1 files changed, 79 insertions, 0 deletions
diff --git a/pw_tokenizer/rust/pw_tokenizer/internal.rs b/pw_tokenizer/rust/pw_tokenizer/internal.rs
new file mode 100644
index 000000000..4b798b973
--- /dev/null
+++ b/pw_tokenizer/rust/pw_tokenizer/internal.rs
@@ -0,0 +1,79 @@
+// Copyright 2023 The Pigweed Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+use core::cmp::min;
+
+use pw_status::Result;
+use pw_stream::{Cursor, Write};
+
+pub fn encode_string(cursor: &mut Cursor<&mut [u8]>, value: &str) -> Result<()> {
+    const MAX_STRING_LENGTH: usize = 0x7f;
+
+    let string_bytes = value.as_bytes();
+
+    // Limit the encoding to the lesser of 127 or the available space in the buffer.
+    let max_len = min(MAX_STRING_LENGTH, cursor.remaining() - 1);
+    let overflow = max_len < string_bytes.len();
+    let len = min(max_len, string_bytes.len());
+
+    // First byte of an encoded string is it's length.
+    let mut header = len as u8;
+
+    // The high bit of the first byte is used to indicate if the string was
+    // truncated.
+    if overflow {
+        header |= 0x80;
+    }
+    cursor.write_all(&[header as u8])?;
+
+    cursor.write_all(&string_bytes[..len])
+}
+
+#[cfg(test)]
+mod test {
+    use pw_stream::{Cursor, Seek};
+
+    use super::encode_string;
+
+    fn do_string_encode_test<const BUFFER_LEN: usize>(value: &str, expected: &[u8]) {
+        let mut buffer = [0u8; BUFFER_LEN];
+        let mut cursor = Cursor::new(&mut buffer[..]);
+        encode_string(&mut cursor, value).unwrap();
+
+        let len = cursor.stream_position().unwrap() as usize;
+        let buffer = cursor.into_inner();
+
+        assert_eq!(len, expected.len());
+        assert_eq!(&buffer[..len], expected);
+    }
+
+    #[test]
+    fn test_string_encode() {
+        do_string_encode_test::<64>("test", b"\x04test");
+        do_string_encode_test::<4>("test", b"\x83tes");
+        do_string_encode_test::<1>("test", b"\x80");
+
+        // Truncates when the string does not fit.
+        do_string_encode_test::<64>(
+            "testtesttesttesttesttesttesttesttesttesttesttesttesttesttesttest",
+            b"\xbftesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttes",
+        );
+
+        // Truncates when string is over 127 bytes.
+        do_string_encode_test::<1024>(
+            "testtesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttest",
+            b"\xfftesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttesttes",
+        );
+    }
+}