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

use crate::{
    DecodeError, DecodeErrorHandler, EncodeErrorHandler, NestedDecode, NestedDecodeInput,
    NestedEncode, NestedEncodeOutput, TopDecode, TopDecodeInput, TopEncode, TopEncodeOutput,
};

impl<T: NestedEncode> NestedEncode for Option<T> {
    fn dep_encode_or_handle_err<O, H>(&self, dest: &mut O, h: H) -> Result<(), H::HandledErr>
    where
        O: NestedEncodeOutput,
        H: EncodeErrorHandler,
    {
        match self {
            Some(v) => {
                dest.push_byte(1u8);
                v.dep_encode_or_handle_err(dest, h)
            },
            None => {
                dest.push_byte(0u8);
                Ok(())
            },
        }
    }
}

impl<T: NestedDecode> NestedDecode for Option<T> {
    fn dep_decode_or_handle_err<I, H>(input: &mut I, h: H) -> Result<Self, H::HandledErr>
    where
        I: NestedDecodeInput,
        H: DecodeErrorHandler,
    {
        match input.read_byte(h)? {
            0 => Ok(None),
            1 => Ok(Some(T::dep_decode_or_handle_err(input, h)?)),
            _ => Err(h.handle_error(DecodeError::INVALID_VALUE)),
        }
    }
}

impl<T: NestedEncode> TopEncode for Option<T> {
    /// Allow None to be serialized to empty bytes, but leave the leading "1" for Some,
    /// to allow disambiguation between e.g. Some(0) and None.
    fn top_encode_or_handle_err<O, H>(&self, output: O, h: H) -> Result<(), H::HandledErr>
    where
        O: TopEncodeOutput,
        H: EncodeErrorHandler,
    {
        match self {
            Some(v) => {
                let mut buffer = output.start_nested_encode();
                buffer.push_byte(1u8);
                v.dep_encode_or_handle_err(&mut buffer, h)?;
                output.finalize_nested_encode(buffer);
            },
            None => {
                output.set_slice_u8(&[]);
            },
        }
        Ok(())
    }
}

impl<T: NestedDecode> TopDecode for Option<T> {
    fn top_decode_or_handle_err<I, H>(input: I, h: H) -> Result<Self, H::HandledErr>
    where
        I: TopDecodeInput,
        H: DecodeErrorHandler,
    {
        let mut buffer = input.into_nested_buffer();
        if buffer.is_depleted() {
            Ok(None) // empty input is none
        } else {
            let first_byte = buffer.read_byte(h)?;
            let value = match first_byte {
                0 => None, // also allow "0x00" to be interpreted as None
                1 => Some(T::dep_decode_or_handle_err(&mut buffer, h)?),
                _ => return Err(h.handle_error(DecodeError::INVALID_VALUE)),
            };

            if buffer.is_depleted() {
                Ok(value)
            } else {
                Err(h.handle_error(DecodeError::INPUT_TOO_LONG))
            }
        }
    }
}

#[cfg(test)]
pub mod tests {
    use alloc::vec::Vec;

    use crate::{
        test_util::{check_top_decode, check_top_encode_decode},
        DecodeError, TopDecode,
    };

    #[test]
    fn test_top() {
        let some_v = Some([1i32, 2i32, 3i32].to_vec());
        let expected: &[u8] = &[
            /*opt*/ 1, /*size*/ 0, 0, 0, 3, /*data*/ 0, 0, 0, 1, 0, 0, 0, 2, 0, 0,
            0, 3,
        ];
        check_top_encode_decode(some_v, expected);

        let none_v: Option<Vec<i32>> = None;
        check_top_encode_decode(none_v, &[]);
    }

    #[test]
    fn test_top_none() {
        // default representation
        assert_eq!(None, check_top_decode::<Option<u32>>(&[][..]));

        // single zero byte also allowed
        assert_eq!(None, check_top_decode::<Option<u32>>(&[0x00u8][..]));

        // invalid None bytes
        assert_eq!(
            Option::<u32>::top_decode(&[0x00u8, 0x00u8][..]),
            Err(DecodeError::INPUT_TOO_LONG)
        );

        // more invalid None bytes
        assert_eq!(
            Option::<u32>::top_decode(&[0x00u8, 0x00u8, 0x00u8][..]),
            Err(DecodeError::INPUT_TOO_LONG)
        );

        // just invalid byte
        assert_eq!(
            Option::<u32>::top_decode(&[0x02u8][..]),
            Err(DecodeError::INVALID_VALUE)
        );
    }
}