nextBitsResult5.ToString())
End Sub
''' <summary>
''' Reads the next <b>n</b> bits from a byte array starting at a specified bit position and sums their values.<br></br>
''' If n bits are to be read across bytes, a new byte is returned, such that the bits of the last byte are considered the leftmost
''' (most significant) bits of the result byte.<br></br>
''' Serves as a look ahead.
''' </summary>
''' <param name="byteArray"></param>
''' <param name="startPosition">The start position (0-based) in bits from which to begin reading.</param>
''' <param name="numBits">The number of bits to read from the byte array.</param>
''' <param name="dest">To specify a position in a temporary output array. This parameter is not relevant for end users.</param>
''' <returns></returns>
Friend Function NextBits(byteArray() As Byte, startPosition As Integer, numBits As Integer, Optional dest As Integer = 0) As UInt32
' https://stackoverflow.com/a/50899946
If IsWithinOneByte(startPosition, numBits) Then
Dim idx As Integer = startPosition \ 8
'Return AnotherFunction(byteArray(idx), startPosition, numBits)
End If
Dim bitmask As Integer = -128
Dim len As Integer = numBits
Dim b As Byte() = New Byte(byteArray.Length - 1) {}
While len > 0
Dim idx As Integer = startPosition \ 8
Dim offset As Integer = startPosition Mod 8
Dim tmp As Byte = byteArray(idx) << offset
Dim next_bits As Integer = offset + len - 8
If len > 8 Then
next_bits += 1
End If
If next_bits < 0 Then
' Don't even need all of the current byte -> remove trailing bits
tmp = CByte(tmp And (bitmask >> (len - 1)))
ElseIf next_bits > 0 Then
' Need to include part of next byte
Dim tmp2 As Byte = CByte(byteArray(idx + 1) And (bitmask >> (next_bits - 1)))
If offset <> 0 Then
tmp = tmp Or (tmp2 >> (8 - offset))
Else
tmp = Math.Min(tmp, tmp2 >> (8 - offset))
End If
End If
' Determine byte index and offset in output byte array
idx = dest \ 8
offset = dest Mod 8
b(idx) = b(idx) Or tmp << (8 - offset)
If CUInt(tmp) << CInt(8UI - CUInt(offset)) > 255UI Then ' In the original C code, this check is redundant because if the value exceeds 255, the assignment b(idx + 1) = 0 effectively handles the overflow. (In C, there are no exceptions)
If idx + 1 < b.Length Then ' The original C code did write an array index too far.
b(idx + 1) = 0
End If
Else
If idx + 1 < b.Length Then
b(idx + 1) = b(idx + 1) Or (tmp << (8 - offset))
End If
End If
' Update start position and length for next pass
If len > 8 Then
len -= 8
dest += 8
startPosition += 8
Else
len -= len
End If
End While
Dim ret As UInt32 = CUInt(b.Sum(Function(x As UInt32) x))
Return ret
End Function
Private Function IsWithinOneByte(startPosition As Integer, numBits As Integer) As Boolean
Dim startByte As Integer = startPosition \ 8
Dim endByte As Integer = (startPosition + numBits - 1) \ 8
Return startByte = endByte
End Function
End Module
输出：
下一个比特 764 = 764
下一个比特 3 = 3
下一个比特 248 = 248
下一个比特 300 = 254
Function NextBits(byteArray() As Byte, startPosition As Integer, numBits As Integer)
Dim s_offs = startPosition \ 8
Dim s_mod = startPosition Mod 8
Dim endPosition = startPosition + numBits
Dim e_offs = endPosition \ 8
Dim e_mod = endPosition Mod 8
Dim sum = 0
If s_mod = 0 Then
sum = byteArray.Skip(s_offs).Take(e_offs - s_offs).Sum(Function(b) b)
If e_mod <> 0 Then
sum += byteArray(e_offs) And (&HFF << (8 - e_mod)) And &HFF
End If
ElseIf s_mod >= e_mod Then
sum = Enumerable.Range(s_offs, e_offs - s_offs - 1).Sum(Function(i) _
((byteArray(i) << s_mod) Or (byteArray(i + 1) >> (8 - s_mod))) And &HFF)
sum += ((byteArray(e_offs - 1) << s_mod) Or (byteArray(e_offs) >> (8 - s_mod))) _
And (&HFF << (s_mod - e_mod)) And &HFF
Else
sum = Enumerable.Range(s_offs, e_offs - s_offs).Sum(Function(i) _
((byteArray(i) << s_mod) Or (byteArray(i + 1) >> (8 - s_mod))) And &HFF)
sum += byteArray(e_offs) And (&HFF << (8 - (e_mod - s_mod))) And &HFF
End If
Return sum
End Function