[parquet] Adding counting method in thrift_enum macro to support ENCODING_SLOTS

parquet/src/basic.rs

@@ -741,7 +741,7 @@ pub struct EncodingMask(i32);
 
 impl EncodingMask {
     /// Highest valued discriminant in the [`Encoding`] enum
-    const MAX_ENCODING: i32 = Encoding::BYTE_STREAM_SPLIT as i32;
+    const MAX_ENCODING: i32 = Encoding::MAX_DISCRIMINANT;
     /// A mask consisting of unused bit positions, used for validation. This includes the never
     /// used GROUP_VAR_INT encoding value of `1`.
     const ALLOWED_MASK: u32 =

parquet/src/column/reader/decoder.rs

@@ -138,7 +138,7 @@ pub trait ColumnValueDecoder {
 ///
 /// This replaces `HashMap` lookups with direct indexing to avoid hashing overhead in the
 /// hot decoding paths.
-const ENCODING_SLOTS: usize = Encoding::BYTE_STREAM_SPLIT as usize + 1;
+const ENCODING_SLOTS: usize = Encoding::MAX_DISCRIMINANT as usize + 1;
 
 /// An implementation of [`ColumnValueDecoder`] for `[T::T]`
 pub struct ColumnValueDecoderImpl<T: DataType> {

parquet/src/parquet_macros.rs

@@ -79,6 +79,35 @@ macro_rules! thrift_enum {
                 Ok(field_id)
             }
         }
+
+        impl $identifier {
+            #[allow(deprecated)]
+            #[doc = "Returns a slice containing every variant of this enum."]
+            #[allow(dead_code)]
+            pub const VARIANTS: &'static [Self] = &[
+                $(Self::$field_name),*
+            ];
+
+            #[allow(deprecated)]
+            const fn max_discriminant_impl() -> i32 {
+                let values: &[i32] = &[$($field_value),*];
+                let mut max = values[0];
+                let mut idx = 1;
+                while idx < values.len() {
+                    let candidate = values[idx];
+                    if candidate > max {
+                        max = candidate;
+                    }
+                    idx += 1;
+                }
+                max
+            }
+
+            #[allow(deprecated)]
+            #[doc = "Returns the largest discriminant value defined for this enum."]
+            #[allow(dead_code)]
+            pub const MAX_DISCRIMINANT: i32 = Self::max_discriminant_impl();
+        }
     }
 }