add: use fixed-size array and no allocs

src/lib.rs

@@ -1,45 +1,55 @@
 use nom::{branch::alt, character::complete, IResult};
 use rand::Rng;
 
-pub struct Roller {
-    expr: Expr,
+pub struct Roller<const S: usize> {
+    exprs: [Option<Cmd>; S],
 }
 
-impl Roller {
+impl<const S: usize> Roller<S> {
     /// parse converts the str, of form:
-    ///     2d8 + 1d8 + 4
-    /// into a parsed expression, returning an Expr enum
-    /// and the remaining unparsed string.
-    pub fn parse(expr: &str) -> Result<Roller, nom::Err<nom::error::Error<&str>>> {
-        let (mut expr, left) = term(expr)?;
-        let mut res = Expr::Term(left);
+    ///     2d8+1d8+4
+    /// into a parsed expression.
+    pub fn parse(mut expr: &str) -> Result<Roller<S>, nom::Err<nom::error::Error<&str>>> {
+        let mut op = Oper::Add;
+        let mut exprs = [const { None }; S];
+        let mut i = 0;
         while expr.len() > 0 {
-            let (e, oper) = oper(expr)?;
-            let (e, term) = term(e)?;
-            res = Expr::Op(Box::new(res), oper, Box::new(Expr::Term(term)));
+            let (e, term) = term(expr)?;
+            expr = e;
+            exprs[i] = Some(Cmd{term, oper: op});
+            i += 1;
+            if i == exprs.len() {
+                return Err(nom::Err::Incomplete(nom::Needed::new(S + 1)));
+            }
+
+            // Get the next oper
+            if e.len() > 0 {
+                let (e, _op) = oper(expr)?;
+                op = _op;
                 expr = e;
             }
-        Ok(Roller{expr: res})
+        }
+        Ok(Roller{exprs})
     }
     pub fn roll<R: Rng>(&self, rng: &mut R) -> u64 {
-        self.expr.val(rng)
+        let mut sum = 0;
+        for expr in &self.exprs {
+            if expr.is_none() {
+                break;
+            }
+            let cmd = expr.as_ref().unwrap();
+            match cmd.oper {
+                Oper::Add => sum += cmd.term.val(rng),
+                Oper::Sub => sum -= cmd.term.val(rng),
+            };
+        }
+        sum
     }
 }
 
-enum Expr {
-    Term(Term),
-    Op(Box<Expr>, Oper, Box<Expr>),
-}
-
-impl Expr {
-    fn val<R: Rng>(&self, rng: &mut R) -> u64 {
-        match self {
-            Expr::Term(Term::Const(x)) => *x,
-            Expr::Term(Term::Roll(r)) => r.val(rng),
-            Expr::Op(a, Oper::Add, b) => a.val(rng) + b.val(rng),
-            Expr::Op(a, Oper::Sub, b) => a.val(rng) - b.val(rng),
-        }
-    }
+struct Cmd {
+    oper: Oper,
+    term: Term,
 }
 
 enum Term {
@@ -47,6 +57,16 @@ enum Term {
     Const(u64),
 }
 
+impl Term {
+    fn val<R: Rng>(&self, rng: &mut R) -> u64 {
+        match self {
+            Term::Const(c) => *c,
+            Term::Roll(r) => r.val(rng),
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
 enum Oper {
     Add,
     Sub,
@@ -106,7 +126,7 @@ mod tests {
     #[test]
     fn roll_many_d6s() {
         let mut rng = StdRng::seed_from_u64(1337);
-        let roller = Roller::parse("1d6").unwrap();
+        let roller = Roller::<1024>::parse("1d6").unwrap();
         let mut results = [0u64; 6];
         for _ in 0..1000 {
             results[(roller.roll(&mut rng)-1) as usize] += 1;
@@ -120,7 +140,7 @@ mod tests {
     #[test]
     fn roll_many_d6s_plus_2() {
         let mut rng = StdRng::seed_from_u64(1337);
-        let roller = Roller::parse("1d6+2").unwrap();
+        let roller = Roller::<1024>::parse("1d6+2").unwrap();
         let mut results = [0u64; 6+2];
         for _ in 0..1000 {
             results[(roller.roll(&mut rng)-1) as usize] += 1;
@@ -136,7 +156,7 @@ mod tests {
     #[test]
     fn roll_many_d6s_plus_1d6() {
         let mut rng = StdRng::seed_from_u64(1337);
-        let roller = Roller::parse("1d6+1d6").unwrap();
+        let roller = Roller::<1024>::parse("1d6+1d6").unwrap();
         let mut results = [0u64; 6+6];
         for _ in 0..1000 {
             results[(roller.roll(&mut rng)-1) as usize] += 1;
@@ -151,7 +171,7 @@ mod tests {
     #[test]
     fn roll_many_d6s_plus_1d6_minus_one() {
         let mut rng = StdRng::seed_from_u64(1337);
-        let roller = Roller::parse("1d6+1d6-1").unwrap();
+        let roller = Roller::<1024>::parse("1d6+1d6-1").unwrap();
         let mut results = [0u64; 6+6];
         for _ in 0..1000 {
             results[(roller.roll(&mut rng)-1) as usize] += 1;