beer

//! Byte codecs the escape-sequence layer leans on: base64 (OSC 52 clipboard),

//! hex (XTGETTCAP capability names), and percent-decoding of `file://` URIs

//! (OSC 7 working-directory reports).

const B64: &[u8; 64] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/// Standard base64 encode (used for OSC 52 query replies).

pub fn base64_encode(data: &[u8]) -> String {

    let mut out = String::with_capacity(data.len().div_ceil(3) * 4);

    for chunk in data.chunks(3) {

        let b = [

            chunk[0],

            *chunk.get(1).unwrap_or(&0),

            *chunk.get(2).unwrap_or(&0),

        ];

        let n = (u32::from(b[0]) << 16) | (u32::from(b[1]) << 8) | u32::from(b[2]);

        out.push(B64[(n >> 18 & 63) as usize] as char);

        out.push(B64[(n >> 12 & 63) as usize] as char);

        out.push(if chunk.len() > 1 {

            B64[(n >> 6 & 63) as usize] as char

        } else {

            '='

        });

        out.push(if chunk.len() > 2 {

            B64[(n & 63) as usize] as char

        } else {

            '='

        });

    }

    out

}

/// Standard base64 decode, ignoring padding and whitespace; `None` on a bad

/// character.

pub fn base64_decode(data: &[u8]) -> Option<Vec<u8>> {

    let val = |c: u8| -> Option<u32> {

        match c {

            b'A'..=b'Z' => Some(u32::from(c - b'A')),

            b'a'..=b'z' => Some(u32::from(c - b'a') + 26),

            b'0'..=b'9' => Some(u32::from(c - b'0') + 52),

            b'+' => Some(62),

            b'/' => Some(63),

            _ => None,

        }

    };

    let filtered: Vec<u8> = data

        .iter()

        .copied()

        .filter(|&c| c != b'=' && !c.is_ascii_whitespace())

        .collect();

    let mut out = Vec::with_capacity(filtered.len() / 4 * 3);

    for chunk in filtered.chunks(4) {

        if chunk.len() == 1 {

            return None; // a lone sextet cannot form a byte

        }

        let mut n = 0u32;

        for &c in chunk {

            n = (n << 6) | val(c)?;

        }

        n <<= 6 * (4 - chunk.len() as u32);

        out.push((n >> 16) as u8);

        if chunk.len() >= 3 {

            out.push((n >> 8) as u8);

        }

        if chunk.len() >= 4 {

            out.push(n as u8);

        }

    }

    Some(out)

}

/// Decode an even-length lowercase/uppercase hex string into bytes (XTGETTCAP

/// names arrive hex-encoded).

pub fn decode_hex(s: &[u8]) -> Option<Vec<u8>> {

    if s.is_empty() || !s.len().is_multiple_of(2) {

        return None;

    }

    s.chunks_exact(2)

        .map(|pair| Some((hex_nibble(pair[0])? << 4) | hex_nibble(pair[1])?))

        .collect()

}

/// Turn a hexadecimal character into its numerical value.

fn hex_nibble(b: u8) -> Option<u8> {

    (b as char).to_digit(16).map(|d| d as u8)

}

/// Percent-decode `%XX` byte escapes in a URI path, passing other bytes through.

pub fn percent_decode(s: &[u8]) -> Vec<u8> {

    let mut out = Vec::with_capacity(s.len());

    let mut i = 0;

    while i < s.len() {

        if s[i] == b'%' && i + 2 < s.len() {

            let hi = hex_nibble(s[i + 1]);

            let lo = hex_nibble(s[i + 2]);

            if let (Some(hi), Some(lo)) = (hi, lo) {

                out.push(hi << 4 | lo);

                i += 3;

                continue;

            }

        }

        out.push(s[i]);

        i += 1;

    }

    out

}

/// Extract the local path from an OSC 7 `file://host/path` URI, percent-decoding

/// `%XX` escapes. The host part is ignored (we only spawn locally). Returns

/// `None` if it is not a usable absolute path.

pub fn file_uri_path(uri: &[u8]) -> Option<String> {

    let rest = uri.strip_prefix(b"file://").unwrap_or(uri);

    // Skip the authority (host) up to the first '/', which begins the path.

    let slash = rest.iter().position(|&b| b == b'/')?;

    let path_bytes = percent_decode(&rest[slash..]);

    let path = String::from_utf8(path_bytes).ok()?;

    path.starts_with('/').then_some(path)

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn base64_round_trips() {

        for s in [

            "",

            "f",

            "fo",

            "foo",

            "foob",

            "fooba",

            "foobar",

            "hi there\n",

        ] {

            let enc = base64_encode(s.as_bytes());

            assert_eq!(base64_decode(enc.as_bytes()).as_deref(), Some(s.as_bytes()));

        }

        assert_eq!(base64_encode(b"foobar"), "Zm9vYmFy");

        assert_eq!(base64_decode(b"Zm9vYmFy").as_deref(), Some(&b"foobar"[..]));

    }

    #[test]

    fn decode_hex_rejects_odd_and_nonhex() {

        assert_eq!(decode_hex(b"544e").as_deref(), Some(&b"TN"[..]));

        assert_eq!(decode_hex(b"54e"), None);

        assert_eq!(decode_hex(b"zz"), None);

    }

    #[test]

    fn file_uri_decodes_percent_and_drops_host() {

        assert_eq!(

            file_uri_path(b"file://hermes/home/user/my%20dir").as_deref(),

            Some("/home/user/my dir")

        );

        assert_eq!(file_uri_path(b"file://host").as_deref(), None);

    }

}