Implement Perchance grammar evaluation with basic lists

Cargo.toml

@@ -8,3 +8,4 @@ license = "MIT"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+rand = "0.7.3"

README.md

@@ -6,7 +6,7 @@ Purrchance is an unofficial Rust implementation of the
 ## Features I might implement eventually
 
 - [ ] Parsing grammars from text format
-- [ ] Basic lists
+- [x] Basic lists
 - [ ] Probability weights
 - [ ] Single-item lists
 - [ ] Escape sequences

src/lib.rs

@@ -1,7 +1,107 @@
+extern crate rand;
+
+use rand::{seq::SliceRandom, thread_rng};
+use std::collections::HashMap;
+
+pub trait Purrchance {
+    fn eval(&self, g: &Grammar) -> Option<String>;
+}
+
+pub enum Symbol {
+    Terminal(String),
+    NonTerminal(String),
+}
+
+impl Purrchance for Symbol {
+    fn eval(&self, g: &Grammar) -> Option<String> {
+        match self {
+            Symbol::Terminal(s) => Some(s.to_string()),
+            Symbol::NonTerminal(label) => g.0.get(label)?.eval(g),
+        }
+    }
+}
+
+pub struct Expr(Vec<Symbol>);
+
+impl Purrchance for Expr {
+    fn eval(&self, g: &Grammar) -> Option<String> {
+        Some(self.0.iter().map(|sym| sym.eval(g)).collect::<Option<Vec<String>>>()?.join(""))
+    }
+}
+
+pub struct List(Vec<Expr>);
+
+impl Purrchance for List {
+    fn eval(&self, g: &Grammar) -> Option<String> {
+        self.0.choose(&mut thread_rng())?.eval(g)
+    }
+}
+
+pub struct Grammar(HashMap<String,List>);
+
 #[cfg(test)]
 mod tests {
+    use super::*;
+
     #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
+    fn eval_terminal() {
+        let sym = Symbol::Terminal("hello world".to_string());
+        let g = Grammar(HashMap::new());
+        assert_eq!(sym.eval(&g), Some("hello world".to_string()));
+    }
+
+    #[test]
+    fn eval_terminals_expr() {
+        let sym1 = Symbol::Terminal("hell".to_string());
+        let sym2 = Symbol::Terminal("o world".to_string());
+        let expr = Expr(vec![sym1, sym2]);
+        let g = Grammar(HashMap::new());
+        assert_eq!(expr.eval(&g), Some("hello world".to_string()));
+    }
+
+    #[test]
+    fn eval_single_terminals_expr_list() {
+        let sym1 = Symbol::Terminal("hell".to_string());
+        let sym2 = Symbol::Terminal("o world".to_string());
+        let expr = Expr(vec![sym1, sym2]);
+        let list = List(vec![expr]);
+        let g = Grammar(HashMap::new());
+        assert_eq!(list.eval(&g), Some("hello world".to_string()));
+    }
+
+    #[test]
+    fn eval_multiple_terminals_expr_list() {
+        let sym1 = Symbol::Terminal("hello".to_string());
+        let sym2 = Symbol::Terminal("goodbye".to_string());
+        let list = List(vec![Expr(vec![sym1]), Expr(vec![sym2])]);
+        let g = Grammar(HashMap::new());
+        assert!(vec![Some("hello".to_string()), Some("goodbye".to_string())].contains(&list.eval(&g)));
+    }
+
+    #[test]
+    fn eval_empty_list() {
+        let list = List(vec![]);
+        let g = Grammar(HashMap::new());
+        assert_eq!(list.eval(&g), None);
+    }
+
+    #[test]
+    fn eval_valid_nonterminal() {
+        let term = Symbol::Terminal("hello world".to_string());
+        let list = List(vec![Expr(vec![term])]);
+        let mut g = Grammar(HashMap::new());
+        g.0.insert("output".to_string(), list);
+        let nt = Symbol::NonTerminal("output".to_string());
+        assert_eq!(nt.eval(&g), Some("hello world".to_string()));
+    }
+
+    #[test]
+    fn eval_missing_nonterminal() {
+        let term = Symbol::Terminal("hello world".to_string());
+        let list = List(vec![Expr(vec![term])]);
+        let mut g = Grammar(HashMap::new());
+        g.0.insert("output".to_string(), list);
+        let nt = Symbol::NonTerminal("missing".to_string());
+        assert_eq!(nt.eval(&g), None);
     }
 }