Move alphabetic name creation into string interner

Author: brendanzab

fathom/src/source.rs

@@ -9,6 +9,7 @@ pub type StringId = string_interner::symbol::SymbolU16;
 
 /// String interner.
 pub struct StringInterner {
+    alphabetic_names: Vec<StringId>,
     tuple_labels: Vec<StringId>,
     strings: string_interner::StringInterner<
         string_interner::backend::BucketBackend<StringId>,
@@ -37,25 +38,62 @@ impl StringInterner {
     /// Construct an empty string interner.
     pub fn new() -> StringInterner {
         StringInterner {
+            alphabetic_names: Vec::new(),
             tuple_labels: Vec::new(),
             strings: string_interner::StringInterner::new(),
         }
     }
 
-    /// Allocate and intern all tuple labels upto `max_index` if they are not already present
+    /// Allocate and intern all alphabetic names up-to and including `max_index`
+    /// if they are not already present.
+    pub fn reserve_alphabetic_names(&mut self, max_index: usize) {
+        fill_vec(&mut self.alphabetic_names, max_index, |index| {
+            self.strings.get_or_intern(alphabetic_name(index))
+        })
+    }
+
+    /// Retrieve an alphabetic name based on a numeric count. This is useful for
+    /// producing human-readable names for unnamed binders.
+    ///
+    /// ## Example
+    ///
+    /// ```rust
+    /// use fathom::source::StringInterner;
+    ///
+    /// let mut interner = StringInterner::new();
+    /// assert_eq!(interner.get_alphabetic_name(0), interner.get_or_intern("a"));
+    /// // ...
+    /// assert_eq!(interner.get_alphabetic_name(25), interner.get_or_intern("z"));
+    /// assert_eq!(interner.get_alphabetic_name(26), interner.get_or_intern("a1"));
+    /// // ...
+    /// assert_eq!(interner.get_alphabetic_name(51), interner.get_or_intern("z1"));
+    /// assert_eq!(interner.get_alphabetic_name(52), interner.get_or_intern("a2"));
+    /// // ...
+    /// ```
+    pub fn get_alphabetic_name(&mut self, index: usize) -> StringId {
+        self.reserve_alphabetic_names(index);
+        self.alphabetic_names[index]
+    }
+
+    /// Allocate and intern all tuple labels up-to and including `max_index`
+    /// if they are not already present.
     pub fn reserve_tuple_labels(&mut self, max_index: usize) {
-        let len = self.tuple_labels.len();
-        let cap = self.tuple_labels.capacity();
-        if max_index >= len {
-            self.tuple_labels.reserve(max_index.saturating_sub(cap));
-            for index in len..=max_index {
-                let label = self.get_or_intern(format!("_{index}"));
-                self.tuple_labels.push(label);
-            }
-        }
+        fill_vec(&mut self.tuple_labels, max_index, |index| {
+            self.strings.get_or_intern(format!("_{index}"))
+        })
     }
 
     /// Get or intern a string in the form `_{index}`.
+    ///
+    /// ## Example
+    ///
+    /// ```rust
+    /// use fathom::source::StringInterner;
+    ///
+    /// let mut interner = StringInterner::new();
+    /// assert_eq!(interner.get_tuple_label(0), interner.get_or_intern("_0"));
+    /// assert_eq!(interner.get_tuple_label(1), interner.get_or_intern("_1"));
+    /// ```
     pub fn get_tuple_label(&mut self, index: usize) -> StringId {
         self.reserve_tuple_labels(index);
         self.tuple_labels[index]
@@ -78,6 +116,21 @@ impl StringInterner {
     }
 }
 
+fn alphabetic_name(index: usize) -> String {
+    let base = index / 26;
+    let letter = index % 26;
+    let letter = (letter as u8 + b'a') as char;
+    if base == 0 {
+        format!("{letter}")
+    } else {
+        format!("{letter}{base}")
+    }
+}
+
+fn fill_vec<T>(vec: &mut Vec<T>, max_index: usize, f: impl FnMut(usize) -> T) {
+    vec.extend((vec.len()..=max_index).map(f))
+}
+
 #[derive(Debug, Clone)]
 pub struct Spanned<T> {
     span: Span,

fathom/src/surface/distillation.rs

@@ -84,37 +84,23 @@ impl<'interner, 'arena, 'env> Context<'interner, 'arena, 'env> {
 
     /// Generate a fresh name that does not appear in `self.local_names`
     fn gen_fresh_name(&mut self) -> StringId {
-        fn to_str(x: u32) -> String {
-            let base = x / 26;
-            let letter = x % 26;
-            let letter = (letter as u8 + b'a') as char;
-            if base == 0 {
-                format!("{letter}")
-            } else {
-                format!("{letter}{base}")
-            }
-        }
-
         let mut counter = 0;
         loop {
-            let name = to_str(counter);
-            let name = self.interner.borrow_mut().get_or_intern(name);
+            let name = self.interner.borrow_mut().get_alphabetic_name(counter);
             match self.local_names.iter().any(|symbol| *symbol == Some(name)) {
-                true => {}
+                true => counter += 1,
                 false => return name,
             }
-            counter += 1;
         }
     }
 
     /// Replace `name` with a fresh name if it is `_` and occurs in `body`
     fn freshen_name(&mut self, name: Option<StringId>, body: &core::Term<'_>) -> Option<StringId> {
         match name {
             Some(name) => Some(name),
-            None => match body.binds_local(Index::last()) {
-                false => None,
-                true => Some(self.gen_fresh_name()),
-            },
+            None => body
+                .binds_local(Index::last())
+                .then(|| self.gen_fresh_name()),
         }
     }