//! Expression evaluator and statement interpreter.
//!
//! To interpret a piece of AbleScript code, you first need to
//! construct an [ExecEnv], which is responsible for storing the stack
//! of local variable and function definitions accessible from an
//! AbleScript snippet. You can then call [ExecEnv::eval_stmts] to
//! evaluate or execute any number of expressions or statements.

#[deny(missing_docs)]
use std::{
    collections::HashMap,
    io::{stdout, Write},
    ops::Range,
    process::exit,
    usize,
};

use rand::random;

use crate::{
    ast::{Expr, Iden, Stmt, StmtKind},
    base_55,
    error::{Error, ErrorKind},
    variables::{Functio, Value, Variable},
};

/// An environment for executing AbleScript code.
pub struct ExecEnv {
    /// The stack, ordered such that `stack[stack.len() - 1]` is the
    /// top-most (newest) stack frame, and `stack[0]` is the
    /// bottom-most (oldest) stack frame.
    stack: Vec<Scope>,
}

/// A set of visible variable and function definitions in a single
/// stack frame.
#[derive(Default)]
struct Scope {
    /// The mapping from variable names to values.
    variables: HashMap<String, Variable>,
    // In the future, this will store functio definitions and possibly
    // other information.
}

/// The reason a successful series of statements halted.
enum HaltStatus {
    /// We ran out of statements to execute.
    Finished,

    /// A `break` statement occurred at the given span, and was not
    /// caught by a `loop` statement up to this point.
    Break(Range<usize>),

    /// A `hopback` statement occurred at the given span, and was not
    /// caught by a `loop` statement up to this point.
    Hopback(Range<usize>),
}

impl ExecEnv {
    /// Create a new Scope with no predefined variable definitions or
    /// other information.
    pub fn new() -> Self {
        Self {
            // We always need at least one stackframe.
            stack: vec![Default::default()],
        }
    }

    /// Execute a set of Statements in the root stack frame. Return an
    /// error if one or more of the Stmts failed to evaluate, or if a
    /// `break` or `hopback` statement occurred at the top level.
    pub fn eval_stmts(&mut self, stmts: &[Stmt]) -> Result<(), Error> {
        match self.eval_stmts_hs(stmts, false)? {
            HaltStatus::Finished => Ok(()),
            HaltStatus::Break(span) | HaltStatus::Hopback(span) => Err(Error {
                // It's an error to issue a `break` outside of a
                // `loop` statement.
                kind: ErrorKind::TopLevelBreak,
                span: span,
            }),
        }
    }

    /// The same as `eval_stmts`, but report "break" and "hopback"
    /// exit codes as normal conditions in a HaltStatus enum, and
    /// create a new stack frame if `stackframe` is true.
    ///
    /// `interpret`-internal code should typically prefer this
    /// function over `eval_stmts`.
    fn eval_stmts_hs(&mut self, stmts: &[Stmt], stackframe: bool) -> Result<HaltStatus, Error> {
        let init_depth = self.stack.len();

        if stackframe {
            self.stack.push(Default::default());
        }

        let mut final_result = Ok(HaltStatus::Finished);
        for stmt in stmts {
            final_result = self.eval_stmt(stmt);
            if !matches!(final_result, Ok(HaltStatus::Finished)) {
                break;
            }
        }

        if stackframe {
            self.stack.pop();
        }

        // Invariant: stack size must have net 0 change.
        debug_assert_eq!(self.stack.len(), init_depth);
        final_result
    }

    /// Evaluate an Expr, returning its value or an error.
    fn eval_expr(&self, expr: &Expr) -> Result<Value, Error> {
        use crate::ast::BinOpKind::*;
        use crate::ast::ExprKind::*;
        use Value::*;

        Ok(match &expr.kind {
            BinOp { lhs, rhs, kind } => {
                let lhs = self.eval_expr(&lhs)?;
                let rhs = self.eval_expr(&rhs)?;
                match kind {
                    // Arithmetic operators.
                    Add | Subtract | Multiply | Divide => {
                        let lhs = lhs.to_i32(&expr.span)?;
                        let rhs = rhs.to_i32(&expr.span)?;

                        let res = match kind {
                            Add => lhs.checked_add(rhs),
                            Subtract => lhs.checked_sub(rhs),
                            Multiply => lhs.checked_mul(rhs),
                            Divide => lhs.checked_div(rhs),
                            _ => unreachable!(),
                        }
                        .ok_or(Error {
                            kind: ErrorKind::ArithmeticError,
                            span: expr.span.clone(),
                        })?;
                        Int(res)
                    }

                    // Numeric comparisons.
                    Less | Greater => {
                        let lhs = lhs.to_i32(&expr.span)?;
                        let rhs = rhs.to_i32(&expr.span)?;

                        let res = match kind {
                            Less => lhs < rhs,
                            Greater => lhs > rhs,
                            _ => unreachable!(),
                        };
                        Bool(res)
                    }

                    // General comparisons.
                    Equal | NotEqual => {
                        let res = match kind {
                            Equal => lhs == rhs,
                            NotEqual => lhs != rhs,
                            _ => unreachable!(),
                        };
                        Bool(res)
                    }

                    // Logical connectives.
                    And | Or => {
                        let lhs = lhs.to_bool();
                        let rhs = rhs.to_bool();
                        let res = match kind {
                            And => lhs && rhs,
                            Or => lhs || rhs,
                            _ => unreachable!(),
                        };
                        Bool(res)
                    }
                }
            }
            Not(expr) => Bool(!self.eval_expr(&expr)?.to_bool()),
            Literal(value) => value.clone(),

            // TODO: not too happy with constructing an artificial
            // Iden here.
            Variable(name) => self.get_var(&Iden {
                iden: name.to_owned(),
                span: expr.span.clone(),
            })?,
        })
    }

    /// Perform the action indicated by a statement.
    fn eval_stmt(&mut self, stmt: &Stmt) -> Result<HaltStatus, Error> {
        match &stmt.kind {
            StmtKind::Print(expr) => {
                println!("{}", self.eval_expr(expr)?);
            }
            StmtKind::Var { iden, init } => {
                let init = match init {
                    Some(e) => self.eval_expr(e)?,
                    None => Value::Nul,
                };

                self.decl_var(&iden.iden, init);
            }
            StmtKind::Functio {
                iden: _,
                args: _,
                body: _,
            } => todo!(),
            StmtKind::BfFunctio {
                iden,
                tape_len,
                code,
            } => {
                self.decl_var(
                    &iden.iden,
                    Value::Functio(Functio::BfFunctio {
                        instructions: code.to_owned(),
                        tape_len: tape_len
                            .as_ref()
                            .map(|tape_len| {
                                self.eval_expr(tape_len)
                                    .and_then(|v| v.to_i32(&stmt.span))
                                    .map(|len| len as usize)
                            })
                            .unwrap_or(Ok(crate::brian::DEFAULT_TAPE_SIZE_LIMIT))?,
                    }),
                );
            }
            StmtKind::If { cond, body } => {
                if self.eval_expr(cond)?.to_bool() {
                    return self.eval_stmts_hs(&body.block, true);
                }
            }
            StmtKind::Call { iden, args } => {
                let func = self.get_var(&iden)?;
                match func {
                    Value::Functio(func) => {
                        match func {
                            Functio::BfFunctio {
                                instructions,
                                tape_len,
                            } => {
                                let mut input: Vec<u8> = vec![];
                                for arg in args {
                                    self.eval_expr(arg)?.bf_write(&mut input);
                                }
                                println!("input = {:?}", input);
                                let mut output = vec![];

                                crate::brian::Interpreter::from_ascii_with_tape_limit(
                                    &instructions,
                                    &input as &[_],
                                    tape_len,
                                )
                                .interpret_with_output(&mut output)
                                .map_err(|e| Error {
                                    kind: ErrorKind::BfInterpretError(e),
                                    span: stmt.span.clone(),
                                })?;

                                // I guess Brainfuck functions write
                                // output to stdout? It's not quite
                                // clear to me what else to do. ~~Alex
                                stdout()
                                    .write_all(&output)
                                    .expect("Failed to write to stdout");
                            }
                            Functio::AbleFunctio(_) => {
                                todo!()
                            }
                        }
                    }
                    _ => {
                        return Err(Error {
                            kind: ErrorKind::TypeError(iden.iden.to_owned()),
                            span: stmt.span.clone(),
                        })
                    }
                }
            }
            StmtKind::Loop { body } => loop {
                let res = self.eval_stmts_hs(&body.block, true)?;
                match res {
                    HaltStatus::Finished => {}
                    HaltStatus::Break(_) => break,
                    HaltStatus::Hopback(_) => continue,
                }
            },
            StmtKind::Assign { iden, value } => {
                self.get_var_mut(&iden)?.value = self.eval_expr(value)?;
            }
            StmtKind::Break => {
                return Ok(HaltStatus::Break(stmt.span.clone()));
            }
            StmtKind::HopBack => {
                return Ok(HaltStatus::Hopback(stmt.span.clone()));
            }
            StmtKind::Melo(iden) => {
                self.get_var_mut(&iden)?.melo = true;
            }
            StmtKind::Rlyeh => {
                // Maybe print a creepy error message or something
                // here at some point. ~~Alex
                exit(random());
            }
            StmtKind::Rickroll => {
                stdout().write_all(b"We're no strangers to love
You know the rules and so do I
A full commitments what I'm thinking of
You wouldn't get this from another guy

I just wanna tell you how I'm feeling
Gotta make you understand

Never gonna give you up
Never gonna let you down
Never gonna run around and desert you
Never gonna make you cry
Never gonna say goodbye
Never gonna tell a lie and hurt you

We've known each other for so long
Your heart's been aching but you're too shy to say it
Inside we both know what's been going on
We know the game and we're gonna play it

And if you ask me how I'm feeling
Don't tell me you're too blind to see

Never gonna give you up
Never gonna let you down
Never gonna run around and desert you
Never gonna make you cry
Never gonna say goodbye
Never gonna tell a lie and hurt you

Never gonna give you up
Never gonna let you down
Never gonna run around and desert you
Never gonna make you cry
Never gonna say goodbye
Never gonna tell a lie and hurt you

Never gonna give, never gonna give
(Give you up)

We've known each other for so long
Your heart's been aching but you're too shy to say it
Inside we both know what's been going on
We know the game and we're gonna play it

I just wanna tell you how I'm feeling
Gotta make you understand

Never gonna give you up
Never gonna let you down
Never gonna run around and desert you
Never gonna make you cry
Never gonna say goodbye
Never gonna tell a lie and hurt you

Never gonna give you up
Never gonna let you down
Never gonna run around and desert you
Never gonna make you cry
Never gonna say goodbye
Never gonna tell a lie and hurt you

Never gonna give you up
Never gonna let you down
Never gonna run around and desert you
Never gonna make you cry
Never gonna say goodbye").expect("Failed to write to stdout")
            }
        }

        Ok(HaltStatus::Finished)
    }

    /// Get the value of a variable. Throw an error if the variable is
    /// inaccessible or banned.
    fn get_var(&self, name: &Iden) -> Result<Value, Error> {
        // One-letter names are reserved as base55 numbers.
        let mut chars = name.iden.chars();
        if let (Some(first), None) = (chars.next(), chars.next()) {
            return Ok(Value::Int(base_55::char2num(first)));
        }

        // Otherwise, search for the name in the stack from top to
        // bottom.
        match self
            .stack
            .iter()
            .rev()
            .find_map(|scope| scope.variables.get(&name.iden))
        {
            Some(var) => {
                if !var.melo {
                    Ok(var.value.clone())
                } else {
                    Err(Error {
                        kind: ErrorKind::MeloVariable(name.iden.to_owned()),
                        span: name.span.clone(),
                    })
                }
            }
            None => Err(Error {
                kind: ErrorKind::UnknownVariable(name.iden.to_owned()),
                span: name.span.clone(),
            }),
        }
    }

    /// Get a mutable reference to a variable. Throw an error if the
    /// variable is inaccessible or banned.
    fn get_var_mut(&mut self, name: &Iden) -> Result<&mut Variable, Error> {
        // This function has a lot of duplicated code with `get_var`,
        // which I feel like is a bad sign...
        match self
            .stack
            .iter_mut()
            .rev()
            .find_map(|scope| scope.variables.get_mut(&name.iden))
        {
            Some(var) => {
                if !var.melo {
                    Ok(var)
                } else {
                    Err(Error {
                        kind: ErrorKind::MeloVariable(name.iden.to_owned()),
                        span: name.span.clone(),
                    })
                }
            }
            None => Err(Error {
                kind: ErrorKind::UnknownVariable(name.iden.to_owned()),
                span: name.span.clone(),
            }),
        }
    }

    /// Declares a new variable, with the given initial value.
    fn decl_var(&mut self, name: &str, value: Value) {
        self.stack
            .iter_mut()
            .last()
            .expect("Declaring variable on empty stack")
            .variables
            .insert(name.to_owned(), Variable { melo: false, value });
    }
}

#[cfg(test)]
mod tests {
    use crate::ast::ExprKind;

    use super::*;

    #[test]
    fn basic_expression_test() {
        // Check that 2 + 2 = 4.
        let env = ExecEnv::new();
        assert_eq!(
            env.eval_expr(&Expr {
                kind: ExprKind::BinOp {
                    lhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Int(2)),
                        span: 1..1,
                    }),
                    rhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Int(2)),
                        span: 1..1,
                    }),
                    kind: crate::ast::BinOpKind::Add,
                },
                span: 1..1
            })
            .unwrap(),
            Value::Int(4)
        )
    }

    #[test]
    fn type_errors() {
        // The sum of an integer and a boolean results in a type
        // error.
        let env = ExecEnv::new();
        assert!(matches!(
            env.eval_expr(&Expr {
                kind: ExprKind::BinOp {
                    lhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Int(2)),
                        span: 1..1,
                    }),
                    rhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Bool(true)),
                        span: 1..1,
                    }),
                    kind: crate::ast::BinOpKind::Add,
                },
                span: 1..1
            }),
            Err(Error {
                kind: ErrorKind::TypeError(_),
                span: _,
            })
        ));
    }

    #[test]
    fn overflow_should_not_panic() {
        // Integer overflow should throw a recoverable error instead
        // of panicking.
        let env = ExecEnv::new();
        assert!(matches!(
            env.eval_expr(&Expr {
                kind: ExprKind::BinOp {
                    lhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Int(i32::MAX)),
                        span: 1..1,
                    }),
                    rhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Int(1)),
                        span: 1..1,
                    }),
                    kind: crate::ast::BinOpKind::Add,
                },
                span: 1..1
            }),
            Err(Error {
                kind: ErrorKind::ArithmeticError,
                span: _,
            })
        ));

        // And the same for divide by zero.
        assert!(matches!(
            env.eval_expr(&Expr {
                kind: ExprKind::BinOp {
                    lhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Int(1)),
                        span: 1..1,
                    }),
                    rhs: Box::new(Expr {
                        kind: ExprKind::Literal(Value::Int(0)),
                        span: 1..1,
                    }),
                    kind: crate::ast::BinOpKind::Divide,
                },
                span: 1..1
            }),
            Err(Error {
                kind: ErrorKind::ArithmeticError,
                span: _,
            })
        ));
    }

    // From here on out, I'll use this function to parse and run
    // expressions, because writing out abstract syntax trees by hand
    // takes forever and is error-prone.
    fn eval(env: &mut ExecEnv, src: &str) -> Result<Value, Error> {
        let mut parser = crate::parser::Parser::new(src);

        // We can assume there won't be any syntax errors in the
        // interpreter tests.
        let ast = parser.init().unwrap();
        env.eval_stmts(&ast).map(|()| Value::Nul)
    }

    #[test]
    #[ignore = "doesn't make sense anymore due to separation of statements & expressions"]
    fn variable_decl_and_assignment() {
        // Declaring and reading from a variable.
        assert_eq!(
            eval(&mut ExecEnv::new(), "var foo = 32; foo + 1").unwrap(),
            Value::Int(33)
        );

        // It should be possible to overwrite variables as well.
        assert_eq!(
            eval(&mut ExecEnv::new(), "var bar = 10; bar = 20; bar").unwrap(),
            Value::Int(20)
        );

        // But variable assignment should be illegal when the variable
        // hasn't been declared in advance.
        eval(&mut ExecEnv::new(), "baz = 10;").unwrap_err();
    }

    #[test]
    #[ignore = "doesn't make sense anymore due to separation of statements & expressions"]
    fn variable_persistence() {
        // Global variables should persist between invocations of
        // ExecEnv::eval_items().
        let mut env = ExecEnv::new();
        eval(&mut env, "var foo = 32;").unwrap();
        assert_eq!(eval(&mut env, "foo").unwrap(), Value::Int(32));
    }

    #[test]
    #[ignore = "doesn't make sense anymore due to separation of statements & expressions"]
    fn scope_visibility_rules() {
        // Declaration and assignment of variables declared in an `if`
        // statement should have no effect on those declared outside
        // of it.
        assert_eq!(
            eval(
                &mut ExecEnv::new(),
                "var foo = 1; if (true) { var foo = 2; foo = 3; } foo"
            )
            .unwrap(),
            Value::Int(1)
        );
    }
}