add polynomial division

src/circuit/quotient.rs

@@ -1,4 +1,4 @@
-use super::dag::{Circuit, NodeId};
+use super::dag::{Circuit, Node, NodeId};
 use crate::poly::{flat::Poly, var::Var};
 
 use itertools::Itertools;
@@ -35,10 +35,10 @@ impl<V: Var> Quotient<V> {
         self.circuit.node(n)
     }
 
-    pub fn add(left: NodeId, right: NodeId) {
+    pub fn add(&mut self, left: NodeId, right: NodeId) -> NodeId {
         self.circuit.add(left, right)
     }
-    pub fn mul(left: NodeId, right: NodeId) {
+    pub fn mul(&mut self, left: NodeId, right: NodeId) -> NodeId {
         self.circuit.mul(left, right)
     }
 }

src/poly/flat.rs

@@ -1,7 +1,46 @@
+use std::cmp::Ordering;
 use std::collections::HashMap;
 
 use super::var::Var;
 
+pub fn lex_cmp<V: Var>(a: &Mono<V>, b: &Mono<V>) -> Ordering {
+    let mut a_it = a.term.iter().peekable();
+    let mut b_it = b.term.iter().peekable();
+
+    loop {
+        match (a_it.peek(), b_it.peek()) {
+            (None, None) => return Ordering::Equal,
+            (Some((_, a_exp)), None) => {
+                return if *a_exp > 0 { Ordering::Greater } else { Ordering::Equal };
+            }
+            (None, Some((_, b_exp))) => {
+                return if *b_exp > 0 { Ordering::Less } else { Ordering::Equal };
+            }
+            (Some((a_var, a_exp)), Some((b_var, b_exp))) => {
+                if a_var < b_var {
+                    if *a_exp > 0 {
+                        return Ordering::Greater;
+                    }
+                    a_it.next();
+                } else if a_var > b_var {
+                    if *b_exp > 0 {
+                        return Ordering::Less;
+                    }
+                    b_it.next();
+                } else {
+                    match a_exp.cmp(b_exp) {
+                        Ordering::Equal => {
+                            a_it.next();
+                            b_it.next();
+                        }
+                        ord => return ord,
+                    }
+                }
+            }
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq)]
 pub struct Poly<V: Var> {
     pub(crate) mono: HashMap<Mono<V>, i32>,
@@ -15,6 +54,20 @@ impl<V: Var> Default for Poly<V> {
     }
 }
 
+impl<V: Var> Poly<V> {
+    pub fn is_zero(&self) -> bool {
+        self.mono.is_empty()
+    }
+
+    /// Returns (leading monomial, leading coefficient) under lex order, or None if zero.
+    pub fn leading_term_lex(&self) -> Option<(Mono<V>, i32)> {
+        self.mono
+            .iter()
+            .max_by(|(m1, _), (m2, _)| lex_cmp(m1, m2))
+            .map(|(m, &c)| (m.clone(), c))
+    }
+}
+
 impl<V: Var, T: IntoIterator> From<T> for Poly<V>
 where
     Poly<V>: FromIterator<<T as IntoIterator>::Item>,
@@ -62,6 +115,36 @@ impl<V: Var> Mono<V> {
 
         true
     }
+
+    /// Divides self by other. Assumes `self.contains(other)`.
+    pub fn div(self, other: &Mono<V>) -> Mono<V> {
+        let mut self_it = self.term.into_iter().peekable();
+        let mut other_it = other.term.iter().peekable();
+        let mut result: Vec<(V, u32)> = vec![];
+
+        loop {
+            match (self_it.peek(), other_it.peek()) {
+                (None, None) => break,
+                (Some(_), None) => result.push(self_it.next().unwrap()),
+                (None, Some(_)) => unreachable!("divisor not contained in dividend"),
+                (Some((s_var, _)), Some((o_var, _))) => {
+                    if s_var < o_var {
+                        result.push(self_it.next().unwrap());
+                    } else if s_var > o_var {
+                        unreachable!("divisor not contained in dividend");
+                    } else {
+                        let (var, s_exp) = self_it.next().unwrap();
+                        let (_, o_exp) = other_it.next().unwrap();
+                        if s_exp > *o_exp {
+                            result.push((var, s_exp - o_exp));
+                        }
+                    }
+                }
+            }
+        }
+
+        Mono { term: result }
+    }
 }
 
 impl<V: Var, T: IntoIterator> From<T> for Mono<V>

src/poly/ops.rs

@@ -72,6 +72,20 @@ impl<V: Var> Mul<Mono<V>> for i32 {
     }
 }
 
+impl<V: Var> Mul<Poly<V>> for i32 {
+    type Output = Poly<V>;
+
+    fn mul(self, mut poly: Poly<V>) -> Self::Output {
+        if self == 0 {
+            return Poly::default();
+        }
+        for coeff in poly.mono.values_mut() {
+            *coeff *= self;
+        }
+        poly
+    }
+}
+
 impl<V: Var> Mul for Poly<V> {
     type Output = Poly<V>;
 
@@ -113,3 +127,49 @@ impl<V: Var> Sub for Poly<V> {
         self
     }
 }
+
+impl<V: Var> Poly<V> {
+    /// Pseudo-division with remainder.
+    ///
+    /// Returns `(d, q, r)` satisfying `lc(divisor)^d * self = q * divisor + r`,
+    /// where no term of `r` has its monomial divisible by `LM(divisor)` under lex order.
+    ///
+    /// Panics if `divisor` is zero.
+    pub fn div_rem(self, divisor: &Poly<V>) -> (u32, Poly<V>, Poly<V>) {
+        let (lt_g_mono, lt_g_coeff) = divisor
+            .leading_term_lex()
+            .expect("divisor must be nonzero");
+
+        let mut p = self;
+        let mut q = Poly::default();
+        let mut r = Poly::default();
+        let mut d = 0u32;
+
+        while !p.is_zero() {
+            let (lt_p_mono, lt_p_coeff) = p.leading_term_lex().unwrap();
+
+            if lt_p_mono.contains(&lt_g_mono) {
+                let t_mono = lt_p_mono.div(&lt_g_mono);
+                if lt_p_coeff % lt_g_coeff == 0 {
+                    // Exact division: no need to multiply through by lc(g)
+                    let t_poly: Poly<V> = (lt_p_coeff / lt_g_coeff) * t_mono;
+                    p = p - t_poly.clone() * divisor.clone();
+                    q = q + t_poly;
+                } else {
+                    // Pseudo-division: multiply through by lc(g) to stay in ℤ
+                    let t_poly: Poly<V> = lt_p_coeff * t_mono;
+                    p = lt_g_coeff * p - t_poly.clone() * divisor.clone();
+                    q = lt_g_coeff * q + t_poly;
+                    r = lt_g_coeff * r;
+                    d += 1;
+                }
+            } else {
+                let lt_poly: Poly<V> = lt_p_coeff * lt_p_mono;
+                p = p - lt_poly.clone();
+                r = r + lt_poly;
+            }
+        }
+
+        (d, q, r)
+    }
+}

src/poly/tests.rs

@@ -1,4 +1,4 @@
-use super::flat::{Mono, Poly};
+use super::flat::{lex_cmp, Mono, Poly};
 use super::var::StaticVar;
 
 #[test]
@@ -99,3 +99,130 @@ fn test_poly_sub() {
     let b: Poly<StaticVar> = [(4, [("x", 2)]), (1, [("y", 1)])].into();
     assert_eq!(a - b, Poly::default());
 }
+
+#[test]
+fn test_lex_cmp() {
+    use std::cmp::Ordering;
+
+    let x2: Mono<StaticVar> = [("x", 2)].into();
+    let xy: Mono<StaticVar> = [("x", 1), ("y", 1)].into();
+    let y2: Mono<StaticVar> = [("y", 2)].into();
+    let x: Mono<StaticVar> = [("x", 1)].into();
+    let one: Mono<StaticVar> = Mono { term: vec![] };
+
+    // x² > xy (x exponent 2 vs 1)
+    assert_eq!(lex_cmp(&x2, &xy), Ordering::Greater);
+    // x > y² (x has higher priority)
+    assert_eq!(lex_cmp(&x, &y2), Ordering::Greater);
+    // xy > y² (x present in xy but not y²)
+    assert_eq!(lex_cmp(&xy, &y2), Ordering::Greater);
+    // 1 < x
+    assert_eq!(lex_cmp(&one, &x), Ordering::Less);
+    // reflexive
+    assert_eq!(lex_cmp(&x2, &x2), Ordering::Equal);
+}
+
+#[test]
+fn test_mono_div() {
+    // x² / x = x
+    let a: Mono<StaticVar> = [("x", 2)].into();
+    let b: Mono<StaticVar> = [("x", 1)].into();
+    assert_eq!(a.div(&b), Mono::from([("x", 1)]));
+
+    // x²y / xy = x
+    let a: Mono<StaticVar> = [("x", 2), ("y", 1)].into();
+    let b: Mono<StaticVar> = [("x", 1), ("y", 1)].into();
+    assert_eq!(a.div(&b), Mono::from([("x", 1)]));
+
+    // x²y / y = x²
+    let a: Mono<StaticVar> = [("x", 2), ("y", 1)].into();
+    let b: Mono<StaticVar> = [("y", 1)].into();
+    assert_eq!(a.div(&b), Mono::from([("x", 2)]));
+
+    // x / x = 1
+    let a: Mono<StaticVar> = [("x", 1)].into();
+    let b: Mono<StaticVar> = [("x", 1)].into();
+    assert_eq!(a.div(&b), Mono { term: vec![] });
+}
+
+fn make_const_poly(c: i32) -> Poly<StaticVar> {
+    Poly { mono: [(Mono { term: vec![] }, c)].into_iter().collect() }
+}
+
+fn verify_div_rem(f: Poly<StaticVar>, g: &Poly<StaticVar>, d: u32, q: Poly<StaticVar>, r: Poly<StaticVar>) {
+    // lc(g)^d * f == q * g + r
+    let (_, lc_g) = g.leading_term_lex().unwrap();
+    let lhs = lc_g.pow(d) * f;
+    let rhs = q * g.clone() + r;
+    assert_eq!(lhs, rhs);
+}
+
+#[test]
+fn test_div_rem() {
+    // x³ / x² = x, r=0, d=0
+    let f: Poly<StaticVar> = [(1, [("x", 3)])].into();
+    let g: Poly<StaticVar> = [(1, [("x", 2)])].into();
+    let expected_q: Poly<StaticVar> = [(1, [("x", 1)])].into();
+    let (d, q, r) = f.clone().div_rem(&g);
+    assert_eq!(q, expected_q);
+    assert!(r.is_zero());
+    verify_div_rem(f, &g, d, q, r);
+
+    // (x³ + x²y) / x² = x + y, r=0
+    // f = x²(x + y), g = x²
+    let f: Poly<StaticVar> = [
+        (1i32, Mono::from([("x", 3u32)])),
+        (1i32, Mono::from([("x", 2u32), ("y", 1u32)])),
+    ].into_iter().collect();
+    let g: Poly<StaticVar> = [(1, [("x", 2)])].into();
+    let expected_q: Poly<StaticVar> = [(1, [("x", 1)]), (1, [("y", 1)])].into();
+    let (d, q, r) = f.clone().div_rem(&g);
+    assert_eq!(q, expected_q);
+    assert!(r.is_zero());
+    verify_div_rem(f, &g, d, q, r);
+
+    // (x³ + y) / x² = x, r=y
+    // LT(x³) divisible by x², LT(y) is not
+    let f: Poly<StaticVar> = [(1, [("x", 3)]), (1, [("y", 1)])].into();
+    let g: Poly<StaticVar> = [(1, [("x", 2)])].into();
+    let expected_q: Poly<StaticVar> = [(1, [("x", 1)])].into();
+    let expected_r: Poly<StaticVar> = [(1, [("y", 1)])].into();
+    let (d, q, r) = f.clone().div_rem(&g);
+    assert_eq!(q, expected_q);
+    assert_eq!(r, expected_r);
+    verify_div_rem(f, &g, d, q, r);
+
+    // 3x² / (2x): 2 ∤ 3, needs pseudo-division
+    // 2¹ · 3x² = 3x · 2x  =>  d=1, q=3x, r=0
+    let f: Poly<StaticVar> = [(3, [("x", 2)])].into();
+    let g: Poly<StaticVar> = [(2, [("x", 1)])].into();
+    let expected_q: Poly<StaticVar> = [(3, [("x", 1)])].into();
+    let (d, q, r) = f.clone().div_rem(&g);
+    assert_eq!(d, 1);
+    assert_eq!(q, expected_q);
+    assert!(r.is_zero());
+    verify_div_rem(f, &g, d, q, r);
+
+    // 6xy / 2 = 3xy, r=0, d=0
+    let f: Poly<StaticVar> = [(6, [("x", 1), ("y", 1)])].into();
+    let g = make_const_poly(2);
+    let expected_q: Poly<StaticVar> = [(3, [("x", 1), ("y", 1)])].into();
+    let (d, q, r) = f.clone().div_rem(&g);
+    assert_eq!(d, 0);
+    assert_eq!(q, expected_q);
+    assert!(r.is_zero());
+    verify_div_rem(f, &g, d, q, r);
+
+    // (x² + xy) / (x + y) = x, r=0
+    // f = x(x + y), g = x + y
+    let f: Poly<StaticVar> = [
+        (1i32, Mono::from([("x", 2u32)])),
+        (1i32, Mono::from([("x", 1u32), ("y", 1u32)])),
+    ].into_iter().collect();
+    let g: Poly<StaticVar> = [(1, [("x", 1)]), (1, [("y", 1)])].into();
+    let expected_q: Poly<StaticVar> = [(1, [("x", 1)])].into();
+    let (d, q, r) = f.clone().div_rem(&g);
+    assert_eq!(q, expected_q);
+    assert!(r.is_zero());
+    verify_div_rem(f, &g, d, q, r);
+}