Added expression degrees.

crates/prover/src/constraint_framework/expr/degree.rs

@@ -0,0 +1,95 @@
+/// Finds a degree bound for an expressions.
+/// Computes the actual degree with the following caveats:
+///     1. The constant expression 0 receives degree 0 like all other constants rather than the
+///        mathematically correcy -infinity. This means, for example, that expresisons of the
+///        type 0 * expr will return degree deg expr. This should be mitigated by
+///        simplification.
+///     2. If expressions p and q cancel out under some operation, this will not be
+///        accounted for, so that (x^2 + 1) - (x^2 + x) will return degree 2.
+use std::collections::HashMap;
+
+use super::{BaseExpr, ExtExpr};
+
+type Degree = usize;
+
+/// A struct of named expressions that can be searched when determining the degree bound for an
+/// expression that contains parameters.
+pub struct NamedExprs {
+    exprs: HashMap<String, BaseExpr>,
+    ext_exprs: HashMap<String, ExtExpr>,
+}
+
+impl NamedExprs {
+    pub fn degree_bound(&self, name: String) -> Degree {
+        if let Some(expr) = self.exprs.get(&name) {
+            expr.degree_bound(self)
+        } else if let Some(expr) = self.ext_exprs.get(&name) {
+            expr.degree_bound(self)
+        } else {
+            // If expression isn't found assume it's an external variable, effectively a const.
+            0
+        }
+    }
+}
+
+impl BaseExpr {
+    pub fn degree_bound(&self, named_exprs: &NamedExprs) -> Degree {
+        match self {
+            BaseExpr::Col(_) => 1,
+            BaseExpr::Const(_) => 0,
+            BaseExpr::Param(name) => named_exprs.degree_bound(name.clone()),
+            BaseExpr::Add(a, b) => a.degree_bound(named_exprs).max(b.degree_bound(named_exprs)),
+            BaseExpr::Sub(a, b) => a.degree_bound(named_exprs).max(b.degree_bound(named_exprs)),
+            BaseExpr::Mul(a, b) => a.degree_bound(named_exprs) + b.degree_bound(named_exprs),
+            BaseExpr::Neg(a) => a.degree_bound(named_exprs),
+            // TODO(alont): Consider handling this in the type system.
+            BaseExpr::Inv(_) => panic!("Cannot compute the degree of an inverse."),
+        }
+    }
+}
+
+impl ExtExpr {
+    pub fn degree_bound(&self, named_exprs: &NamedExprs) -> Degree {
+        match self {
+            ExtExpr::SecureCol(coefs) => coefs
+                .iter()
+                .cloned()
+                .map(|coef| coef.degree_bound(named_exprs))
+                .max()
+                .unwrap(),
+            ExtExpr::Const(_) => 0,
+            ExtExpr::Param(name) => named_exprs.degree_bound(name.clone()),
+            ExtExpr::Add(a, b) => a.degree_bound(named_exprs).max(b.degree_bound(named_exprs)),
+            ExtExpr::Sub(a, b) => a.degree_bound(named_exprs).max(b.degree_bound(named_exprs)),
+            ExtExpr::Mul(a, b) => a.degree_bound(named_exprs) + b.degree_bound(named_exprs),
+            ExtExpr::Neg(a) => a.degree_bound(named_exprs),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use crate::constraint_framework::expr::degree::NamedExprs;
+    use crate::constraint_framework::expr::utils::*;
+
+    #[test]
+    fn test_degree_bound() {
+        let intermediate = (felt!(12) + col!(1, 1, 0)) * var!("a") * col!(1, 0, 0);
+        let qintermediate = secure_col!(intermediate.clone(), felt!(12), var!("b"), felt!(0));
+
+        let named_exprs = NamedExprs {
+            exprs: [("intermediate".to_string(), intermediate.clone())].into(),
+            ext_exprs: [("qintermediate".to_string(), qintermediate.clone())].into(),
+        };
+
+        let expr = var!("intermediate") * col!(2, 1, 0);
+        let qexpr =
+            var!("qintermediate") * secure_col!(col!(2, 1, 0), expr.clone(), felt!(0), felt!(1));
+
+        assert_eq!(intermediate.degree_bound(&named_exprs), 2);
+        assert_eq!(qintermediate.degree_bound(&named_exprs), 2);
+        assert_eq!(expr.degree_bound(&named_exprs), 3);
+        assert_eq!(qexpr.degree_bound(&named_exprs), 5);
+    }
+}

crates/prover/src/constraint_framework/expr/mod.rs

@@ -1,4 +1,5 @@
 pub mod assignment;
+pub mod degree;
 pub mod evaluator;
 pub mod format;
 pub mod simplify;