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
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166

use std::{slice::ChunksExact, sync::OnceLock};

use thiserror::Error;

const ALPHABET: &[u8] = b"0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";

#[derive(Debug, Error)]
pub enum GwilymDecodeError {
    #[error("Does not contain version")]
    NoVersion,
    #[error("Only version 1 is supported")]
    WrongVersion,
    #[error("Input must be a multiple of 3 but have {0}")]
    LengthWrong(usize),
}

pub fn gwilym_decode(input: &str) -> Result<GwilymDecodeIter<'_>, GwilymDecodeError> {
    GwilymDecodeIter::new(input)
}

pub struct GwilymDecodeIter<'a> {
    chunks: ChunksExact<'a, u8>,
}

impl<'a> GwilymDecodeIter<'a> {
    fn new(input: &'a str) -> Result<Self, GwilymDecodeError> {
        let input = input
            .strip_prefix("https://agbrs.dev/crash#")
            .unwrap_or(input);

        let Some((input, version)) = input.rsplit_once('v') else {
            return Err(GwilymDecodeError::NoVersion);
        };

        if version != "1" {
            return Err(GwilymDecodeError::WrongVersion);
        }

        if input.len() % 3 != 0 {
            return Err(GwilymDecodeError::LengthWrong(input.len()));
        }

        Ok(Self {
            chunks: input.as_bytes().chunks_exact(3),
        })
    }
}

impl<'a> Iterator for GwilymDecodeIter<'a> {
    type Item = u32;

    fn next(&mut self) -> Option<Self::Item> {
        let chunk = self.chunks.next()?;

        let value = decode_chunk(chunk);
        if value & (1 << 16) != 0 {
            let upper_bits = value << 16;
            let lower_bits = self.next().unwrap_or(0) & 0xffff;

            return Some(upper_bits | lower_bits);
        }

        Some(value | 0x0800_0000)
    }
}

fn decode_chunk(chunk: &[u8]) -> u32 {
    let a = get_value_for_char(chunk[0]);
    let b = get_value_for_char(chunk[1]);
    let c = get_value_for_char(chunk[2]);

    (a << (16 - 5)) | (b << (16 - 10)) | c
}

fn get_value_for_char(input: u8) -> u32 {
    static REVERSE_ALHPABET: OnceLock<[u8; 128]> = OnceLock::new();

    REVERSE_ALHPABET.get_or_init(|| {
        let mut result = [0; 128];
        for (i, &c) in ALPHABET.iter().enumerate() {
            result[c as usize] = i as u8;
        }

        result
    })[input as usize] as u32
}

#[cfg(test)]
mod test {
    use super::*;
    use std::fmt::Write;

    #[test]
    fn should_correctly_decode_16s() -> Result<(), GwilymDecodeError> {
        assert_eq!(
            &gwilym_decode("2QI65Q69306Kv1")?.collect::<Vec<_>>(),
            &[0x0800_16d3, 0x0800_315b, 0x0800_3243, 0x0800_0195]
        );

        Ok(())
    }

    fn encode_16(input: u16) -> [u8; 3] {
        let input = input as usize;
        [
            ALPHABET[input >> (16 - 5)],
            ALPHABET[(input >> (16 - 10)) & 0b11111],
            ALPHABET[input & 0b111111],
        ]
    }

    fn encode_32(input: u32) -> [u8; 6] {
        let input = input as usize;
        let output_lower_16 = encode_16(input as u16);
        let input_upper_16 = input >> 16;
        [
            ALPHABET[(input_upper_16 >> (16 - 5)) | (1 << 5)],
            ALPHABET[(input_upper_16 >> (16 - 10)) & 0b11111],
            ALPHABET[input_upper_16 & 0b111111],
            output_lower_16[0],
            output_lower_16[1],
            output_lower_16[2],
        ]
    }

    #[test]
    fn should_correctly_decode_16s_and_32s() -> Result<(), Box<dyn std::error::Error>> {
        let trace: &[u32] = &[
            0x0300_2990,
            0x0800_3289,
            0x0500_2993,
            0x3829_2910,
            0xffff_ffff,
            0x0000_0000,
        ];

        let mut result = String::new();
        for &ip in trace {
            if ip & 0xFFFF_0000 == 0x0800_0000 {
                let encoded = encode_16(ip as u16);
                let encoded_s = std::str::from_utf8(&encoded)?;
                write!(&mut result, "{encoded_s}")?
            } else {
                let encoded = encode_32(ip);
                let encoded_s = std::str::from_utf8(&encoded)?;
                write!(&mut result, "{encoded_s}")?
            }
        }

        write!(&mut result, "v1")?;

        assert_eq!(&gwilym_decode(&result)?.collect::<Vec<_>>(), trace);

        Ok(())
    }

    #[test]
    fn should_strip_the_agbrsdev_prefix() -> Result<(), Box<dyn std::error::Error>> {
        assert_eq!(
            &gwilym_decode("https://agbrs.dev/crash#2QI65Q69306Kv1")?.collect::<Vec<_>>(),
            &[0x0800_16d3, 0x0800_315b, 0x0800_3243, 0x0800_0195]
        );

        Ok(())
    }
}