1 files changed, 163 insertions, 0 deletions
diff --git a/common/json_writer/src/lib.rs b/common/json_writer/src/lib.rs
new file mode 100644
index 00000000..c3c372bf
--- /dev/null
+++ b/common/json_writer/src/lib.rs
@@ -0,0 +1,163 @@
+use std::num::FpCategory;
+
+#[derive(Debug, Clone)]
+pub struct JsonWriter {
+    buffer: Vec<u8>,
+}
+
+#[derive(thiserror::Error, Debug)]
+pub enum JsonWriterError {
+    #[error("JsonWriter produced invalid UTF8 string")]
+    InvalidUtf8Conversion(#[from] std::string::FromUtf8Error),
+
+    #[error("IoError")]
+    IoError(#[from] std::io::Error),
+
+    #[error("Serde Json error")]
+    SerdeJsonError(#[from] serde_json::Error),
+
+    #[error("Invalid f64 value {value:?}")]
+    InvalidF64Value { value: f64 },
+}
+
+impl JsonWriter {
+    pub fn new() -> Self {
+        Self { buffer: Vec::new() }
+    }
+
+    pub fn with_capacity(capacity: usize) -> Self {
+        Self {
+            buffer: Vec::with_capacity(capacity),
+        }
+    }
+
+    pub fn write_key(&mut self, key: &str) -> Result<(), JsonWriterError> {
+        self.write_str(key)?;
+        self.buffer.push(b':');
+        Ok(())
+    }
+
+    pub fn write_str(&mut self, s: &str) -> Result<(), JsonWriterError> {
+        Ok(serde_json::to_writer(&mut self.buffer, s)?)
+    }
+
+    pub fn write_f64(&mut self, value: f64) -> Result<(), JsonWriterError> {
+        match value.classify() {
+            FpCategory::Normal | FpCategory::Zero | FpCategory::Subnormal => {
+                Ok(serde_json::to_writer(&mut self.buffer, &value)?)
+            }
+            FpCategory::Infinite | FpCategory::Nan => {
+                Err(JsonWriterError::InvalidF64Value { value })
+            }
+        }
+    }
+
+    pub fn write_separator(&mut self) {
+        self.buffer.push(b',');
+    }
+
+    pub fn write_open_obj(&mut self) {
+        self.buffer.push(b'{');
+    }
+
+    pub fn write_close_obj(&mut self) {
+        self.buffer.push(b'}');
+    }
+
+    pub fn into_string(self) -> Result<String, JsonWriterError> {
+        Ok(String::from_utf8(self.buffer)?)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn write_empty_message() -> anyhow::Result<()> {
+        let mut jw = JsonWriter::new();
+        jw.write_open_obj();
+        jw.write_close_obj();
+        assert_eq!(jw.into_string()?, "{}");
+        Ok(())
+    }
+
+    #[test]
+    fn write_invalid_f64_message() -> anyhow::Result<()> {
+        let mut jw = JsonWriter::new();
+        let value = 1.0 / 0.0;
+        let error = jw.write_f64(value).unwrap_err();
+        assert_eq!(error.to_string(), "Invalid f64 value inf");
+        Ok(())
+    }
+
+    #[test]
+    fn write_key_with_quote() -> anyhow::Result<()> {
+        let mut jw = JsonWriter::with_capacity(128);
+        jw.write_key("va\"lue")?;
+        assert_eq!(jw.into_string()?, "\"va\\\"lue\":");
+        Ok(())
+    }
+
+    #[test]
+    fn write_timestamp_message() -> anyhow::Result<()> {
+        let mut jw = JsonWriter::with_capacity(128);
+        jw.write_open_obj();
+        jw.write_key("time")?;
+        jw.write_str("2013-06-22T17:03:14.123+02:00")?;
+        jw.write_close_obj();
+        assert_eq!(
+            jw.into_string()?,
+            r#"{"time":"2013-06-22T17:03:14.123+02:00"}"#
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn write_single_value_message() -> anyhow::Result<()> {
+        let mut jw = JsonWriter::with_capacity(128);
+        jw.write_open_obj();
+        jw.write_key("time")?;
+        jw.write_str("2013-06-22T17:03:14.123+02:00")?;
+        jw.write_separator();
+        jw.write_key("temperature")?;
+        jw.write_f64(128.0)?;
+        jw.write_close_obj();
+        assert_eq!(
+            jw.into_string()?,
+            r#"{"time":"2013-06-22T17:03:14.123+02:00","temperature":128.0}"#
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn write_multivalue_message() -> anyhow::Result<()> {
+        let mut jw = JsonWriter::with_capacity(128);
+        jw.write_open_obj();
+        jw.write_key("time")?;
+        jw.write_str("2013-06-22T17:03:14.123+02:00")?;
+        jw.write_separator();
+        jw.write_key("temperature")?;
+        jw.write_f64(128.0)?;
+        jw.write_separator();
+        jw.write_key("location")?;
+        jw.write_open_obj();
+        jw.write_key("altitude")?;
+        jw.write_f64(1028.0)?;
+        jw.write_separator();
+        jw.write_key("longitude")?;
+        jw.write_f64(1288.0)?;
+        jw.write_separator();
+        jw.write_key("longitude")?;
+        jw.write_f64(1280.0)?;
+        jw.write_close_obj();
+        jw.write_close_obj();
+
+        assert_eq!(
+            jw.into_string()?,
+            r#"{"time":"2013-06-22T17:03:14.123+02:00","temperature":128.0,"location":{"altitude":1028.0,"longitude":1288.0,"longitude":1280.0}}"#
+        );
+
+        Ok(())
+    }
+}