use proptest::{
    prelude::*,
    string::{string_regex, RegexGeneratorStrategy},
};
use serde::{Deserialize, Serialize};
use std::fmt::{self, Display};

#[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
pub enum Value {
    Num(i32),
    Bool(bool),
    Sym(String),
    App(Box<Value>, Box<Value>),
    Lam(String, Box<Value>),
    Def(String, Box<Value>),
    Let(String, Box<Value>, Box<Value>),
}

impl Display for Value {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Value::Num(i) => write!(f, "{}", i),
            Value::Bool(b) => write!(f, "{}", b),
            Value::Sym(s) => write!(f, "{}", s),
            Value::App(l, r) => {
                if let App(l1, l2) = *l.clone() {
                    write!(f, "({} {} {})", l1, l2, r)
                } else {
                    write!(f, "({} {})", l, r)
                }
            }
            Value::Lam(var, body) => write!(f, "(lam {} {})", var, body),
            Value::Def(var, value) => write!(f, "(def {} {})", var, value),
            Value::Let(var, value, body) => write!(f, "(let ({} {}) {})", var, value, body),
        }
    }
}

use Value::*;

pub const IDENTIFIER: &str = "\\pL(\\pL|\\pN)*";

pub fn identifier() -> RegexGeneratorStrategy<String> {
    string_regex(IDENTIFIER).unwrap()
}

pub fn ascii_identifier() -> RegexGeneratorStrategy<String> {
    string_regex("[a-zA-Z][a-zA-Z0-9]*").unwrap()
}

impl Arbitrary for Value {
    type Parameters = ();
    type Strategy = BoxedStrategy<Self>;

    fn arbitrary_with(_args: ()) -> Self::Strategy {
        let any_num = any::<i32>().prop_map(Num);
        let any_bool = any::<bool>().prop_map(Bool);
        let leaf = prop_oneof![
            any_num,
            any_bool,
            // see https://unicode.org/reports/tr18/#General_Category_Property for one letter unicode categories
            identifier().prop_map(Sym),
        ];
        let expr = leaf.prop_recursive(4, 128, 5, move |inner| {
            prop_oneof![
                (inner.clone(), inner.clone()).prop_map(|(l, r)| App(Box::new(l), Box::new(r))),
                (identifier(), inner.clone()).prop_map(|(var, body)| Lam(var, Box::new(body))),
                (identifier(), inner.clone(), inner.clone()).prop_map(|(var, body, expr)| {
                    Value::Let(var, Box::new(body), Box::new(expr))
                }),
            ]
        });
        prop_oneof![
            expr.clone(),
            (identifier(), expr).prop_map(|(var, body)| Def(var, Box::new(body)))
        ]
        .boxed()
    }
}

#[cfg(test)]
mod ast_tests {

    use super::{
        identifier,
        Value::{self, *},
    };
    use proptest::collection::vec;
    use proptest::prelude::*;

    fn any_sym() -> impl Strategy<Value = Value> {
        identifier().prop_map(Sym)
    }

    proptest! {

       #[test]
        fn display_multiple_applications_as_a_sequence(atoms in vec("[a-z]".prop_map(Sym), 2..10)) {
            let init  = atoms.first().unwrap().clone();
            let value = atoms.iter().skip(1).fold(init, |acc, expr| {
              Value::App(Box::new(acc.clone()), Box::new(expr.clone()))
            });
            assert_eq!(value.to_string(),
                       format!("({})",
                               atoms.iter().map(|v| v.to_string()).collect::<Vec<String>>().join(" ")));
      }
    }
}