Serialize custom persistent fields as base64

This still takes up more space than plain JSON would, but I don't know
if serde_json would allow inlining that JSON. Presumably not. It would
get hairy quickly as a regular embedded JSON string due to the encoding.

Cargo.lock

@@ -8,6 +8,12 @@ version = "1.0.53"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "94a45b455c14666b85fc40a019e8ab9eb75e3a124e05494f5397122bc9eb06e0"
 
+[[package]]
+name = "base64"
+version = "0.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "904dfeac50f3cdaba28fc6f57fdcddb75f49ed61346676a78c4ffe55877802fd"
+
 [[package]]
 name = "gain"
 version = "0.1.0"
@@ -31,6 +37,7 @@ checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
 name = "nih_plug"
 version = "0.1.0"
 dependencies = [
+ "base64",
  "lazy_static",
  "nih_plug_derive",
  "serde",

Cargo.toml

@@ -10,6 +10,7 @@ members = ["nih_plug_derive", "plugins/gain", "xtask"]
 
 [dependencies]
 nih_plug_derive = { path = "nih_plug_derive" }
+base64 = "0.13"
 lazy_static = "1.4"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"

src/wrapper/state.rs

@@ -16,6 +16,7 @@
 
 //! Utilities for saving a [crate::plugin::Plugin]'s state.
 
+use serde::de::Error;
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 
@@ -39,7 +40,38 @@ pub(crate) struct State {
     /// on the [Params] struct that's annotated with `#[persist = "stable_name"]` will be persisted
     /// this way.
     ///
-    /// TODO: Serialize this as base64 or some other more densely packed representation. Currently
-    ///       every bute takes up 2-4 ASCII characters
+    /// The individual JSON-serialized fields are encoded as base64 strings so they don't take up as
+    /// much space in the preset. Storing them as a plain JSON string would have also been possible,
+    /// but that can get messy with escaping since those will likely also contain double quotes.
+    #[serde(serialize_with = "encode_fields")]
+    #[serde(deserialize_with = "decode_fields")]
     pub fields: HashMap<String, Vec<u8>>,
 }
+
+fn encode_fields<S>(bytes: &HashMap<String, Vec<u8>>, serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: serde::Serializer,
+{
+    serializer.collect_map(
+        bytes
+            .into_iter()
+            .map(|(id, json)| (id, base64::encode(json))),
+    )
+}
+
+fn decode_fields<'de, D>(deserializer: D) -> Result<HashMap<String, Vec<u8>>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    let base64_map: HashMap<String, String> = HashMap::deserialize(deserializer)?;
+    let decoded_map: Result<HashMap<String, Vec<u8>>, D::Error> = base64_map
+        .into_iter()
+        .map(|(id, base64)| {
+            base64::decode(base64)
+                .map(|decoded| (id, decoded))
+                .map_err(|err| D::Error::custom(format!("base64 decode failed: {}", err)))
+        })
+        .collect();
+
+    Ok(decoded_map?)
+}