diff --git a/Cargo.lock b/Cargo.lock
index 72e69a2..59e6cd1 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,162 @@
 # It is not intended for manual editing.
 version = 3
 
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+ "libc",
+ "wasi",
+]
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+]
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 [[package]]
 name = "rust-roller"
 version = "0.1.0"
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diff --git a/Cargo.toml b/Cargo.toml
index 3ba0aaf..2d6ea56 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,3 +4,5 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+nom = "7.1.3"
+rand = "0.8.5"
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..556b0f8
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,165 @@
+use nom::{branch::alt, character::complete, IResult};
+use rand::Rng;
+
+pub struct Roller {
+    expr: Expr,
+}
+
+impl Roller {
+    /// 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);
+        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)));
+            expr = e;
+        }
+        Ok(Roller{expr: res})
+    }
+    pub fn roll<R: Rng>(&self, rng: &mut R) -> u64 {
+        self.expr.val(rng)
+    }
+}
+
+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),
+        }
+    }
+}
+
+enum Term {
+    Roll(Roll),
+    Const(u64),
+}
+
+enum Oper {
+    Add,
+    Sub,
+}
+
+pub struct Roll{
+    reps: u64,
+    dice: u64,
+}
+
+impl Roll {
+    fn val<R: Rng>(&self, rng: &mut R) -> u64 {
+        let mut total = 0;
+        for _ in 0..self.reps {
+            total += rng.gen::<u64>() % self.dice + 1;
+        }
+        total
+    }
+}
+
+
+fn term(e: &str) -> IResult<&str, Term> {
+    alt((roll, cnst))(e)
+}   
+
+fn roll(e: &str) -> IResult<&str, Term> {
+    let (e, reps) = complete::u64(e)?;
+    let (e, _) = complete::char('d')(e)?;
+    let (e, dice) = complete::u64(e)?;
+    Ok((e, Term::Roll(Roll{reps, dice})))
+}
+
+fn cnst(e: &str) -> IResult<&str, Term> {
+    let (e, val) = complete::u64(e)?;
+    Ok((e, Term::Const(val)))
+}
+
+fn oper(e: &str) -> IResult<&str, Oper> {
+    alt((add, sub))(e)
+}
+
+fn add(e: &str) -> IResult<&str, Oper> {
+    let (e, _) = complete::char('+')(e)?;
+    Ok((e, Oper::Add))
+}
+
+fn sub(e: &str) -> IResult<&str, Oper> {
+    let (e, _) = complete::char('-')(e)?;
+    Ok((e, Oper::Sub))
+}
+
+#[cfg(test)]
+mod tests {
+    use rand::{rngs::StdRng, SeedableRng};
+
+    use super::*;
+    #[test]
+    fn roll_many_d6s() {
+        let mut rng = StdRng::seed_from_u64(1337);
+        let roller = Roller::parse("1d6").unwrap();
+        let mut results = [0u64; 6];
+        for _ in 0..1000 {
+            results[(roller.roll(&mut rng)-1) as usize] += 1;
+        }
+        // We should have some in each bucket
+        for x in results {
+            assert!(x > 50);
+        }
+    }
+
+    #[test]
+    fn roll_many_d6s_plus_2() {
+        let mut rng = StdRng::seed_from_u64(1337);
+        let roller = Roller::parse("1d6+2").unwrap();
+        let mut results = [0u64; 6+2];
+        for _ in 0..1000 {
+            results[(roller.roll(&mut rng)-1) as usize] += 1;
+        }
+        // Can't get a 1 or 2, but should have many others
+        assert!(results[0] == 0);
+        assert!(results[1] == 0);
+        for x in &results[2..] {
+            assert!(*x > 50);
+        }
+    }
+
+    #[test]
+    fn roll_many_d6s_plus_1d6() {
+        let mut rng = StdRng::seed_from_u64(1337);
+        let roller = Roller::parse("1d6+1d6").unwrap();
+        let mut results = [0u64; 6+6];
+        for _ in 0..1000 {
+            results[(roller.roll(&mut rng)-1) as usize] += 1;
+        }
+        // Its impossible to roll 1
+        assert!(results[0] == 0);
+        for x in &results[1..] {
+            assert!(*x > 1);
+        }
+    }
+
+    #[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 mut results = [0u64; 6+6];
+        for _ in 0..1000 {
+            results[(roller.roll(&mut rng)-1) as usize] += 1;
+        }
+        // Its impossible to roll 12
+        assert!(results[11] == 0);
+        for x in &results[..11] {
+            assert!(*x > 1);
+        }
+    }
+}
\ No newline at end of file