Repository: Peternator7/strum (rust)

Issue:
EnumString would be faster by using phf
When using EnumString to implement FromStr on large enums, since the generated code uses a match whose complexity is O(n), the performance is pretty low.

It looks like this could be much improved by using the phf crate (https://lib.rs/crates/phf) instead of a simple match.

Pull request description:
Add support for PHF in EnumString
Resolves #218

Via an additional `use_phf` attribute at the enum metadata level.

Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6215afc..050bf40 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+* [#220](https://github.com/Peternator7/strum/pull/220). Add support for PHF in `EnumString` (opt-in runtime
+  performance improvements for large enums as `#[strum(use_phf)]`, requires `phf` feature and increases MSRV to `1.46`)
+
 ## 0.24.0
 
 * [#212](https://github.com/Peternator7/strum/pull/212). Fix some clippy lints
diff --git a/strum/Cargo.toml b/strum/Cargo.toml
index 276c8fa..ca4dd8f 100644
--- a/strum/Cargo.toml
+++ b/strum/Cargo.toml
@@ -16,6 +16,7 @@ readme = "../README.md"
 
 [dependencies]
 strum_macros = { path = "../strum_macros", optional = true, version = "0.24" }
+phf = { version = "0.10", features = ["macros"], optional = true }
 
 [dev-dependencies]
 strum_macros = { path = "../strum_macros", version = "0.24" }
diff --git a/strum/src/lib.rs b/strum/src/lib.rs
index 0ce6e53..2711912 100644
--- a/strum/src/lib.rs
+++ b/strum/src/lib.rs
@@ -30,6 +30,10 @@
 // only for documentation purposes
 pub mod additional_attributes;
 
+#[cfg(feature = "phf")]
+#[doc(hidden)]
+pub use phf as _private_phf_reexport_for_macro_if_phf_feature;
+
 /// The `ParseError` enum is a collection of all the possible reasons
 /// an enum can fail to parse from a string.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
diff --git a/strum_macros/src/helpers/metadata.rs b/strum_macros/src/helpers/metadata.rs
index 08ca384..56e4c78 100644
--- a/strum_macros/src/helpers/metadata.rs
+++ b/strum_macros/src/helpers/metadata.rs
@@ -5,7 +5,8 @@ use syn::{
     parse2, parse_str,
     punctuated::Punctuated,
     spanned::Spanned,
-    Attribute, DeriveInput, Ident, Lit, LitBool, LitStr, Meta, MetaNameValue, Path, Token, Variant, Visibility,
+    Attribute, DeriveInput, Ident, Lit, LitBool, LitStr, Meta, MetaNameValue, Path, Token, Variant,
+    Visibility,
 };
 
 use super::case_style::CaseStyle;
@@ -16,6 +17,7 @@ pub mod kw {
 
     // enum metadata
     custom_keyword!(serialize_all);
+    custom_keyword!(use_phf);
 
     // enum discriminant metadata
     custom_keyword!(derive);
@@ -43,6 +45,7 @@ pub enum EnumMeta {
         kw: kw::Crate,
         crate_module_path: Path,
     },
+    UsePhf(kw::use_phf),
 }
 
 impl Parse for EnumMeta {
@@ -64,8 +67,9 @@ impl Parse for EnumMeta {
                 crate_module_path,
             })
         } else if lookahead.peek(kw::ascii_case_insensitive) {
-            let kw = input.parse()?;
-            Ok(EnumMeta::AsciiCaseInsensitive(kw))
+            Ok(EnumMeta::AsciiCaseInsensitive(input.parse()?))
+        } else if lookahead.peek(kw::use_phf) {
+            Ok(EnumMeta::UsePhf(input.parse()?))
         } else {
             Err(lookahead.error())
         }
@@ -78,6 +82,7 @@ impl Spanned for EnumMeta {
             EnumMeta::SerializeAll { kw, .. } => kw.span(),
             EnumMeta::AsciiCaseInsensitive(kw) => kw.span(),
             EnumMeta::Crate { kw, .. } => kw.span(),
+            EnumMeta::UsePhf(use_phf) => use_phf.span(),
         }
     }
 }
@@ -275,14 +280,19 @@ pub trait VariantExt {
 impl VariantExt for Variant {
     fn get_metadata(&self) -> syn::Result<Vec<VariantMeta>> {
         let result = get_metadata_inner("strum", &self.attrs)?;
-        self.attrs.iter()
+        self.attrs
+            .iter()
             .filter(|attr| attr.path.is_ident("doc"))
             .try_fold(result, |mut vec, attr| {
-            if let Meta::NameValue(MetaNameValue { lit: Lit::Str(value), .. }) = attr.parse_meta()? {
-                vec.push(VariantMeta::Documentation { value })
-            }
-            Ok(vec)
-        })
+                if let Meta::NameValue(MetaNameValue {
+                    lit: Lit::Str(value),
+                    ..
+                }) = attr.parse_meta()?
+                {
+                    vec.push(VariantMeta::Documentation { value })
+                }
+                Ok(vec)
+            })
     }
 }
 
diff --git a/strum_macros/src/helpers/type_props.rs b/strum_macros/src/helpers/type_props.rs
index cdca79f..0d49e04 100644
--- a/strum_macros/src/helpers/type_props.rs
+++ b/strum_macros/src/helpers/type_props.rs
@@ -20,6 +20,7 @@ pub struct StrumTypeProperties {
     pub discriminant_name: Option<Ident>,
     pub discriminant_others: Vec<TokenStream>,
     pub discriminant_vis: Option<Visibility>,
+    pub use_phf: bool,
 }
 
 impl HasTypeProperties for DeriveInput {
@@ -31,6 +32,7 @@ impl HasTypeProperties for DeriveInput {
 
         let mut serialize_all_kw = None;
         let mut ascii_case_insensitive_kw = None;
+        let mut use_phf_kw = None;
         let mut crate_module_path_kw = None;
         for meta in strum_meta {
             match meta {
@@ -50,6 +52,14 @@ impl HasTypeProperties for DeriveInput {
                     ascii_case_insensitive_kw = Some(kw);
                     output.ascii_case_insensitive = true;
                 }
+                EnumMeta::UsePhf(kw) => {
+                    if let Some(fst_kw) = use_phf_kw {
+                        return Err(occurrence_error(fst_kw, kw, "use_phf"));
+                    }
+
+                    use_phf_kw = Some(kw);
+                    output.use_phf = true;
+                }
                 EnumMeta::Crate {
                     crate_module_path,
                     kw,
diff --git a/strum_macros/src/lib.rs b/strum_macros/src/lib.rs
index 32f9a78..de16c9c 100644
--- a/strum_macros/src/lib.rs
+++ b/strum_macros/src/lib.rs
@@ -47,6 +47,9 @@ fn debug_print_generated(ast: &DeriveInput, toks: &TokenStream) {
 /// See the [Additional Attributes](https://docs.rs/strum/0.22/strum/additional_attributes/index.html)
 /// Section for more information on using this feature.
 ///
+/// If you have a large enum, you may want to consider using the `use_phf` attribute here. It leverages
+/// perfect hash functions to parse much quicker than a standard `match`. (MSRV 1.46)
+///
 /// # Example howto use `EnumString`
 /// ```
 /// use std::str::FromStr;
@@ -471,11 +474,11 @@ pub fn from_repr(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
 /// Encode strings into the enum itself. The `strum_macros::EmumMessage` macro implements the `strum::EnumMessage` trait.
 /// `EnumMessage` looks for `#[strum(message="...")]` attributes on your variants.
 /// You can also provided a `detailed_message="..."` attribute to create a seperate more detailed message than the first.
-/// 
+///
 /// `EnumMessage` also exposes the variants doc comments through `get_documentation()`. This is useful in some scenarios,
 /// but `get_message` should generally be preferred. Rust doc comments are intended for developer facing documentation,
 /// not end user messaging.
-/// 
+///
 /// ```
 /// // You need to bring the trait into scope to use it
 /// use strum::EnumMessage;
diff --git a/strum_macros/src/macros/strings/from_string.rs b/strum_macros/src/macros/strings/from_string.rs
index d0754d2..a4cc4d1 100644
--- a/strum_macros/src/macros/strings/from_string.rs
+++ b/strum_macros/src/macros/strings/from_string.rs
@@ -18,8 +18,15 @@ pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
     let strum_module_path = type_properties.crate_module_path();
 
     let mut default_kw = None;
-    let mut default = quote! { _ => ::core::result::Result::Err(#strum_module_path::ParseError::VariantNotFound) };
-    let mut arms = Vec::new();
+    let mut default =
+        quote! { ::core::result::Result::Err(#strum_module_path::ParseError::VariantNotFound) };
+    let mut phf_exact_match_arms = Vec::new();
+    // We'll use the first one if there are many variants
+    let mut phf_lowercase_arms = Vec::new();
+    // However if there are few variants we'll want to integrate these in the standard match to avoid alloc
+    let mut case_insensitive_arms_alternative = Vec::new();
+    // Later we can add custom arms in there
+    let mut standard_match_arms = Vec::new();
     for variant in variants {
         let ident = &variant.ident;
         let variant_properties = variant.get_variant_properties()?;
@@ -45,26 +52,11 @@ pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
 
             default_kw = Some(kw);
             default = quote! {
-                default => ::core::result::Result::Ok(#name::#ident(default.into()))
+                ::core::result::Result::Ok(#name::#ident(s.into()))
             };
             continue;
         }
 
-        let is_ascii_case_insensitive = variant_properties
-            .ascii_case_insensitive
-            .unwrap_or(type_properties.ascii_case_insensitive);
-        // If we don't have any custom variants, add the default serialized name.
-        let attrs = variant_properties
-            .get_serializations(type_properties.case_style)
-            .into_iter()
-            .map(|serialization| {
-                if is_ascii_case_insensitive {
-                    quote! { s if s.eq_ignore_ascii_case(#serialization) }
-                } else {
-                    quote! { #serialization }
-                }
-            });
-
         let params = match &variant.fields {
             Fields::Unit => quote! {},
             Fields::Unnamed(fields) => {
@@ -81,19 +73,95 @@ pub fn from_string_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
             }
         };
 
-        arms.push(quote! { #(#attrs => ::core::result::Result::Ok(#name::#ident #params)),* });
+        let is_ascii_case_insensitive = variant_properties
+            .ascii_case_insensitive
+            .unwrap_or(type_properties.ascii_case_insensitive);
+
+        // If we don't have any custom variants, add the default serialized name.
+        for serialization in variant_properties.get_serializations(type_properties.case_style) {
+            if type_properties.use_phf {
+                if !is_ascii_case_insensitive {
+                    phf_exact_match_arms.push(quote! { #serialization => #name::#ident #params, });
+                } else {
+                    // In that case we'll store the lowercase values in phf, and lowercase at runtime
+                    // before searching
+                    // Unless there are few such variants, in that case we'll use the standard match with
+                    // eq_ignore_ascii_case to avoid allocating
+                    case_insensitive_arms_alternative.push(quote! { s if s.eq_ignore_ascii_case(#serialization) => #name::#ident #params, });
+
+                    let mut ser_string = serialization.value();
+                    ser_string.make_ascii_lowercase();
+                    let serialization = syn::LitStr::new(&ser_string, serialization.span());
+                    phf_lowercase_arms.push(quote! { #serialization => #name::#ident #params, });
+                }
+            } else {
+                standard_match_arms.push(if !is_ascii_case_insensitive {
+                    quote! { #serialization => #name::#ident #params, }
+                } else {
+                    quote! { s if s.eq_ignore_ascii_case(#serialization) => #name::#ident #params, }
+                });
+            }
+        }
+    }
+
+    // Probably under that string allocation is more expensive than matching few times
+    // Proper threshold is not benchmarked - feel free to do so :)
+    if phf_lowercase_arms.len() <= 3 {
+        standard_match_arms.extend(case_insensitive_arms_alternative);
+        phf_lowercase_arms.clear();
     }
 
-    arms.push(default);
+    let use_phf = if phf_exact_match_arms.is_empty() && phf_lowercase_arms.is_empty() {
+        quote!()
+    } else {
+        quote! {
+            use #strum_module_path::_private_phf_reexport_for_macro_if_phf_feature as phf;
+        }
+    };
+    let phf_body = if phf_exact_match_arms.is_empty() {
+        quote!()
+    } else {
+        quote! {
+            static PHF: phf::Map<&'static str, #name> = phf::phf_map! {
+                #(#phf_exact_match_arms)*
+            };
+            if let Some(value) = PHF.get(s).cloned() {
+                return ::core::result::Result::Ok(value);
+            }
+        }
+    };
+    let phf_lowercase_body = if phf_lowercase_arms.is_empty() {
+        quote!()
+    } else {
+        quote! {
+            static PHF_LOWERCASE: phf::Map<&'static str, #name> = phf::phf_map! {
+                #(#phf_lowercase_arms)*
+            };
+            if let Some(value) = PHF_LOWERCASE.get(&s.to_ascii_lowercase()).cloned() {
+                return ::core::result::Result::Ok(value);
+            }
+        }
+    };
+    let standard_match_body = if standard_match_arms.is_empty() {
+        default
+    } else {
+        quote! {
+            ::core::result::Result::Ok(match s {
+                #(#standard_match_arms)*
+                _ => return #default,
+            })
+        }
+    };
 
     let from_str = quote! {
         #[allow(clippy::use_self)]
         impl #impl_generics ::core::str::FromStr for #name #ty_generics #where_clause {
             type Err = #strum_module_path::ParseError;
             fn from_str(s: &str) -> ::core::result::Result< #name #ty_generics , <Self as ::core::str::FromStr>::Err> {
-                match s {
-                    #(#arms),*
-                }
+                #use_phf
+                #phf_body
+                #phf_lowercase_body
+                #standard_match_body
             }
         }
     };


Test patch:
diff --git a/strum_tests/Cargo.toml b/strum_tests/Cargo.toml
index 796a069..872a3b0 100644
--- a/strum_tests/Cargo.toml
+++ b/strum_tests/Cargo.toml
@@ -4,6 +4,10 @@ version = "0.24.0"
 edition = "2018"
 authors = ["Peter Glotfelty <peglotfe@microsoft.com>"]
 
+[features]
+default = ["test_phf"]
+test_phf = ["strum/phf"]
+
 [dependencies]
 strum = { path = "../strum", features = ["derive"] }
 strum_macros = { path = "../strum_macros", features = [] }
diff --git a/strum_tests/tests/phf.rs b/strum_tests/tests/phf.rs
new file mode 100644
index 0000000..163418e
--- /dev/null
+++ b/strum_tests/tests/phf.rs
@@ -0,0 +1,33 @@
+#[cfg(feature = "test_phf")]
+#[test]
+fn from_str_with_phf() {
+    #[derive(Debug, PartialEq, Eq, Clone, strum::EnumString)]
+    #[strum(use_phf)]
+    enum Color {
+        #[strum(ascii_case_insensitive)]
+        Blue,
+        Red,
+    }
+    assert_eq!("Red".parse::<Color>().unwrap(), Color::Red);
+    assert_eq!("bLuE".parse::<Color>().unwrap(), Color::Blue);
+}
+
+#[cfg(feature = "test_phf")]
+#[test]
+fn from_str_with_phf_big() {
+    // This tests PHF when there are many case insensitive variants
+    #[derive(Debug, PartialEq, Eq, Clone, strum::EnumString)]
+    #[strum(use_phf, ascii_case_insensitive)]
+    enum Enum {
+        Var1,
+        Var2,
+        Var3,
+        Var4,
+        Var5,
+        Var6,
+        Var7,
+        Var8,
+        Var9,
+    }
+    assert_eq!("vAr2".parse::<Enum>().unwrap(), Enum::Var2);
+}