// EndBASIC
// Copyright 2020 Julio Merino
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License.  You may obtain a copy
// of the License at:
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
// License for the specific language governing permissions and limitations
// under the License.

//! Execution engine for EndBASIC programs.

use crate::ast::{Expr, Statement, Value, VarRef, VarType};
use crate::eval;
use crate::parser::{self, Parser};
use crate::syms::{CallError, CallableMetadata, Command, Function, Symbol, Symbols};
use std::future::Future;
use std::io;
use std::pin::Pin;
use std::rc::Rc;

/// Execution errors.
#[derive(Debug, thiserror::Error)]
pub enum Error {
    /// Evaluation error during execution.
    #[error("{0}")]
    EvalError(#[from] eval::Error),

    /// I/O error during execution.
    #[error("{0}")]
    IoError(#[from] io::Error),

    /// Parsing error during execution.
    #[error("{0}")]
    ParseError(#[from] parser::Error),

    /// Syntax error.
    #[error("{0}")]
    SyntaxError(String),
}

impl Error {
    /// Annotates a call evaluation error with the command's metadata.
    // TODO(jmmv): This is a hack to support the transition to a better Command abstraction within
    // Symbols and exists to minimize the amount of impacted tests.  Should be removed and/or
    // somehow unified with the equivalent function in eval::Error.
    pub(crate) fn from_call_error(md: &CallableMetadata, e: CallError) -> Self {
        match e {
            CallError::ArgumentError(e) => Self::SyntaxError(e),
            CallError::EvalError(e) => Self::EvalError(e),
            CallError::InternalError(e) => Self::SyntaxError(e),
            CallError::IoError(e) => Self::IoError(e),
            CallError::SyntaxError => Self::SyntaxError(format!(
                "Syntax error in call to {}: expected {}",
                md.name(),
                md.syntax()
            )),
        }
    }
}

/// Result for execution return values.
pub type Result<T> = std::result::Result<T, Error>;

/// Instantiates a new `Err(Error::SyntaxError(...))` from a message.  Syntactic sugar.
fn new_syntax_error<T, S: Into<String>>(message: S) -> Result<T> {
    Err(Error::SyntaxError(message.into()))
}

/// Describes how the machine stopped execution while it was running a script via `exec()`.
#[derive(Clone, Debug, PartialEq)]
#[must_use]
pub enum StopReason {
    /// Execution terminates because the machine reached the end of the input.
    Eof,

    /// Execution terminated because the machine was asked to terminate with `exit()`.
    Exited(u8),
}

impl StopReason {
    /// Converts the stop reason into a process exit code.
    pub fn as_exit_code(&self) -> i32 {
        match self {
            StopReason::Eof => 0,
            StopReason::Exited(i) => *i as i32,
        }
    }
}

/// Executes an EndBASIC program and tracks its state.
#[derive(Default)]
pub struct Machine {
    symbols: Symbols,
    stop_reason: Option<StopReason>,
}

impl Machine {
    /// Registers the given builtin command, which must not yet be registered.
    pub fn add_command(&mut self, command: Rc<dyn Command>) {
        self.symbols.add_command(command)
    }

    /// Registers the given builtin function, which must not yet be registered.
    pub fn add_function(&mut self, function: Rc<dyn Function>) {
        self.symbols.add_function(function)
    }

    /// Resets the state of the machine by clearing all variable.
    pub fn clear(&mut self) {
        self.symbols.clear()
    }

    /// Tells the machine to stop execution at the next statement boundary.
    ///
    /// The `exec()` call that's stopped by this invocation will return the `code` given to this
    /// call.
    pub fn exit(&mut self, code: u8) {
        self.stop_reason = Some(StopReason::Exited(code));
    }

    /// Obtains immutable access to the state of the symbols.
    pub fn get_symbols(&self) -> &Symbols {
        &self.symbols
    }

    /// Obtains mutable access to the state of the symbols.
    pub fn get_mut_symbols(&mut self) -> &mut Symbols {
        &mut self.symbols
    }

    /// Retrieves the variable `name` as a boolean.  Fails if it is some other type or if it's not
    /// defined.
    pub fn get_var_as_bool(&self, name: &str) -> Result<bool> {
        match self.symbols.get_var(&VarRef::new(name, VarType::Boolean))? {
            Value::Boolean(b) => Ok(*b),
            _ => panic!("Invalid type check in get()"),
        }
    }

    /// Retrieves the variable `name` as an integer.  Fails if it is some other type or if it's not
    /// defined.
    pub fn get_var_as_int(&self, name: &str) -> Result<i32> {
        match self.symbols.get_var(&VarRef::new(name, VarType::Integer))? {
            Value::Integer(i) => Ok(*i),
            _ => panic!("Invalid type check in get()"),
        }
    }

    /// Retrieves the variable `name` as a string.  Fails if it is some other type or if it's not
    /// defined.
    pub fn get_var_as_string(&self, name: &str) -> Result<&str> {
        match self.symbols.get_var(&VarRef::new(name, VarType::Text))? {
            Value::Text(s) => Ok(s),
            _ => panic!("Invalid type check in get()"),
        }
    }

    /// Assigns the value of `expr` to the variable `vref`.
    fn assign(&mut self, vref: &VarRef, expr: &Expr) -> Result<()> {
        let value = expr.eval(&mut self.symbols)?;
        self.symbols.set_var(&vref, value)?;
        Ok(())
    }

    /// Assigns the value of `expr` to the array `vref` in the position `subscripts`.
    fn assign_array(&mut self, vref: &VarRef, subscripts: &[Expr], expr: &Expr) -> Result<()> {
        let mut ds = Vec::with_capacity(subscripts.len());
        for ss_expr in subscripts {
            match ss_expr.eval(&mut self.symbols)? {
                Value::Integer(i) => ds.push(i),
                v => return new_syntax_error(format!("Subscript {:?} must be an integer", v)),
            }
        }

        let value = expr.eval(&mut self.symbols)?;

        match self.symbols.get_mut(vref)? {
            Some(Symbol::Array(array)) => {
                array.assign(&ds, value)?;
                Ok(())
            }
            Some(_) => new_syntax_error(format!("Cannot index non-array {}", vref.name())),
            None => new_syntax_error(format!("Cannot index undefined array {}", vref.name())),
        }
    }

    /// Defines a new array `name` of type `subtype` with `dimensions`.  The array must not yet
    /// exist, and the name may not overlap function or variable names.
    pub fn dim_array(&mut self, name: &str, subtype: &VarType, dimensions: &[Expr]) -> Result<()> {
        let mut ds = Vec::with_capacity(dimensions.len());
        for dim_expr in dimensions {
            match dim_expr.eval(&mut self.symbols)? {
                Value::Integer(i) => {
                    if i <= 0 {
                        return new_syntax_error("Dimensions in DIM array must be positive");
                    }
                    ds.push(i as usize);
                }
                _ => return new_syntax_error("Dimensions in DIM array must be integers"),
            }
        }
        self.symbols.dim_array(name, *subtype, ds)?;
        Ok(())
    }

    /// Executes an `IF` statement.
    async fn do_if(&mut self, branches: &[(Expr, Vec<Statement>)]) -> Result<()> {
        for (expr, stmts) in branches {
            match expr.eval(&mut self.symbols)? {
                Value::Boolean(true) => {
                    for s in stmts {
                        self.exec_one(s).await?;
                    }
                    break;
                }
                Value::Boolean(false) => (),
                _ => return new_syntax_error("IF/ELSEIF require a boolean condition"),
            };
        }
        Ok(())
    }

    /// Executes a `FOR` loop.
    async fn do_for(
        &mut self,
        iterator: &VarRef,
        start: &Expr,
        end: &Expr,
        next: &Expr,
        body: &[Statement],
    ) -> Result<()> {
        debug_assert!(
            iterator.ref_type() == VarType::Auto || iterator.ref_type() == VarType::Integer
        );
        let start_value = start.eval(&mut self.symbols)?;
        match start_value {
            Value::Integer(_) => self.symbols.set_var(iterator, start_value)?,
            _ => return new_syntax_error("FOR supports integer iteration only"),
        }

        loop {
            match end.eval(&mut self.symbols)? {
                Value::Boolean(false) => {
                    break;
                }
                Value::Boolean(true) => (),
                _ => panic!("Built-in condition should have evaluated to a boolean"),
            }

            for s in body {
                self.exec_one(s).await?;
            }

            self.assign(iterator, next)?;
        }
        Ok(())
    }

    /// Executes a `WHILE` loop.
    async fn do_while(&mut self, condition: &Expr, body: &[Statement]) -> Result<()> {
        loop {
            match condition.eval(&mut self.symbols)? {
                Value::Boolean(true) => {
                    for s in body {
                        self.exec_one(s).await?;
                    }
                }
                Value::Boolean(false) => break,
                _ => return new_syntax_error("WHILE requires a boolean condition"),
            }
        }
        Ok(())
    }

    /// Executes a single statement.
    async fn exec_one<'a>(&'a mut self, stmt: &'a Statement) -> Result<()> {
        if self.stop_reason.is_some() {
            return Ok(());
        }

        match stmt {
            Statement::ArrayAssignment(vref, subscripts, value) => {
                self.assign_array(vref, subscripts, value)?
            }
            Statement::Assignment(vref, expr) => self.assign(vref, expr)?,
            Statement::BuiltinCall(name, args) => {
                let cmd = match self.symbols.get(&VarRef::new(name, VarType::Auto))? {
                    Some(Symbol::Command(cmd)) => cmd.clone(),
                    Some(_) => return new_syntax_error(format!("{} is not a command", name)),
                    None => return new_syntax_error(format!("Unknown builtin {}", name)),
                };
                cmd.exec(&args, self)
                    .await
                    .map_err(|e| Error::from_call_error(cmd.metadata(), e))?;
            }
            Statement::Dim(varname, vartype) => self.symbols.dim(varname, *vartype)?,
            Statement::DimArray(varname, dimensions, subtype) => {
                self.dim_array(varname, subtype, dimensions)?
            }
            Statement::If(branches) => {
                // Change this to using FutureExt::boxed_local if we ever depend on the futures or
                // futures_lite crate directly.
                let f: Pin<Box<dyn Future<Output = Result<()>>>> = Box::pin(self.do_if(branches));
                f.await?;
            }
            Statement::For(iterator, start, end, next, body) => {
                // Change this to using FutureExt::boxed_local if we ever depend on the futures or
                // futures_lite crate directly.
                let f: Pin<Box<dyn Future<Output = Result<()>>>> =
                    Box::pin(self.do_for(iterator, start, end, next, body));
                f.await?;
            }
            Statement::While(condition, body) => {
                // Change this to using FutureExt::boxed_local if we ever depend on the futures or
                // futures_lite crate directly.
                let f: Pin<Box<dyn Future<Output = Result<()>>>> =
                    Box::pin(self.do_while(condition, body));
                f.await?;
            }
        }
        Ok(())
    }

    /// Executes a program extracted from the `input` readable.
    ///
    /// Note that this does not consume `self`.  As a result, it is possible to execute multiple
    /// different programs on the same machine, all sharing state.
    pub async fn exec(&mut self, input: &mut dyn io::Read) -> Result<StopReason> {
        debug_assert!(self.stop_reason.is_none());
        let mut parser = Parser::from(input);
        while self.stop_reason.is_none() {
            match parser.parse()? {
                Some(stmt) => self.exec_one(&stmt).await?,
                None => break,
            }
        }
        Ok(self.stop_reason.take().unwrap_or(StopReason::Eof))
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::testutils::*;
    use futures_lite::future::block_on;
    use std::cell::RefCell;
    use std::rc::Rc;

    #[test]
    fn test_clear() {
        let mut machine = Machine::default();
        assert_eq!(
            StopReason::Eof,
            block_on(machine.exec(&mut b"a = TRUE: b = 1".as_ref())).expect("Execution failed")
        );
        assert!(machine.get_var_as_bool("a").is_ok());
        assert!(machine.get_var_as_int("b").is_ok());
        machine.clear();
        assert!(machine.get_var_as_bool("a").is_err());
        assert!(machine.get_var_as_int("b").is_err());
    }

    #[test]
    fn test_get_var_as_bool() {
        let mut machine = Machine::default();
        assert_eq!(
            StopReason::Eof,
            block_on(machine.exec(&mut b"a = TRUE: b = 1".as_ref())).expect("Execution failed")
        );
        assert!(machine.get_var_as_bool("a").expect("Failed to query a"));
        assert_eq!(
            "Incompatible types in b? reference",
            format!("{}", machine.get_var_as_bool("b").expect_err("Querying b succeeded"))
        );
        assert_eq!(
            "Undefined variable c",
            format!("{}", machine.get_var_as_bool("c").expect_err("Querying c succeeded"))
        );
    }

    #[test]
    fn test_get_var_as_int() {
        let mut machine = Machine::default();
        assert_eq!(
            StopReason::Eof,
            block_on(machine.exec(&mut b"a = 1: b = \"foo\"".as_ref())).expect("Execution failed")
        );
        assert_eq!(1, machine.get_var_as_int("a").expect("Failed to query a"));
        assert_eq!(
            "Incompatible types in b% reference",
            format!("{}", machine.get_var_as_int("b").expect_err("Querying b succeeded"))
        );
        assert_eq!(
            "Undefined variable c",
            format!("{}", machine.get_var_as_int("c").expect_err("Querying c succeeded"))
        );
    }

    #[test]
    fn test_get_var_as_string() {
        let mut machine = Machine::default();
        assert_eq!(
            StopReason::Eof,
            block_on(machine.exec(&mut b"a = \"foo\": b = FALSE".as_ref()))
                .expect("Execution failed")
        );
        assert_eq!("foo", machine.get_var_as_string("a").expect("Failed to query a"));
        assert_eq!(
            "Incompatible types in b$ reference",
            format!("{}", machine.get_var_as_string("b").expect_err("Querying b succeeded"))
        );
        assert_eq!(
            "Undefined variable c",
            format!("{}", machine.get_var_as_string("c").expect_err("Querying c succeeded"))
        );
    }

    /// Runs the `input` code on a new test machine.
    ///
    /// `golden_in` is the sequence of values to yield by `IN`.
    /// `expected_out` is updated with the sequence of calls to `OUT`.
    fn run(
        input: &str,
        golden_in: &'static [&'static str],
        captured_out: Rc<RefCell<Vec<String>>>,
    ) -> Result<StopReason> {
        let mut machine = Machine::default();
        machine.add_command(ExitCommand::new());
        machine.add_command(InCommand::new(Box::from(RefCell::from(golden_in.iter()))));
        machine.add_command(OutCommand::new(captured_out));
        machine.add_function(SumFunction::new());
        block_on(machine.exec(&mut input.as_bytes()))
    }

    /// Runs the `input` code on a new test machine and verifies its output.
    ///
    /// `golden_in` is the sequence of values to yield by `IN`.
    /// `expected_out` is the sequence of expected calls to `OUT`.
    fn do_ok_test(
        input: &str,
        golden_in: &'static [&'static str],
        expected_out: &'static [&'static str],
    ) {
        let captured_out = Rc::from(RefCell::from(vec![]));
        assert_eq!(
            StopReason::Eof,
            run(input, golden_in, captured_out.clone()).expect("Execution failed")
        );
        assert_eq!(expected_out, captured_out.borrow().as_slice());
    }

    /// Runs the `input` code on a new test machine and verifies that it fails with `expected_err`.
    ///
    /// Given that the code has side-effects until it fails, this follows the same process as
    /// `do_ok_test` regarding `golden_in` and `expected_out`.
    fn do_error_test(
        input: &str,
        golden_in: &'static [&'static str],
        expected_out: &'static [&'static str],
        expected_err: &str,
    ) {
        let captured_out = Rc::from(RefCell::from(vec![]));
        let err = run(input, golden_in, captured_out.clone()).expect_err("Execution did not fail");
        assert_eq!(expected_err, format!("{}", err));
        assert_eq!(expected_out, captured_out.borrow().as_slice());
    }

    /// Runs the `input` code on a new machine and verifies that it fails with `expected_err`.
    ///
    /// This is a syntactic wrapper over `do_error_test` to simplify those tests that are not
    /// expected to request any input nor generate any output.
    fn do_simple_error_test(input: &str, expected_err: &str) {
        do_error_test(input, &[], &[], expected_err);
    }

    #[test]
    fn test_array_assignment_ok() {
        do_ok_test("DIM a(3)\na(1) = 5 + 1\nOUT a(0); a(1); a(2)", &[], &["0 6 0"]);
        do_ok_test("DIM a(3) AS STRING\na$(1) = \"x\"\nOUT a(0); a(1); a$(2)", &[], &[" x "]);
        do_ok_test(
            "DIM a(3, 8, 2) AS BOOLEAN\na(3 - 3, 2 * 2, 1) = TRUE\nOUT a(0, 4, 1)",
            &[],
            &["TRUE"],
        );
    }

    #[test]
    fn test_array_assignment_ok_case_insensitive() {
        do_ok_test("DIM a(3)\nA(1) = 5\na(2) = 1\nOUT A(0); a(1); A(2)", &[], &["0 5 1"]);
    }

    #[test]
    fn test_array_assignment_errors() {
        do_simple_error_test("a() = 3\n", "Cannot index undefined array a");
        do_simple_error_test("a = 3\na(0) = 3\n", "Cannot index non-array a");
        do_simple_error_test("DIM a(2)\na() = 3\n", "Cannot index array with 0 subscripts; need 1");
        do_simple_error_test("DIM a(1)\na(-1) = 3\n", "Subscript -1 cannot be negative");
        do_simple_error_test(
            "DIM a(1)\na(1, 3.0) = 3\n",
            "Subscript Double(3.0) must be an integer",
        );
        do_simple_error_test("DIM a(2)\na$(1) = 3", "Incompatible types in a$ reference");
    }

    #[test]
    fn test_assignment_ok_types() {
        do_ok_test("a = TRUE\nOUT a; a?", &[], &["TRUE TRUE"]);
        do_ok_test("a? = FALSE\nOUT a; a?", &[], &["FALSE FALSE"]);

        do_ok_test("a = 3\nOUT a; a%", &[], &["3 3"]);
        do_ok_test("a% = 3\nOUT a; a%", &[], &["3 3"]);

        do_ok_test("a = \"some text\"\nOUT a; a$", &[], &["some text some text"]);
        do_ok_test("a$ = \"some text\"\nOUT a; a$", &[], &["some text some text"]);

        do_ok_test("a = 1\na = a + 1\nOUT a", &[], &["2"]);
    }

    #[test]
    fn test_assignment_ok_case_insensitive() {
        do_ok_test("foo = 32\nOUT FOO", &[], &["32"]);
    }

    #[test]
    fn test_assignment_errors() {
        do_simple_error_test("a =\n", "Missing expression in assignment");
        do_simple_error_test("a = b\n", "Undefined variable b");
        do_simple_error_test("a = 3\na = TRUE\n", "Incompatible types in a assignment");
        do_simple_error_test("a? = 3", "Incompatible types in a? assignment");
    }

    #[test]
    fn test_dim_ok() {
        do_ok_test("DIM foo\nDIM bar AS BOOLEAN\nOUT foo%; bar?", &[], &["0 FALSE"]);
    }

    #[test]
    fn test_dim_errors() {
        do_simple_error_test("DIM i\nDIM i", "Cannot DIM already-defined symbol i");
        do_simple_error_test("DIM i\nDIM I", "Cannot DIM already-defined symbol I");
        do_simple_error_test("i = 0\nDIM i", "Cannot DIM already-defined symbol i");
    }

    #[test]
    fn test_dim_array_ok() {
        do_ok_test(
            "DIM foo(3, 4)\nDIM bar(1) AS BOOLEAN\nOUT foo%(2, 2); bar?(0)",
            &[],
            &["0 FALSE"],
        );
    }

    #[test]
    fn test_dim_array_errors() {
        do_simple_error_test("DIM i()", "Arrays require at least one dimension");
        do_simple_error_test("DIM i(FALSE)", "Dimensions in DIM array must be integers");
        do_simple_error_test("DIM i(-3)", "Dimensions in DIM array must be positive");
        do_simple_error_test("DIM i\nDIM i(3)", "Cannot DIM already-defined symbol i");
    }

    #[test]
    fn test_exit_simple() {
        let captured_out = Rc::from(RefCell::from(vec![]));
        assert_eq!(
            StopReason::Exited(5),
            run("OUT 1\nEXIT 5\nOUT 2", &[], captured_out.clone()).expect("Execution failed")
        );
        assert_eq!(&["1"], captured_out.borrow().as_slice());
    }

    #[test]
    fn test_exit_zero_is_not_special() {
        let captured_out = Rc::from(RefCell::from(vec![]));
        assert_eq!(
            StopReason::Exited(0),
            run("OUT 1\nOUT 2\nEXIT 0\nOUT 3", &[], captured_out.clone())
                .expect("Execution failed")
        );
        assert_eq!(&["1", "2"], captured_out.borrow().as_slice());
    }

    #[test]
    fn test_exit_nested() {
        let captured_out = Rc::from(RefCell::from(vec![]));
        assert_eq!(
            StopReason::Exited(42),
            run(
                "FOR a = 0 TO 10\nOUT a\nIF a = 3 THEN\nEXIT 42\nEND IF\nNEXT",
                &[],
                captured_out.clone()
            )
            .expect("Execution failed")
        );
        assert_eq!(&["0", "1", "2", "3"], captured_out.borrow().as_slice());
    }

    #[test]
    fn test_exit_can_resume() {
        let captured_out = Rc::from(RefCell::from(vec![]));
        let mut machine = Machine::default();
        machine.add_command(ExitCommand::new());
        machine.add_command(OutCommand::new(captured_out.clone()));
        machine.add_function(SumFunction::new());

        assert_eq!(
            StopReason::Exited(10),
            block_on(machine.exec(&mut "OUT 1\nEXIT 10\nOUT 2".as_bytes()))
                .expect("Execution failed")
        );
        assert_eq!(&["1"], captured_out.borrow().as_slice());

        captured_out.borrow_mut().clear();
        assert_eq!(
            StopReason::Exited(11),
            block_on(machine.exec(&mut "OUT 2\nEXIT 11\nOUT 3".as_bytes()))
                .expect("Execution failed")
        );
        assert_eq!(&["2"], captured_out.borrow().as_slice());
    }

    #[test]
    fn test_if_ok() {
        let code = r#"
            IN n
            IF n = 3 THEN
                OUT "match"
            END IF
            IF n <> 3 THEN
                OUT "no match"
            END IF
        "#;
        do_ok_test(code, &["3"], &["match"]);
        do_ok_test(code, &["5"], &["no match"]);

        let code = r#"
            IN n
            IF n = 1 THEN
                OUT "first"
            ELSEIF n = 2 THEN
                OUT "second"
            ELSEIF n = 3 THEN
                OUT "third"
            ELSE
                OUT "fourth"
            END IF
        "#;
        do_ok_test(code, &["1"], &["first"]);
        do_ok_test(code, &["2"], &["second"]);
        do_ok_test(code, &["3"], &["third"]);
        do_ok_test(code, &["4"], &["fourth"]);
    }

    #[test]
    fn test_if_ok_on_malformed_branch() {
        let code = r#"
            IN n
            IF n = 3 THEN
                OUT "match"
            ELSEIF "foo" THEN 'Invalid expression type but not evaluated.
                OUT "no match"
            END IF
        "#;
        do_ok_test(code, &["3"], &["match"]);
        do_error_test(code, &["5"], &[], "IF/ELSEIF require a boolean condition");
    }

    #[test]
    fn test_if_errors() {
        do_simple_error_test("IF TRUE THEN END IF", "Expecting newline after THEN");
        do_simple_error_test(
            "IF TRUE THEN\nELSE IF TRUE THEN\nEND IF",
            "Expecting newline after ELSE",
        );
        do_simple_error_test("IF TRUE\nEND IF\nOUT 3", "No THEN in IF statement");

        do_simple_error_test("IF 2\nEND IF", "No THEN in IF statement");
        do_simple_error_test("IF 2 THEN\nEND IF", "IF/ELSEIF require a boolean condition");
        do_simple_error_test(
            "IF FALSE THEN\nELSEIF 2 THEN\nEND IF",
            "IF/ELSEIF require a boolean condition",
        );
    }

    #[test]
    fn test_for_incrementing() {
        let code = r#"
            IN first
            IN last
            FOR a = first TO last
                OUT "a is"; a
            NEXT
        "#;
        do_ok_test(code, &["0", "0"], &["a is 0"]);
        do_ok_test(code, &["0", "3"], &["a is 0", "a is 1", "a is 2", "a is 3"]);
    }

    #[test]
    fn test_for_incrementing_with_step() {
        let code = r#"
            IN first
            IN last
            FOR a = first TO last STEP 3
                OUT "a is"; a
            NEXT
        "#;
        do_ok_test(code, &["0", "0"], &["a is 0"]);
        do_ok_test(code, &["0", "2"], &["a is 0"]);
        do_ok_test(code, &["0", "3"], &["a is 0", "a is 3"]);
        do_ok_test(code, &["0", "10"], &["a is 0", "a is 3", "a is 6", "a is 9"]);
    }

    #[test]
    fn test_for_decrementing_with_step() {
        let code = r#"
            IN first
            IN last
            FOR a = first TO last STEP -2
                OUT "a is"; a
            NEXT
        "#;
        do_ok_test(code, &["0", "0"], &["a is 0"]);
        do_ok_test(code, &["2", "0"], &["a is 2", "a is 0"]);
        do_ok_test(code, &["-2", "-6"], &["a is -2", "a is -4", "a is -6"]);
        do_ok_test(code, &["10", "1"], &["a is 10", "a is 8", "a is 6", "a is 4", "a is 2"]);
    }

    #[test]
    fn test_for_already_done() {
        do_ok_test("FOR i = 10 TO 9\nOUT i\nNEXT", &[], &[]);
        do_ok_test("FOR i = 9 TO 10 STEP -1\nOUT i\nNEXT", &[], &[]);
    }

    #[test]
    fn test_for_iterator_is_visible_after_next() {
        let code = r#"
            FOR something = 1 TO 10 STEP 8
            NEXT
            OUT something
        "#;
        do_ok_test(code, &[], &["17"]);
    }

    #[test]
    fn test_for_iterator_can_be_modified() {
        let code = r#"
            FOR something = 1 TO 5
                OUT something
                something = something + 1
            NEXT
        "#;
        do_ok_test(code, &[], &["1", "3", "5"]);
    }

    #[test]
    fn test_for_errors() {
        do_simple_error_test("FOR\nNEXT", "No iterator name in FOR statement");
        do_simple_error_test("FOR a = 1 TO 10\nEND IF", "Unexpected token End in statement");

        do_simple_error_test("FOR i = \"a\" TO 3\nNEXT", "FOR supports integer iteration only");
        do_simple_error_test(
            "FOR i = 1 TO \"a\"\nNEXT",
            "Cannot compare Integer(1) and Text(\"a\") with <=",
        );

        do_simple_error_test(
            "FOR i = \"b\" TO 7 STEP -8\nNEXT",
            "FOR supports integer iteration only",
        );
        do_simple_error_test(
            "FOR i = 1 TO \"b\" STEP -8\nNEXT",
            "Cannot compare Integer(1) and Text(\"b\") with >=",
        );

        do_simple_error_test("FOR a = 1.0 TO 10.0\nNEXT", "FOR supports integer iteration only");
    }

    #[test]
    fn test_function_call_ok() {
        do_ok_test("x = 3\nOUT SUM(x, Sum%(4, 5), 1, sum())", &[], &["13"]);
    }

    #[test]
    fn test_function_call_errors() {
        do_simple_error_test("OUT OUT()", "OUT is not an array or a function");
        do_simple_error_test("OUT SUM?()", "Incompatible types in SUM? reference");
    }

    #[test]
    fn test_while_ok() {
        let code = r#"
            IN n
            WHILE n > 0
                OUT "n is"; n
                n = n - 1
            WEND
        "#;
        do_ok_test(code, &["0"], &[]);
        do_ok_test(code, &["3"], &["n is 3", "n is 2", "n is 1"]);

        do_ok_test("WHILE FALSE\nOUT 1\nWEND", &[], &[]);
    }

    #[test]
    fn test_while_errors() {
        do_simple_error_test("WHILE\nWEND", "No expression in WHILE statement");
        do_simple_error_test("WHILE\nEND IF", "WHILE without WEND");

        do_simple_error_test("WHILE 2\n", "WHILE without WEND");
        do_simple_error_test("WHILE 2\nWEND", "WHILE requires a boolean condition");
    }

    #[test]
    fn test_misc_comments_and_spaces() {
        let code = r#"
            REM This is the start of the program.

            OUT "Hello" 'Some remark here.

            IF TRUE THEN

                OUT "Bye" 'And another remark here after a blank line.
            END IF
        "#;
        do_ok_test(code, &[], &["Hello", "Bye"]);
    }

    #[test]
    fn test_top_level_errors() {
        do_simple_error_test("FOO BAR", "Unknown builtin FOO");
        do_error_test("OUT \"a\"\nFOO BAR\nOUT \"b\"", &[], &["a"], "Unknown builtin FOO");

        do_simple_error_test("+ b", "Unexpected token Plus in statement");
        do_error_test(
            "OUT \"a\"\n+ b\nOUT \"b\"",
            &[],
            &["a"],
            "Unexpected token Plus in statement",
        );
    }

    #[test]
    fn test_exec_shares_state() {
        let mut machine = Machine::default();
        assert_eq!(
            StopReason::Eof,
            block_on(machine.exec(&mut b"a = 10".as_ref())).expect("Execution failed")
        );
        assert_eq!(
            StopReason::Eof,
            block_on(machine.exec(&mut b"b = a".as_ref())).expect("Execution failed")
        );
    }
}