BASE64 Decode & Encode (Rosetta Code Task)

[SpriggsySpriggs]

Inspired by @AndyA , I decided to try my hand at one of the tasks for Rosetta Code. I've already posted it on their site but here is the code I sent off. I translated the C++ implementation.

https://rosettacode.org/wiki/Base64_decode_data#QB64

Code: QB64: [Select]

1. Option _Explicit
2.  
3. Dim As String udata, decoded
4. udata = "VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVwIHlvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLVBhdWwgUi5FaHJsaWNo"
5.  
6. decoded = decode(udata)
7.  
8. Print udata
9. Print decoded
10.  
11. Function findIndex& (value As _Unsigned _Byte)
12.     If Asc("A") <= value And value <= Asc("Z") Then
13.         findIndex = value - Asc("A")
14.         Exit Function
15.     End If
16.     If Asc("a") <= value And value <= Asc("z") Then
17.         findIndex = value - Asc("a") + 26
18.         Exit Function
19.     End If
20.     If Asc("0") <= value And value <= Asc("9") Then
21.         findIndex = value - Asc("0") + 52
22.         Exit Function
23.     End If
24.     If value = Asc("+") Then
25.         findIndex = 62
26.         Exit Function
27.     End If
28.     If value = Asc("/") Then
29.         findIndex = 63
30.         Exit Function
31.     End If
32.     findIndex = -1
33. End Function
34.  
35. Function encode$ (source As String)
36.     Dim As String Base64: Base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
37.     Dim As _Unsigned _Integer64 length: length = Len(source)
38.     Dim As _Unsigned _Integer64 it, strend
39.     Dim As Long acc
40.     Dim As String sink
41.     strend = length
42.     While it <> strend
43.         Dim As _Unsigned _Byte b1, b2, b3, b4
44.         it = it + 1
45.         b1 = Asc(Mid$(source, it, 1))
46.         sink = sink + Mid$(Base64, _SHR(b1, 2), 1)
47.         acc = _SHL(b1 And &H3, 4)
48.         If it <> strend Then
49.             it = it + 1
50.             b2 = Asc(Mid$(source, it, 1))
51.             acc = acc Or _SHR(b2, 4)
52.             sink = sink + Mid$(Base64, acc, 1)
53.             acc = _SHL(b2 And &HF, 2)
54.             If it <> strend Then
55.                 it = it + 1
56.                 b3 = Asc(Mid$(source, it, 1))
57.                 acc = acc Or _SHR(b3, 6)
58.                 sink = sink + Mid$(Base64, acc, 1)
59.                 sink = sink + Mid$(Base64, b3 And &H3F, 1)
60.             Else
61.                 sink = sink + Mid$(Base64, acc, 1)
62.                 sink = sink + "="
63.             End If
64.         Else
65.             sink = sink + Mid$(Base64, acc, 1)
66.             sink = sink + "="
67.             sink = sink + "="
68.         End If
69.     Wend
70.     encode = sink
71. End Function
72.  
73. Function decode$ (source As String)
74.     Dim As _Unsigned _Integer64 length: length = Len(source)
75.     Dim As _Unsigned _Integer64 it, strend
76.     Dim As Long acc
77.     Dim As String sink
78.     strend = length
79.     While it <> strend
80.         Dim As _Unsigned _Byte b1, b2, b3, b4
81.         it = it + 1
82.         b1 = Asc(Mid$(source, it, 1))
83.         it = it + 1
84.         b2 = Asc(Mid$(source, it, 1))
85.         it = it + 1
86.         b3 = Asc(Mid$(source, it, 1))
87.         it = it + 1
88.         b4 = Asc(Mid$(source, it, 1))
89.         Dim As Long i1, i2
90.         i1 = findIndex(b1)
91.         i2 = findIndex(b2)
92.         acc = _SHL(i1, 2)
93.         acc = acc Or _SHR(i2, 4)
94.         sink = sink + Chr$(acc)
95.         If b3 <> Asc("=") Then
96.             Dim As Long i3
97.             i3 = findIndex(b3)
98.             acc = _SHL(i2 And &HF, 4)
99.             acc = acc Or _SHR(i3, 2)
100.             sink = sink + Chr$(acc)
101.             If b4 <> Asc("=") Then
102.                 Dim As Long i4
103.                 i4 = findIndex(b4)
104.                 acc = _SHL(i3 And &H3, 6)
105.                 acc = acc Or i4
106.                 sink = sink + Chr$(acc)
107.             End If
108.         End If
109.     Wend
110.     decode = sink
111. End Function