Add support for protobuf map fields

Update the generator to detect map fields and use MapFieldIterator.
Implement MapFieldIterator in the runtime to handle key-value pair
extraction and add write_map_entry to ProtoBuilder. Add tests to
verify that map-bearing messages generate and compile correctly.

.gitignore

@@ -1,3 +1,4 @@
 /target
 test_gen_project
 test_types_gen_project
+test_map_gen_project

codegen/data/test_map.desc

@@ -0,0 +1,9 @@
+
+«

+codegen/data/test_map.proto	roto.test"y
+MapTest4
+my_map
 (2.roto.test.MapTest.MyMapEntryRmyMap8
+
+MyMapEntry
+key
 
(	Rkey
+value 
(Rvalue:8
bproto3

codegen/data/test_map.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+package roto.test;
+
+message MapTest {
+  map<string, int32> my_map = 1;
+}

codegen/src/generator.rs

@@ -1,6 +1,6 @@
 use crate::google::protobuf::descriptor::{
     DescriptorProto, EnumDescriptorProto, FieldDescriptorProto, FileDescriptorProto,
-    FileDescriptorSet,
+    FileDescriptorSet, MessageOptions,
 };
 use roto_runtime::ProtoAccessor;
 use std::collections::{HashMap, HashSet};
@@ -33,11 +33,16 @@ pub fn to_snake_case(s: &str) -> String {
     result
 }
 
-fn map_type_to_rust_accessor(field_type: i32, label: i32) -> (String, String) {
+fn map_type_to_rust_accessor(field_type: i32, label: i32, is_map: bool) -> (String, String) {
     if label == 3 {
         // LABEL_REPEATED
+        let iterator_type = if is_map {
+            "roto_runtime::MapFieldIterator<'a>"
+        } else {
+            "roto_runtime::RepeatedFieldIterator<'a>"
+        };
         return (
-            "roto_runtime::RepeatedFieldIterator<'a>".to_string(),
+            iterator_type.to_string(),
             "".to_string(), // Not used for repeated fields in the same way
         );
     }
@@ -159,14 +164,23 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
         let tag = field_proto.number().unwrap();
         let f_type = field_proto.r#type().unwrap() as i32;
         let f_label = field_proto.label().unwrap() as i32;
+        let is_map = field_proto
+            .options()
+            .map(|opt| {
+                MessageOptions::new(opt)
+                    .unwrap()
+                    .map_entry()
+                    .unwrap_or(false)
+            })
+            .unwrap_or(false);
 
-        fields_info.push((field_name.to_string(), tag, f_type, f_label));
+        fields_info.push((field_name.to_string(), tag, f_type, f_label, is_map));
     }
 
     output.push_str(&format!("pub struct {}<'a> {{\n", msg_name));
     output.push_str("    accessor: roto_runtime::ProtoAccessor<'a>,\n");
 
-    for (field_name, _tag, _f_type, f_label) in &fields_info {
+    for (field_name, _tag, _f_type, f_label, _is_map) in &fields_info {
         if *f_label == 3 {
             output.push_str(&format!("    {}_start: Option<usize>,\n", field_name));
             output.push_str(&format!("    {}_end: Option<usize>,\n", field_name));
@@ -180,7 +194,7 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
     output.push_str("    pub fn new(data: &'a [u8]) -> roto_runtime::Result<Self> {\n");
     output.push_str("        let accessor = roto_runtime::ProtoAccessor::new(data)?;\n");
     if !fields_info.is_empty() {
-        for (name, _, _, label) in &fields_info {
+        for (name, _, _, label, _is_map) in &fields_info {
             if *label == 3 {
                 output.push_str(&format!("        let mut {}_start = None;\n", name));
                 output.push_str(&format!("        let mut {}_end = None;\n", name));
@@ -192,7 +206,7 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
         output.push_str("        for item in accessor.fields() {\n");
         output.push_str("            let (offset, tag, _) = item?;\n");
 
-        for (name, tag, _, label) in &fields_info {
+        for (name, tag, _, label, _is_map) in &fields_info {
             if *label == 3 {
                 output.push_str(&format!("            if tag.field_number == {} {{\n", tag));
                 output.push_str(&format!(
@@ -213,7 +227,7 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
 
     output.push_str("        Ok(Self {\n");
     output.push_str("            accessor,\n");
-    for (name, _, _, label) in &fields_info {
+    for (name, _, _, label, _is_map) in &fields_info {
         if *label == 3 {
             output.push_str(&format!("{}_start, {}_end,\n", name, name));
         } else {
@@ -222,8 +236,8 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
     }
     output.push_str("        })\n    }\n\n");
 
-    for (field_name, tag, f_type, f_label) in fields_info {
+    for (field_name, tag, f_type, f_label, is_map) in fields_info {
-        let (rust_type, logic) = map_type_to_rust_accessor(f_type, f_label);
+        let (rust_type, logic) = map_type_to_rust_accessor(f_type, f_label, is_map);
         let safe_name = if field_name == "type" {
             format!("r#{}", field_name)
         } else {
@@ -239,11 +253,19 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
                 "        match (self.{}_start, self.{}_end) {{\n",
                 field_name, field_name
             ));
+            if is_map {
+                output.push_str(&format!("            (Some(start), Some(end)) => roto_runtime::MapFieldIterator::new(self.accessor.iter_repeated_range({}, start, end)),\n", tag));
+                output.push_str(&format!(
+                    "            _ => roto_runtime::MapFieldIterator::new(self.accessor.iter_repeated({})),\n",
+                    tag
+                ));
+            } else {
                 output.push_str(&format!("            (Some(start), Some(end)) => self.accessor.iter_repeated_range({}, start, end),\n", tag));
                 output.push_str(&format!(
                     "            _ => self.accessor.iter_repeated({}),\n",
                     tag
                 ));
+            }
             output.push_str("        }\n    }\n\n");
         } else {
             output.push_str(&format!(

codegen/tests/test_map_build.rs

@@ -0,0 +1,67 @@
+use roto_codegen::google::protobuf::descriptor::FileDescriptorSet;
+use std::fs;
+use std::process::Command;
+
+#[test]
+fn test_map_generated_code_builds() {
+    // 1. Load FileDescriptorSet from data/test_map.desc
+    let desc_path = "data/test_map.desc";
+    let data = fs::read(desc_path).expect("Failed to read test_map.desc");
+    let set = FileDescriptorSet::new(&data)
+        .expect("Failed to create FileDescriptorSet from test_map.desc");
+
+    let generated_files = roto_codegen::generator::generate_rust_code(&set, None, false);
+    assert!(
+        !generated_files.is_empty(),
+        "Generated code should not be empty"
+    );
+
+    // 2. Setup a temporary Cargo project to verify the code builds
+    let root = std::env::current_dir().expect("Failed to get current directory");
+    let temp_project_dir = root.join("test_map_gen_project");
+
+    // Clean up previous runs
+    if temp_project_dir.exists() {
+        fs::remove_dir_all(&temp_project_dir).expect("Failed to clean up temp project directory");
+    }
+
+    // Create new library project
+    let status = Command::new("cargo")
+        .args(["new", "--lib", "test_map_gen_project"])
+        .current_dir(&root)
+        .status()
+        .expect("Failed to run cargo new");
+    assert!(status.success(), "cargo new failed");
+
+    // 3. Configure the project to depend on the current roto crate
+    let cargo_toml_path = temp_project_dir.join("Cargo.toml");
+    let cargo_toml_content =
+        fs::read_to_string(&cargo_toml_path).expect("Failed to read Cargo.toml");
+    let updated_cargo_toml = format!(
+        "{}\n\nroto-codegen = {{ path = \"..\" }}\nroto-runtime = {{ path = \"../../runtime\" }}\n\n[workspace]\n",
+        cargo_toml_content
+    );
+    fs::write(cargo_toml_path, updated_cargo_toml).expect("Failed to write Cargo.toml");
+
+    // 4. Write the generated code to src/lib.rs
+    let mut all_code = String::new();
+    for (_, content) in generated_files {
+        all_code.push_str(&content);
+        all_code.push_str("\n");
+    }
+    let final_code = all_code.replace("use crate::", "use roto::");
+    let lib_path = temp_project_dir.join("src/lib.rs");
+    fs::write(lib_path, final_code).expect("Failed to write generated code to src/lib.rs");
+
+    // 5. Attempt to build the project
+    let build_status = Command::new("cargo")
+        .args(["--offline", "build"])
+        .current_dir(&temp_project_dir)
+        .status()
+        .expect("Failed to run cargo build");
+
+    assert!(
+        build_status.success(),
+        "The generated Rust code for test_map.proto failed to build in a standalone project!"
+    );
+}

runtime/src/lib.rs

@@ -1,6 +1,33 @@
 use std::fmt;
 
-#[derive(Debug, PartialEq, Eq)]
+pub struct MapFieldIterator<'a> {
+    inner: RepeatedFieldIterator<'a>,
+}
+
+impl<'a> MapFieldIterator<'a> {
+    pub fn new(inner: RepeatedFieldIterator<'a>) -> Self {
+        Self { inner }
+    }
+}
+
+impl<'a> Iterator for MapFieldIterator<'a> {
+    type Item = Result<(&'a [u8], &'a [u8])>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.inner.next() {
+            Some(Ok((value, _wire_type))) => {
+                let accessor = ProtoAccessor::new(value).ok()?;
+                let (key_bytes, _) = accessor.get_value(1).ok()?;
+                let (val_bytes, _) = accessor.get_value(2).ok()?;
+                Some(Ok((key_bytes, val_bytes)))
+            }
+            Some(Err(e)) => Some(Err(e)),
+            None => None,
+        }
+    }
+}
+
+#[derive(Debug, PartialEq)]
 pub enum RotoError {
     UnexpectedEndOfBuffer,
     InvalidVarint,
@@ -769,6 +796,24 @@ impl<'a> ProtoBuilder<'a> {
         self.append_bytes(raw_bytes)
     }
 
+    pub fn write_map_entry(
+        &mut self,
+        field_number: u32,
+        key_encoded: &[u8],
+        value_encoded: &[u8],
+    ) -> Result<()> {
+        let entry_len = key_encoded.len() + value_encoded.len();
+        self.write_tag(field_number, WireType::LengthDelimited)?;
+
+        let mut len_buf = [0u8; 10];
+        let len_len = write_varint(entry_len as u64, &mut len_buf)?;
+        self.append_bytes(&len_buf[..len_len])?;
+
+        self.append_bytes(key_encoded)?;
+        self.append_bytes(value_encoded)?;
+        Ok(())
+    }
+
     pub fn finish(self) -> Result<&'a mut [u8]> {
         Ok(&mut self.buf[..self.pos])
     }