roto/src/lib.rs

use std::fmt;

#[derive(Debug, PartialEq, Eq)]
pub enum RotoError {
    UnexpectedEndOfBuffer,
    InvalidVarint,
    InvalidWireType(u8),
    BufferOverflow,
    FieldNotFound,
    WireFormatViolation,
}

impl fmt::Display for RotoError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            RotoError::UnexpectedEndOfBuffer => write!(f, "Unexpected end of buffer"),
            RotoError::InvalidVarint => write!(f, "Invalid varint encoding"),
            RotoError::InvalidWireType(t) => write!(f, "Invalid wire type: {t}"),
            RotoError::BufferOverflow => write!(f, "Buffer overflow during write"),
            RotoError::FieldNotFound => write!(f, "Requested field not found in message"),
            RotoError::WireFormatViolation => write!(f, "Wire format violation"),
        }
    }
}

impl std::error::Error for RotoError {}

pub type Result<T> = std::result::Result<T, RotoError>;

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum WireType {
    Varint = 0,
    Fixed64 = 1,
    LengthDelimited = 2,
    StartGroup = 3, // Deprecated
    EndGroup = 4,   // Deprecated
    Fixed32 = 5,
}

impl WireType {
    pub fn from_u8(value: u8) -> Result<Self> {
        match value {
            0 => Ok(WireType::Varint),
            1 => Ok(WireType::Fixed64),
            2 => Ok(WireType::LengthDelimited),
            3 => Ok(WireType::StartGroup),
            4 => Ok(WireType::EndGroup),
            5 => Ok(WireType::Fixed32),
            _ => Err(RotoError::InvalidWireType(value)),
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Tag {
    pub field_number: u32,
    pub wire_type: WireType,
}

impl Tag {
    /// Decodes a tag from the buffer, returning the tag and the number of bytes read.
    pub fn decode(data: &[u8]) -> Result<(Self, usize)> {
        let (val, len) = read_varint(data)?;
        let wire_type_raw = (val & 0x7) as u8;
        let field_number = (val >> 3) as u32;

        Ok((
            Tag {
                field_number,
                wire_type: WireType::from_u8(wire_type_raw)?,
            },
            len,
        ))
    }

    /// Encodes a tag into the provided buffer.
    pub fn encode(field_number: u32, wire_type: WireType, buf: &mut [u8]) -> Result<usize> {
        let val = ((field_number as u64) << 3) | (wire_type as u64);
        write_varint(val, buf)
    }
}

/// Reads a varint from the start of the buffer.
pub fn read_varint(data: &[u8]) -> Result<(u64, usize)> {
    let mut result = 0u64;
    let mut shift = 0;
    let mut bytes_read = 0;

    for &byte in data {
        bytes_read += 1;
        if bytes_read > 10 {
            return Err(RotoError::InvalidVarint);
        }

        let value = (byte & 0x7F) as u64;
        if shift >= 64 {
            return Err(RotoError::InvalidVarint);
        }
        result |= value << shift;
        shift += 7;

        if (byte & 0x80) == 0 {
            return Ok((result, bytes_read));
        }
    }

    Err(RotoError::UnexpectedEndOfBuffer)
}

/// Writes a varint into the buffer.
pub fn write_varint(mut value: u64, buf: &mut [u8]) -> Result<usize> {
    let mut bytes_written = 0;
    while value >= 0x80 {
        if bytes_written >= buf.len() {
            return Err(RotoError::BufferOverflow);
        }
        buf[bytes_written] = (value as u8 & 0x7F) | 0x80;
        value >>= 7;
        bytes_written += 1;
    }

    if bytes_written >= buf.len() {
        return Err(RotoError::BufferOverflow);
    }
    buf[bytes_written] = value as u8;
    bytes_written += 1;
    Ok(bytes_written)
}

/// Returns the number of bytes that should be skipped for a given wire type and the current data slice.
pub fn skip_value(wire_type: WireType, data: &[u8]) -> Result<usize> {
    match wire_type {
        WireType::Varint => {
            let (_, len) = read_varint(data)?;
            Ok(len)
        }
        WireType::Fixed64 => {
            if data.len() < 8 {
                return Err(RotoError::UnexpectedEndOfBuffer);
            }
            Ok(8)
        }
        WireType::LengthDelimited => {
            let (len, varint_len) = read_varint(data)?;
            let total_len = varint_len + len as usize;
            if data.len() < total_len {
                return Err(RotoError::UnexpectedEndOfBuffer);
            }
            Ok(total_len)
        }
        WireType::Fixed32 => {
            if data.len() < 4 {
                return Err(RotoError::UnexpectedEndOfBuffer);
            }
            Ok(4)
        }
        WireType::StartGroup | WireType::EndGroup => {
            // These are deprecated and not fully supported in this runtime.
            Err(RotoError::WireFormatViolation)
        }
    }
}

pub struct ProtoAccessor<'a> {
    data: &'a [u8],
}

impl<'a> ProtoAccessor<'a> {
    pub fn new(data: &'a [u8]) -> Result<Self> {
        Ok(Self { data })
    }

    /// Returns an iterator over all fields in the message.
    pub fn fields(&self) -> FieldIterator<'a> {
        FieldIterator {
            data: self.data,
            cursor: 0,
        }
    }

    /// Returns the value and wire type of the last occurrence of the specified field.
    pub fn get_value(&self, field_number: u32) -> Result<(&[u8], WireType)> {
        let mut last_value = None;
        for item in self.fields() {
            let (tag, value) = item?;
            if tag.field_number == field_number {
                last_value = Some((value, tag.wire_type));
            }
        }
        last_value.ok_or(RotoError::FieldNotFound)
    }

    /// Returns an iterator that scans the entire buffer for all occurrences of the specified field.
    pub fn iter_repeated(&self, field_number: u32) -> RepeatedFieldIterator<'a> {
        RepeatedFieldIterator::new(self.data, field_number)
    }
}

pub struct FieldIterator<'a> {
    data: &'a [u8],
    cursor: usize,
}

impl<'a> Iterator for FieldIterator<'a> {
    type Item = Result<(Tag, &'a [u8])>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.cursor >= self.data.len() {
            return None;
        }

        let (tag, tag_len) = match Tag::decode(&self.data[self.cursor..]) {
            Ok(t) => t,
            Err(e) => {
                self.cursor = self.data.len();
                return Some(Err(e));
            }
        };

        let cursor_after_tag = self.cursor + tag_len;
        if cursor_after_tag > self.data.len() {
            self.cursor = self.data.len();
            return Some(Err(RotoError::UnexpectedEndOfBuffer));
        }

        let value_len = match skip_value(tag.wire_type, &self.data[cursor_after_tag..]) {
            Ok(l) => l,
            Err(e) => {
                self.cursor = self.data.len();
                return Some(Err(e));
            }
        };

        let (value_offset, actual_value_len) = match tag.wire_type {
            WireType::LengthDelimited => {
                let (_, varint_len) = match read_varint(&self.data[cursor_after_tag..]) {
                    Ok(v) => v,
                    Err(e) => {
                        self.cursor = self.data.len();
                        return Some(Err(e));
                    }
                };
                (cursor_after_tag + varint_len, value_len - varint_len)
            }
            _ => (cursor_after_tag, value_len),
        };

        self.cursor = cursor_after_tag + value_len;

        Some(Ok((tag, &self.data[value_offset..value_offset + actual_value_len])))
    }
}

pub struct RepeatedFieldIterator<'a> {
    iterator: FieldIterator<'a>,
    field_number: u32,
}

impl<'a> RepeatedFieldIterator<'a> {
    fn new(data: &'a [u8], field_number: u32) -> Self {
        Self {
            iterator: FieldIterator {
                data,
                cursor: 0,
            },
            field_number,
        }
    }
}

impl<'a> Iterator for RepeatedFieldIterator<'a> {
    type Item = Result<(&'a [u8], WireType)>;

    fn next(&mut self) -> Option<Self::Item> {
        while let Some(item) = self.iterator.next() {
            match item {
                Ok((tag, value)) if tag.field_number == self.field_number => {
                    return Some(Ok((value, tag.wire_type)));
                }
                Ok(_) => continue,
                Err(e) => return Some(Err(e)),
            }
        }
        None
    }
}

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

    #[test]
    fn test_varint_read_write() {
        let mut buf = [0u8; 10];
        let val = 300u64;
        let len = write_varint(val, &mut buf).unwrap();
        assert_eq!(len, 2);
        assert_eq!(&buf[..2], &[0xAC, 0x02]);

        let (read_val, read_len) = read_varint(&buf[..2]).unwrap();
        assert_eq!(read_val, val);
        assert_eq!(read_len, 2);
    }

    #[test]
    fn test_tag_decode() {
        // Field 1, WireType Varint: (1 << 3) | 0 = 8
        let data = [8u8];
        let (tag, len) = Tag::decode(&data).unwrap();
        assert_eq!(tag.field_number, 1);
        assert_eq!(tag.wire_type, WireType::Varint);
        assert_eq!(len, 1);

        // Field 15, WireType LengthDelimited: (15 << 3) | 2 = 120 | 2 = 122
        let data2 = [122u8];
        let (tag2, len2) = Tag::decode(&data2).unwrap();
        assert_eq!(tag2.field_number, 15);
        assert_eq!(tag2.wire_type, WireType::LengthDelimited);
        assert_eq!(len2, 1);
    }

    #[test]
    fn test_skip_value() {
        // Varint: 300 (2 bytes)
        let data_varint = [0xAC, 0x02];
        assert_eq!(skip_value(WireType::Varint, &data_varint).unwrap(), 2);

        // Fixed32: 4 bytes
        let data_fixed32 = [0u8; 4];
        assert_eq!(skip_value(WireType::Fixed32, &data_fixed32).unwrap(), 4);

        // Length delimited: len=3, data=[1,2,3] (1 byte varint for length + 3 bytes)
        let data_len = [3, 1, 2, 3];
        assert_eq!(skip_value(WireType::LengthDelimited, &data_len).unwrap(), 4);
    }

    #[test]
    fn test_accessor_basic() {
        // Field 1 (Varint): 150
        // Tag: (1 << 3) | 0 = 8. Value: 150 = [150, 1]
        // Field 2 (LengthDelimited): "hi"
        // Tag: (2 << 3) | 2 = 18. Length: 2. Value: [104, 105]
        let data = [8, 150, 1, 18, 2, 104, 105];
        let acc = ProtoAccessor::new(&data).unwrap();

        let (val1, type1) = acc.get_value(1).unwrap();
        assert_eq!(type1, WireType::Varint);
        assert_eq!(val1, &[150, 1]);

        let (val2, type2) = acc.get_value(2).unwrap();
        assert_eq!(type2, WireType::LengthDelimited);
        assert_eq!(val2, &[104, 105]);
    }

    #[test]
    fn test_accessor_repeated() {
        // Field 1: 10, Field 1: 20, Field 1: 30
        // Tags: 8, 8, 8. Values: 10, 20, 30
        let data = [8, 10, 8, 20, 8, 30];
        let acc = ProtoAccessor::new(&data).unwrap();

        // Last value should be 30
        let (val, _) = acc.get_value(1).unwrap();
        assert_eq!(val, &[30]);

        // Iteration should find all three
        let results: Vec<_> = acc.iter_repeated(1).collect();
        assert_eq!(results.len(), 3);
        assert_eq!(results[0].as_ref().unwrap().0, &[10]);
        assert_eq!(results[1].as_ref().unwrap().0, &[20]);
        assert_eq!(results[2].as_ref().unwrap().0, &[30]);
    }

    #[test]
    fn test_builder_basic() {
        let mut buf = [0u8; 1024];
        let mut builder = ProtoBuilder::new(&mut buf);
        builder.write_string(1, "hello").unwrap();
        builder.write_int32(2, 42).unwrap();
        let data = builder.finish().unwrap();

        let acc = ProtoAccessor::new(data).unwrap();
        let (val1, _) = acc.get_value(1).unwrap();
        assert_eq!(val1, "hello".as_bytes());
        let (val2, _) = acc.get_value(2).unwrap();
        assert_eq!(val2, &[42]);
    }

    #[test]
    fn test_builder_overflow() {
        let mut buf = [0u8; 2];
        let mut builder = ProtoBuilder::new(&mut buf);
        let result = builder.write_string(1, "too long");
        assert_eq!(result, Err(RotoError::BufferOverflow));
    }
}

pub struct ProtoBuilder<'a> {
    buf: &'a mut [u8],
    pos: usize,
}

impl<'a> ProtoBuilder<'a> {
    pub fn new(buf: &'a mut [u8]) -> Self {
        Self { buf, pos: 0 }
    }

    fn write_tag(&mut self, field_number: u32, wire_type: WireType) -> Result<()> {
        let mut temp = [0u8; 10];
        let len = Tag::encode(field_number, wire_type, &mut temp)?;
        self.append_bytes(&temp[..len])
    }

    fn append_bytes(&mut self, bytes: &[u8]) -> Result<()> {
        if self.pos + bytes.len() > self.buf.len() {
            return Err(RotoError::BufferOverflow);
        }
        self.buf[self.pos..self.pos + bytes.len()].copy_from_slice(bytes);
        self.pos += bytes.len();
        Ok(())
    }

    pub fn write_varint(&mut self, field_number: u32, value: u64) -> Result<()> {
        self.write_tag(field_number, WireType::Varint)?;
        let mut temp = [0u8; 10];
        let len = write_varint(value, &mut temp)?;
        self.append_bytes(&temp[..len])
    }

    pub fn write_int32(&mut self, field_number: u32, value: i32) -> Result<()> {
        self.write_varint(field_number, value as u64)
    }

    pub fn write_string(&mut self, field_number: u32, value: &str) -> Result<()> {
        self.write_tag(field_number, WireType::LengthDelimited)?;
        let bytes = value.as_bytes();
        let mut len_buf = [0u8; 10];
        let len_len = write_varint(bytes.len() as u64, &mut len_buf)?;
        self.append_bytes(&len_buf[..len_len])?;
        self.append_bytes(bytes)
    }

    pub fn write_fixed32(&mut self, field_number: u32, value: u32) -> Result<()> {
        self.write_tag(field_number, WireType::Fixed32)?;
        self.append_bytes(&value.to_le_bytes())
    }

    pub fn write_fixed64(&mut self, field_number: u32, value: u64) -> Result<()> {
        self.write_tag(field_number, WireType::Fixed64)?;
        self.append_bytes(&value.to_le_bytes())
    }

    pub fn write_bytes(&mut self, field_number: u32, value: &[u8]) -> Result<()> {
        self.write_tag(field_number, WireType::LengthDelimited)?;
        let mut len_buf = [0u8; 10];
        let len_len = write_varint(value.len() as u64, &mut len_buf)?;
        self.append_bytes(&len_buf[..len_len])?;
        self.append_bytes(value)
    }

    pub fn finish(self) -> Result<&'a mut [u8]> {
        Ok(&mut self.buf[..self.pos])
    }
}