1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136

const ALPHABET: [u8; 64] = [
    0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50,
    0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66,
    0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76,
    0x77, 0x78, 0x79, 0x7a, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x2b, 0x2f,
];

const INDEX: [u8; 123] = [
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x3e, 0xff, 0xff, 0xff, 0x3f,
    0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e,
    0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28,
    0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0x33,
];

pub fn encode(value: &str) -> String {
    let mut data = String::new();
    let mut bytes = value.bytes();

    let len = bytes.len() / 3;
    let rem = bytes.len() % 3;

    for _ in 0..len {
        let chunk: u32 = (bytes.next().unwrap() as u32) << 0x10
            | (bytes.next().unwrap() as u32) << 0x08
            | bytes.next().unwrap() as u32;

        data.push(ALPHABET[(chunk >> 0x12 & 0x3f) as usize] as char);
        data.push(ALPHABET[(chunk >> 0x0c & 0x3f) as usize] as char);
        data.push(ALPHABET[(chunk >> 0x06 & 0x3f) as usize] as char);
        data.push(ALPHABET[(chunk & 0x3f) as usize] as char);
    }

    if rem == 1 {
        let chunk: u32 = (bytes.next().unwrap() as u32) << 0x04;

        data.push(ALPHABET[(chunk >> 0x06 & 0x3f) as usize] as char);
        data.push(ALPHABET[(chunk & 0x3f) as usize] as char);
        data.push('=');
        data.push('=');
    } else if rem == 2 {
        let chunk = (bytes.next().unwrap() as u32) << 0x0a | (bytes.next().unwrap() as u32) << 0x02;

        data.push(ALPHABET[(chunk >> 0x0c & 0x3f) as usize] as char);
        data.push(ALPHABET[(chunk >> 0x06 & 0x3f) as usize] as char);
        data.push(ALPHABET[(chunk & 0x3f) as usize] as char);
        data.push('=');
    }

    data
}

pub fn decode(value: &str) -> String {
    let mut data = vec![];

    let len = &value.len();
    let padding = if &value[len - 2..] == "==" {
        2
    } else if &value[len - 1..] == "=" {
        1
    } else {
        0
    };

    let mut bytes = value.bytes();
    let len = (bytes.len() - padding) / 4;

    for _ in 0..len {
        let chunk: u32 = ((INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f) << 0x12
            | (INDEX[bytes.next().unwrap() as usize] as u32) << 0x0c
            | ((INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f) << 0x06
            | (INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f;

        data.push((chunk >> 0x10) as u8);
        data.push((chunk >> 0x08) as u8);
        data.push(chunk as u8);
    }

    if padding == 1 {
        let chunk = ((INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f) << 0x0c
            | ((INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f) << 0x06
            | (INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f;

        data.push((chunk >> 0x0a) as u8);
        data.push((chunk >> 0x02) as u8);
    } else if padding == 2 {
        let chunk = ((INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f) << 0x06
            | (INDEX[bytes.next().unwrap() as usize] as u32) & 0x3f;

        data.push((chunk >> 0x04) as u8);
    }

    String::from_utf8(data).unwrap()
}

#[cfg(test)]
mod test {
    use super::{decode, encode, ALPHABET, INDEX};

    struct B64Pair(pub &'static str, pub &'static str);

    fn test_data() -> Vec<B64Pair> {
        vec![
            B64Pair("Test String", "VGVzdCBTdHJpbmc="),
            B64Pair("bea is cool", "YmVhIGlzIGNvb2w="),
            B64Pair("🥺", "8J+lug=="),
            B64Pair("おはいよう！私の名前はBea!元気ですか？", "44GK44Gv44GE44KI44GG77yB56eB44Gu5ZCN5YmN44GvQmVhIeWFg+awl+OBp+OBmeOBi++8nw=="),
            B64Pair("These sentences feel random but they're not, I promise. I am just making sure to test as many different silly things I can!", "VGhlc2Ugc2VudGVuY2VzIGZlZWwgcmFuZG9tIGJ1dCB0aGV5J3JlIG5vdCwgSSBwcm9taXNlLiBJIGFtIGp1c3QgbWFraW5nIHN1cmUgdG8gdGVzdCBhcyBtYW55IGRpZmZlcmVudCBzaWxseSB0aGluZ3MgSSBjYW4h"),
            B64Pair("I need to test padding of each length, you see!!", "SSBuZWVkIHRvIHRlc3QgcGFkZGluZyBvZiBlYWNoIGxlbmd0aCwgeW91IHNlZSEh"),
            ]
    }

    #[test]
    fn encode_string() {
        test_data()
            .iter()
            .for_each(|B64Pair(i, o)| assert_eq!(encode(i), *o));
    }

    #[test]
    fn decode_string() {
        test_data()
            .iter()
            .for_each(|B64Pair(i, o)| assert_eq!(decode(o), *i));
    }

    #[test]
    fn lookup_tables() {
        for i in 0..ALPHABET.len() {
            assert_eq!(INDEX[ALPHABET[i] as usize], i as u8);
        }
    }
}