Method decode_from_base58

publication/code/chapter10/base58.rs:104–192

(&self)

// 实现 base58 解码
impl Decoder for str {
    fn decode_from_base58(&self) -> Result<String, DecodeError> {
        // 保存转换字符
        let mut bin = [0u8; 132];
        let mut out = [0u32; (132 + 3) / 4];

        // 以 4 为单位处理数据后剩余的比特数
        let bytes_left = (bin.len() % 4) as u8;
        let zero_mask = match bytes_left {
            0 => 0u32,
            _ => 0xffffffff << (bytes_left * 8),
        };

        // 统计前置 0 个数
        let zero_count = self.chars()
                             .take_while(|&x| x == ALPHABET_INDEX_0)
                             .count();
        let mut i = zero_count;
        let b58: Vec<u8> = self.bytes().collect();
        while i < self.len() {
            // 错误字符
            if (b58[i] & 0x80) != 0 {
                return Err(DecodeError::InvalidCharacter(b58[i] as char, i));
            }

            if DIGITS_MAP[b58[i] as usize] == 255 {
                return Err(DecodeError::InvalidCharacter(b58[i] as char, i));
            }

            // 进制转换
            let mut j = out.len();
            let mut c = DIGITS_MAP[b58[i] as usize] as u64;
            while j != 0 {
                j -= 1;
                let t = out[j] as u64 * (BIG_RADIX as u64) + c;
                c = (t & 0x3f00000000) >> 32;
                out[j] = (t & 0xffffffff) as u32;
            }

            // 数据太长
            if c != 0 {
                return Err(DecodeError::InvalidLength);
            }

            if (out[0] & zero_mask) != 0 {
                return Err(DecodeError::InvalidLength);
            }

            i += 1;
        }

        // 处理剩余的比特
        let mut i = 1;
        let mut j = 0;
        bin[0] = match bytes_left {
            3 => ((out[0] & 0xff0000) >> 16) as u8,
            2 => ((out[0] & 0xff00) >> 8) as u8,
            1 => {
                j = 1;