diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..30421de
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "roto"
+version = "0.1.0"
diff --git a/README.md b/README.md
index a08dc6e..55da5ba 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # roto
 
-Rust protos without the P.
+Rust protos without the pointers.
 
 The codegen is different; we don't create data structures.
 We mark what where each field is, and only read it when asked.
@@ -28,7 +28,7 @@ message Hello {
 fn parse_proto(data: &[u8]) -> Result<String> {
     // Scans the data, marks where each flag is as an offset
     // into the proto.
-    let accessor = HelloProto::new(accessor)?;
+    let accessor = HelloProto::new(data)?;
     // Load the hello world string; returns bytes, not
     // a Rust string.
     let hello_world = accessor.hello_world()?;
@@ -40,6 +40,16 @@ fn parse_proto(data: &[u8]) -> Result<String> {
 } 
 ```
 
+### Sample builder usage
+
+```rust
+let mut buf = [0u8; 1024];
+let mut builder = ProtoBuilder::new(&mut buf);
+builder.write_string(1, "hello world")?;
+builder.write_int32(2, 42)?;
+let data = builder.finish()?; // returns the used slice of the buffer
+```
+
 ### High level design
 
 The runtime library offers an iterator over the fields in a message, using the protobuf wire format provide
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..d7333e1
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,465 @@
+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])
+    }
+}