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use escape8259::escape; use serde::{ser, Serialize}; use std::{error, fmt::Display}; pub struct Serializer { // This string starts empty and JSON is appended as values are serialized. output: String, } #[derive(Debug)] pub struct JaclSerError; impl Display for JaclSerError { fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { unreachable!(); } } impl error::Error for JaclSerError {} impl ser::Error for JaclSerError { fn custom<T>(_: T) -> Self where T: std::fmt::Display, { unreachable!(); } } // By convention, the public API of a Serde serializer is one or more `to_abc` // functions such as `to_string`, `to_bytes`, or `to_writer` depending on what // Rust types the serializer is able to produce as output. // // This basic serializer supports only `to_string`. pub fn to_string<T>(value: &T) -> Result<String, JaclSerError> where T: Serialize, { let mut serializer = Serializer { output: String::new(), }; value.serialize(&mut serializer)?; Ok(serializer.output) } impl<'a> ser::Serializer for &'a mut Serializer { // The output type produced by this `Serializer` during successful // serialization. Most serializers that produce text or binary output should // set `Ok = ()` and serialize into an `io::Write` or buffer contained // within the `Serializer` instance, as happens here. Serializers that build // in-memory data structures may be simplified by using `Ok` to propagate // the data structure around. type Ok = (); // The error type when some error occurs during serialization. type Error = JaclSerError; // Associated types for keeping track of additional state while serializing // compound data structures like sequences and maps. In this case no // additional state is required beyond what is already stored in the // Serializer struct. type SerializeSeq = Self; type SerializeTuple = Self; type SerializeTupleStruct = Self; type SerializeTupleVariant = Self; type SerializeMap = Self; type SerializeStruct = Self; type SerializeStructVariant = Self; // Here we go with the simple methods. The following 12 methods receive one // of the primitive types of the data model and map it to JSON by appending // into the output string. fn serialize_bool(self, v: bool) -> Result<(), JaclSerError> { self.output += if v { "true" } else { "false" }; Ok(()) } // JSON does not distinguish between different sizes of integers, so all // signed integers will be serialized the same and all unsigned integers // will be serialized the same. Other formats, especially compact binary // formats, may need independent logic for the different sizes. fn serialize_i8(self, v: i8) -> Result<(), JaclSerError> { self.serialize_i64(i64::from(v)) } fn serialize_i16(self, v: i16) -> Result<(), JaclSerError> { self.serialize_i64(i64::from(v)) } fn serialize_i32(self, v: i32) -> Result<(), JaclSerError> { self.serialize_i64(i64::from(v)) } // Not particularly efficient but this is example code anyway. A more // performant approach would be to use the `itoa` crate. fn serialize_i64(self, v: i64) -> Result<(), JaclSerError> { self.output += &v.to_string(); Ok(()) } fn serialize_u8(self, v: u8) -> Result<(), JaclSerError> { self.serialize_u64(u64::from(v)) } fn serialize_u16(self, v: u16) -> Result<(), JaclSerError> { self.serialize_u64(u64::from(v)) } fn serialize_u32(self, v: u32) -> Result<(), JaclSerError> { self.serialize_u64(u64::from(v)) } fn serialize_u64(self, v: u64) -> Result<(), JaclSerError> { self.output += &v.to_string(); Ok(()) } fn serialize_f32(self, v: f32) -> Result<(), JaclSerError> { self.serialize_f64(f64::from(v)) } fn serialize_f64(self, v: f64) -> Result<(), JaclSerError> { self.output += &v.to_string(); Ok(()) } // Serialize a char as a single-character string. Other formats may // represent this differently. fn serialize_char(self, v: char) -> Result<(), JaclSerError> { self.serialize_str(&v.to_string()) } // This only works for strings that don't require escape sequences but you // get the idea. For example it would emit invalid JSON if the input string // contains a '"' character. fn serialize_str(self, v: &str) -> Result<(), JaclSerError> { self.output += "\""; self.output += &escape(v); self.output += "\""; Ok(()) } // Serialize a byte array as an array of bytes. Could also use a base64 // string here. Binary formats will typically represent byte arrays more // compactly. fn serialize_bytes(self, v: &[u8]) -> Result<(), JaclSerError> { use serde::ser::SerializeSeq; let mut seq = self.serialize_seq(Some(v.len()))?; for byte in v { seq.serialize_element(byte)?; } seq.end() } // An absent optional is represented as the JSON `null`. fn serialize_none(self) -> Result<(), JaclSerError> { self.serialize_unit() } // A present optional is represented as just the contained value. Note that // this is a lossy representation. For example the values `Some(())` and // `None` both serialize as just `null`. Unfortunately this is typically // what people expect when working with JSON. Other formats are encouraged // to behave more intelligently if possible. fn serialize_some<T>(self, value: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { value.serialize(self) } // In Serde, unit means an anonymous value containing no data. Map this to // JSON as `null`. fn serialize_unit(self) -> Result<(), JaclSerError> { self.output += "null"; Ok(()) } // Unit struct means a named value containing no data. Again, since there is // no data, map this to JSON as `null`. There is no need to serialize the // name in most formats. fn serialize_unit_struct(self, _name: &'static str) -> Result<(), JaclSerError> { self.serialize_unit() } // When serializing a unit variant (or any other kind of variant), formats // can choose whether to keep track of it by index or by name. Binary // formats typically use the index of the variant and human-readable formats // typically use the name. fn serialize_unit_variant( self, _name: &'static str, _variant_index: u32, _variant: &'static str, ) -> Result<(), JaclSerError> { return Err(JaclSerError); } // As is done here, serializers are encouraged to treat newtype structs as // insignificant wrappers around the data they contain. fn serialize_newtype_struct<T>(self, _name: &'static str, value: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { value.serialize(self) } // Note that newtype variant (and all of the other variant serialization // methods) refer exclusively to the "externally tagged" enum // representation. // // Serialize this to JSON in externally tagged form as `{ NAME: VALUE }`. fn serialize_newtype_variant<T>( self, _name: &'static str, _variant_index: u32, _variant: &'static str, _value: &T, ) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { return Err(JaclSerError); } // Now we get to the serialization of compound types. // // The start of the sequence, each value, and the end are three separate // method calls. This one is responsible only for serializing the start, // which in JSON is `[`. // // The length of the sequence may or may not be known ahead of time. This // doesn't make a difference in JSON because the length is not represented // explicitly in the serialized form. Some serializers may only be able to // support sequences for which the length is known up front. fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq, JaclSerError> { self.output += "["; Ok(self) } // Tuples look just like sequences in JSON. Some formats may be able to // represent tuples more efficiently by omitting the length, since tuple // means that the corresponding `Deserialize implementation will know the // length without needing to look at the serialized data. fn serialize_tuple(self, len: usize) -> Result<Self::SerializeTuple, JaclSerError> { self.serialize_seq(Some(len)) } // Tuple structs look just like sequences in JSON. fn serialize_tuple_struct( self, _name: &'static str, len: usize, ) -> Result<Self::SerializeTupleStruct, JaclSerError> { self.serialize_seq(Some(len)) } // Tuple variants are represented in JSON as `{ NAME: [DATA...] }`. Again // this method is only responsible for the externally tagged representation. fn serialize_tuple_variant( self, _name: &'static str, _variant_index: u32, _variant: &'static str, _len: usize, ) -> Result<Self::SerializeTupleVariant, JaclSerError> { return Err(JaclSerError); } // Maps are represented in JSON as `{ K: V, K: V, ... }`. fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap, JaclSerError> { self.output += "{"; Ok(self) } // Structs look just like maps in JSON. In particular, JSON requires that we // serialize the field names of the struct. Other formats may be able to // omit the field names when serializing structs because the corresponding // Deserialize implementation is required to know what the keys are without // looking at the serialized data. fn serialize_struct( self, _name: &'static str, _len: usize, ) -> Result<Self::SerializeStruct, JaclSerError> { self.output += "("; Ok(self) } // Struct variants are represented in JSON as `{ NAME: { K: V, ... } }`. // This is the externally tagged representation. fn serialize_struct_variant( self, _name: &'static str, _variant_index: u32, _variant: &'static str, _len: usize, ) -> Result<Self::SerializeStructVariant, JaclSerError> { return Err(JaclSerError); } } impl<'a> ser::SerializeTupleVariant for &'a mut Serializer { type Ok = (); type Error = JaclSerError; fn serialize_field<T: ?Sized>(&mut self, _value: &T) -> Result<(), Self::Error> where T: Serialize, { return Err(JaclSerError); } fn end(self) -> Result<Self::Ok, Self::Error> { return Err(JaclSerError); } } impl<'a> ser::SerializeStructVariant for &'a mut Serializer { type Ok = (); type Error = JaclSerError; fn serialize_field<T: ?Sized>( &mut self, _key: &'static str, _value: &T, ) -> Result<(), Self::Error> where T: Serialize, { return Err(JaclSerError); } fn end(self) -> Result<Self::Ok, Self::Error> { return Err(JaclSerError); } } // The following 7 impls deal with the serialization of compound types like // sequences and maps. Serialization of such types is begun by a Serializer // method and followed by zero or more calls to serialize individual elements of // the compound type and one call to end the compound type. // // This impl is SerializeSeq so these methods are called after `serialize_seq` // is called on the Serializer. impl<'a> ser::SerializeSeq for &'a mut Serializer { // Must match the `Ok` type of the serializer. type Ok = (); // Must match the `Error` type of the serializer. type Error = JaclSerError; // Serialize a single element of the sequence. fn serialize_element<T>(&mut self, value: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { if !self.output.ends_with('[') { self.output += " "; } value.serialize(&mut **self) } // Close the sequence. fn end(self) -> Result<(), JaclSerError> { self.output += "]"; Ok(()) } } // Same thing but for tuples. impl<'a> ser::SerializeTuple for &'a mut Serializer { type Ok = (); type Error = JaclSerError; fn serialize_element<T>(&mut self, value: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { if !self.output.ends_with('[') { self.output += " "; } value.serialize(&mut **self) } fn end(self) -> Result<(), JaclSerError> { self.output += "]"; Ok(()) } } // Same thing but for tuple structs. impl<'a> ser::SerializeTupleStruct for &'a mut Serializer { type Ok = (); type Error = JaclSerError; fn serialize_field<T>(&mut self, value: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { if !self.output.ends_with('[') { self.output += " "; } value.serialize(&mut **self) } fn end(self) -> Result<(), JaclSerError> { self.output += "]"; Ok(()) } } // Some `Serialize` types are not able to hold a key and value in memory at the // same time so `SerializeMap` implementations are required to support // `serialize_key` and `serialize_value` individually. // // There is a third optional method on the `SerializeMap` trait. The // `serialize_entry` method allows serializers to optimize for the case where // key and value are both available simultaneously. In JSON it doesn't make a // difference so the default behavior for `serialize_entry` is fine. impl<'a> ser::SerializeMap for &'a mut Serializer { type Ok = (); type Error = JaclSerError; // The Serde data model allows map keys to be any serializable type. JSON // only allows string keys so the implementation below will produce invalid // JSON if the key serializes as something other than a string. // // A real JSON serializer would need to validate that map keys are strings. // This can be done by using a different Serializer to serialize the key // (instead of `&mut **self`) and having that other serializer only // implement `serialize_str` and return an error on any other data type. fn serialize_key<T>(&mut self, key: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { if !self.output.ends_with('{') { self.output += " "; } key.serialize(&mut **self) } // It doesn't make a difference whether the colon is printed at the end of // `serialize_key` or at the beginning of `serialize_value`. In this case // the code is a bit simpler having it here. fn serialize_value<T>(&mut self, value: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { self.output += ":"; value.serialize(&mut **self) } fn end(self) -> Result<(), JaclSerError> { self.output += "}"; Ok(()) } } // Structs are like maps in which the keys are constrained to be compile-time // constant strings. impl<'a> ser::SerializeStruct for &'a mut Serializer { type Ok = (); type Error = JaclSerError; fn serialize_field<T>(&mut self, key: &'static str, value: &T) -> Result<(), JaclSerError> where T: ?Sized + Serialize, { if !self.output.ends_with('(') { self.output += " "; } self.output += key; self.output += ":"; value.serialize(&mut **self) } fn end(self) -> Result<(), JaclSerError> { self.output += ")"; Ok(()) } } #[test] fn test_struct() { #[derive(Serialize)] struct Test { int: u32, seq: Vec<&'static str>, } let test = Test { int: 1, seq: vec![r#" "a" "#, "b"], }; assert_eq!(to_string(&test).unwrap(), r#"(int:1 seq:[" \"a\" " "b"])"#); }