1 files changed, 165 insertions, 0 deletions
diff --git a/src/nibble/convert.rs b/src/nibble/convert.rs
new file mode 100644
index 0000000..3e47208
--- /dev/null
+++ b/src/nibble/convert.rs
@@ -0,0 +1,165 @@
+use std::collections::HashMap;
+
+use syn::punctuated::Pair;
+
+use crate::chomp::ast;
+
+use super::cst::{Alt, Call, Cat, Fix, Ident, ParenExpression, Term};
+
+#[derive(Clone, Copy, Debug)]
+pub enum Binding {
+    Variable(usize),
+    Parameter(usize),
+    Global,
+}
+
+#[derive(Debug)]
+pub struct Context {
+    names: HashMap<String, Binding>,
+    vars: usize,
+}
+
+impl Context {
+    pub fn new<I: IntoIterator<Item = Ident>>(globals: &[Ident], params: I) -> Self {
+        let mut names = HashMap::new();
+        for global in globals {
+            names.insert(global.to_string(), Binding::Global);
+        }
+
+        for (index, param) in params.into_iter().enumerate() {
+            names.insert(param.to_string(), Binding::Parameter(index));
+        }
+
+        Self { names, vars: 0 }
+    }
+
+    pub fn lookup(&self, name: &Ident) -> Option<Binding> {
+        // we make variable binding cheaper by inserting wrong and pulling right.
+        match self.names.get(&name.to_string()).copied() {
+            Some(Binding::Variable(index)) => Some(Binding::Variable(self.vars - index - 1)),
+            x => x,
+        }
+    }
+
+    pub fn with_variable<F: FnOnce(&mut Self) -> R, R>(&mut self, name: &Ident, f: F) -> R {
+        let old = self
+            .names
+            .insert(name.to_string(), Binding::Variable(self.vars));
+
+        // we make variable binding cheaper by inserting wrong and pulling right.
+        // we should increment all values in names instead, but that's slow
+        self.vars += 1;
+        let res = f(self);
+        self.vars -= 1;
+
+        if let Some(val) = old {
+            self.names.insert(name.to_string(), val);
+        } else {
+            self.names.remove(&name.to_string());
+        }
+
+        res
+    }
+}
+
+pub trait Convert {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression>;
+}
+
+impl Convert for Ident {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression> {
+        let span = self.span();
+
+        match context.lookup(&self)? {
+            Binding::Variable(index) => Some(ast::Variable::new(self, index).into()),
+            Binding::Parameter(index) => Some(ast::Parameter::new(self, index).into()),
+            Binding::Global => Some(ast::Call::new(self, Vec::new(), Some(span)).into()),
+        }
+    }
+}
+
+impl Convert for Call {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression> {
+        let span = self.span();
+        let args = self
+            .args
+            .into_iter()
+            .map(|arg| arg.convert(context))
+            .collect::<Option<_>>()?;
+        Some(ast::Call::new(self.name, args, span).into())
+    }
+}
+
+impl Convert for Fix {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression> {
+        let span = self.span();
+        let expr = self.expr;
+        let inner = context.with_variable(&self.arg, |context| expr.convert(context))?;
+        Some(ast::Fix::new(self.arg, inner, span).into())
+    }
+}
+
+impl Convert for ParenExpression {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression> {
+        self.expr.convert(context)
+    }
+}
+
+impl Convert for Term {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression> {
+        match self {
+            Self::Epsilon(e) => Some(e.into()),
+            Self::Ident(i) => i.convert(context),
+            Self::Literal(l) => Some(l.into()),
+            Self::Call(c) => c.convert(context),
+            Self::Fix(f) => f.convert(context),
+            Self::Parens(p) => p.convert(context),
+        }
+    }
+}
+
+impl Convert for Cat {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression> {
+        let mut iter = self.0.into_pairs();
+        let mut out = match iter.next().unwrap() {
+            Pair::Punctuated(t, p) => (t.convert(context)?, Some(p)),
+            Pair::End(t) => (t.convert(context)?, None),
+        };
+
+        for pair in iter {
+            let (fst, punct) = out;
+            out = match pair {
+                Pair::Punctuated(t, p) => (
+                    ast::Cat::new(fst, punct, t.convert(context)?).into(),
+                    Some(p),
+                ),
+                Pair::End(t) => (ast::Cat::new(fst, punct, t.convert(context)?).into(), None),
+            };
+        }
+
+        Some(out.0)
+    }
+}
+
+impl Convert for Alt {
+    fn convert(self, context: &mut Context) -> Option<ast::Expression> {
+        let mut iter = self.0.into_pairs();
+        let mut out = match iter.next().unwrap() {
+            Pair::Punctuated(t, p) => (t.convert(context)?, Some(p)),
+            Pair::End(t) => (t.convert(context)?, None),
+        };
+
+        for pair in iter {
+            let (fst, punct) = out;
+            out = match pair {
+                Pair::Punctuated(t, p) => (
+                    ast::Alt::new(fst, punct, t.convert(context)?).into(),
+                    Some(p),
+                ),
+                Pair::End(t) => (ast::Alt::new(fst, punct, t.convert(context)?).into(), None),
+            };
+        }
+
+        Some(out.0)
+    }
+}