add comment to [ScalarCompressor]

src/composer/compiler/scalar.rs

@@ -12,6 +12,27 @@ use hashbrown::HashMap;
 ///
 /// It will compress [i8] equivalent values into a single byte, and perform a
 /// full serialization for values that won't fit [i8].
+///
+/// The compressor contains an internal mapping for indexed [i8] values. Here
+/// follows a couple of example values to better illustrate the concept:
+///
+/// - `-128_i8` is not mapped as it is [i8::MIN]
+/// - `-127_i8` is mapped to `2`
+/// - `-126_i8` is mapped to `3`
+/// - `-125_i8` is mapped to `4`
+/// - (...)
+/// - `-1_i8` is mapped to `128`
+/// - `0_i8` is mapped to `129`
+/// - `1_i8` is mapped to `130`
+/// - (...)
+/// - `124_i8` is mapped to `253`
+/// - `125_i8` is mapped to `254`
+/// - `126_i8` is mapped to `255`
+///
+/// A `BlsScalar::zero() - BlsScalar::from(126)` will be serialized as `3_u8`.
+///
+/// A [BlsScalar] that is not in the map will be serialized as `0_u8`, followed
+/// by the result of [BlsScalar::to_bytes].
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ScalarCompressor {
     map: HashMap<BlsScalar, u8>,