multiavatar 1.0.10

//! Base64 编码（标准 alphabet，带 `=` padding，无换行）。
//!
//! 手写实现（RFC 4648 §4），零 base64 crate 依赖；供 PNG data URI 使用。

const ALPHABET: &[u8; 64] =
    b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/// 编码为标准 Base64 字符串。
pub fn encode(data: &[u8]) -> String {
    if data.is_empty() {
        return String::new();
    }
    let mut out = String::with_capacity((data.len() + 2) / 3 * 4);
    for chunk in data.chunks(3) {
        let b0 = u32::from(chunk[0]);
        let b1 = u32::from(*chunk.get(1).unwrap_or(&0));
        let b2 = u32::from(*chunk.get(2).unwrap_or(&0));
        let n = (b0 << 16) | (b1 << 8) | b2;
        out.push(char::from(ALPHABET[(n >> 18) as usize & 63]));
        out.push(char::from(ALPHABET[(n >> 12) as usize & 63]));
        out.push(if chunk.len() > 1 {
            char::from(ALPHABET[(n >> 6) as usize & 63])
        } else {
            '='
        });
        out.push(if chunk.len() > 2 {
            char::from(ALPHABET[n as usize & 63])
        } else {
            '='
        });
    }
    out
}

#[cfg(test)]
mod tests {
    use super::*;

    /// RFC 4648 §10 测试向量。
    #[test]
    fn rfc4648_vectors() {
        let cases = [
            ("", ""),
            ("f", "Zg=="),
            ("fo", "Zm8="),
            ("foo", "Zm9v"),
            ("foob", "Zm9vYg=="),
            ("fooba", "Zm9vYmE="),
            ("foobar", "Zm9vYmFy"),
        ];
        for (input, expect) in cases {
            assert_eq!(encode(input.as_bytes()), expect, "input={input}");
        }
    }

    /// 二进制输入：与标准实现逐字节对照。
    #[test]
    fn binary_roundtrip() {
        let data = [0u8, 1, 2, 3, 250, 251, 252, 253, 254, 255];
        assert_eq!(encode(&data), "AAECA/r7/P3+/w==");
        assert_eq!(encode(&data).len(), 16);
    }
}