use std::collections::HashMap;
use std::ops::Index;

#[derive(Clone, Debug, PartialEq)]
pub enum OpCode {
    Num(i32),
    Str(usize, usize),
    Call(usize),
    TCall(usize), // tail call, really just ‘jmp’, but named to indicate desired usage.
    If(usize, Option<usize>), // true word index, optional false word index.
    TIf(usize, Option<usize>), // tail version
    Add,
    Sub,
    // todo: maybe get rid of ret? it's only used in word definitions,
    // which are bounded, so running of the end can be treated as an
    // implicit ‘ret’.
    Ret,
}

#[derive(Debug)]
pub(super) struct ByteCode(pub(super) Vec<OpCode>);

impl ByteCode {
    pub fn len(&self) -> usize {
        self.0.len()
    }
}

impl Index<usize> for ByteCode {
    type Output = OpCode;

    fn index(&self, index: usize) -> &Self::Output {
        &self.0[index]
    }
}

#[derive(Debug, Copy, Clone, PartialEq)]
pub(super) struct InstructionPointer {
    pub(super) word: usize,
    pub(super) offset: usize,
}

impl InstructionPointer {
    pub fn new() -> Self {
        Self {
            word: 0,
            offset: 0,
        }
    }
}

#[derive(Debug)]
pub(super) struct DataStack(pub(super) Vec<i32>);

#[derive(Debug)]
pub(super) struct CallStack(pub(super) Vec<InstructionPointer>);

#[derive(Debug)]
pub(super) struct WordList(pub(super) Vec<ByteCode>);

#[derive(Debug)]
pub(super) struct WordCatalog<'a>(pub(super) HashMap<&'a str, usize>);

#[derive(Debug)]
pub struct Interp {
    stack: DataStack,
    callstack: CallStack,
    wordlist: WordList,
    ip: InstructionPointer,
}

#[derive(Debug)]
pub enum RuntimeError {
    StackUnderflow,
}

impl Interp {
    pub fn new() -> Self {
        Self {
            stack: DataStack(Vec::new()),
            callstack: CallStack(Vec::new()),
            wordlist: WordList(vec![]),
            ip: InstructionPointer::new(),
        }
    }

    pub fn jmp(&mut self, word: usize) {
        self.ip.word = word;
        self.ip.offset = 0;
    }

    pub fn call(&mut self, word: usize) {
        self.callstack.0.push(self.ip);
        self.jmp(word);
    }

    pub fn tick(&mut self) -> Result<(), RuntimeError> {
        let bc = &self.wordlist.0[self.ip.word];
        eprintln!("debug: executing {:?}", bc[self.ip.offset]);
        match bc[self.ip.offset] {
            OpCode::Num(n) => self.stack.0.push(n),
            OpCode::Str(start, end) => eprintln!("got str: {} to {}", start, end),
            OpCode::Add => {
                let n1 = self.stack.0.pop().ok_or(RuntimeError::StackUnderflow)?;
                let n2 = self.stack.0.pop().ok_or(RuntimeError::StackUnderflow)?;
                self.stack.0.push(n2 + n1);
            },
            OpCode::Sub => {
                let n1 = self.stack.0.pop().ok_or(RuntimeError::StackUnderflow)?;
                let n2 = self.stack.0.pop().ok_or(RuntimeError::StackUnderflow)?;
                self.stack.0.push(n2 - n1);
            },
            OpCode::Ret => {
                self.ip = self.callstack.0.pop().ok_or(RuntimeError::StackUnderflow)?;
            },
            OpCode::Call(i) => {
                eprintln!("should call word based on index {}", i);
                self.call(i);
                // skip the offset increment
                return Ok(())
            },
            OpCode::TCall(i) => {
                eprintln!("should jump to word based on index {}", i);
                self.jmp(i);
                // skip the offset increment
                return Ok(())
            },
            OpCode::If(true_clause, false_clause) => {
                let flag = self.stack.0.pop().ok_or(RuntimeError::StackUnderflow)?;
                if flag != 0 {
                    eprintln!("should call true branch {}", true_clause);
                    self.call(true_clause);
                    return Ok(())
                } else if let Some(false_clause) = false_clause {
                    eprintln!("should call false branch {}", false_clause);
                    self.call(false_clause);
                    return Ok(())
                } else {
                    eprintln!("should false fall through");
                }
            },
            OpCode::TIf(true_clause, false_clause) => {
                let flag = self.stack.0.pop().ok_or(RuntimeError::StackUnderflow)?;
                if flag != 0 {
                    eprintln!("should call true branch {}", true_clause);
                    self.jmp(true_clause);
                    return Ok(())
                } else if let Some(false_clause) = false_clause {
                    eprintln!("should call false branch {}", false_clause);
                    self.jmp(false_clause);
                    return Ok(())
                } else {
                    eprintln!("should false fall through");
                }
            },
        }
        self.ip.offset += 1;
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use super::OpCode;

    #[test]
    fn simple_ticks() -> Result<(), RuntimeError> {

        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(2), OpCode::Num(3), OpCode::Add]));
        interp.tick()?;
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 1);
        assert_eq!(interp.stack.0.len(), 1);
        assert_eq!(interp.stack.0[0], 2, "first argument");
        interp.tick()?;
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 2);
        assert_eq!(interp.stack.0.len(), 2);
        assert_eq!(interp.stack.0[1], 3, "second argument");
        interp.tick()?;
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 3);
        assert_eq!(interp.stack.0.len(), 1);
        assert_eq!(interp.stack.0[0], 5, "result of addition");

        Ok(())
    }

    #[test]
    fn custom_word() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![
            OpCode::Num(2), OpCode::Num(3), OpCode::Call(1), OpCode::Num(-2), OpCode::Add,
            OpCode::Sub, OpCode::Ret,
        ]));
        // "sub" definition
        interp.wordlist.0.push(ByteCode(vec![
            OpCode::Sub, OpCode::Ret,
        ]));

        interp.tick()?;
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 1);
        assert_eq!(interp.stack.0.len(), 1);
        assert_eq!(interp.stack.0[0], 2, "first argument");
        interp.tick()?;
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 2);
        assert_eq!(interp.stack.0.len(), 2);
        assert_eq!(interp.stack.0[1], 3, "second argument");

        interp.tick()?; // call sub
        assert_eq!(interp.ip.word, 1);
        assert_eq!(interp.ip.offset, 0);
        assert_eq!(interp.stack.0.len(), 2);

        interp.tick()?; // -
        assert_eq!(interp.ip.word, 1);
        assert_eq!(interp.ip.offset, 1);
        assert_eq!(interp.stack.0.len(), 1);

        interp.tick()?; // ret
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 3);
        assert_eq!(interp.stack.0.len(), 1);
        assert_eq!(interp.stack.0[0], -1, "result of sub word");

        interp.tick()?; // 2
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 4);
        assert_eq!(interp.stack.0.len(), 2);
        assert_eq!(interp.stack.0[1], -2, "post sub arg");

        interp.tick()?; // -
        assert_eq!(interp.ip.word, 0);
        assert_eq!(interp.ip.offset, 5);
        assert_eq!(interp.stack.0.len(), 1);
        assert_eq!(interp.stack.0[0], -3, "add opcode result");

        Ok(())
    }

    #[test]
    fn tail_call() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Call(1)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::TCall(2), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Ret]));


        interp.tick()?; // call
        assert_eq!(interp.callstack.0.len(), 1, "callstack after call");
        interp.tick()?; // tcall
        assert_eq!(interp.callstack.0.len(), 1, "callstack after tcall");
        interp.tick()?; // ret
        assert_eq!(interp.callstack.0.len(), 0, "callstack after ret");
        Ok(())
    }

    #[test]
    fn if_then_true() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(-1), OpCode::If(1, None), OpCode::Num(0)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push -1 stack len");
        assert_eq!(interp.stack.0[0], -1, "push -1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "if stack ");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 1 stack len");
        assert_eq!(interp.stack.0[0], 1, "push 1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "ret stack len");
        assert_eq!(interp.stack.0[0], 1, "ret stack val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 2, "push 0 stack len");
        assert_eq!(interp.stack.0[1], 0, "push 0 stack val");

        Ok(())
    }

    #[test]
    fn if_then_false() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(0), OpCode::If(1, None), OpCode::Num(-1)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 0 stack len");
        assert_eq!(interp.stack.0[0], 0, "push 0 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "if stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push -1 stack len");
        assert_eq!(interp.stack.0[0], -1, "push -1 val");

        Ok(())
    }

    #[test]
    fn if_else_then_true() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(-1), OpCode::If(1, Some(2)), OpCode::Num(-2)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(2), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push -1 stack len");
        assert_eq!(interp.stack.0[0], -1, "push -1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "if stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 1 stack len");
        assert_eq!(interp.stack.0[0], 1, "push 1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "ret stack len");
        assert_eq!(interp.stack.0[0], 1, "ret val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 2, "push -2 stack len");
        assert_eq!(interp.stack.0[1], -2, "push -2 val");

        Ok(())
    }

    #[test]
    fn if_else_then_false() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(0), OpCode::If(1, Some(2)), OpCode::Num(-2)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(2), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 0 stack len");
        assert_eq!(interp.stack.0[0], 0, "push 0 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "if");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 2 stack len");
        assert_eq!(interp.stack.0[0], 2, "push 2 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "ret");
        assert_eq!(interp.stack.0[0], 2, "ret");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 2, "push -2 stack len");
        assert_eq!(interp.stack.0[1], -2, "push -2 val");

        Ok(())
    }

    #[test]
    fn tail_if_then_true() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Call(1), OpCode::Num(0)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(-1), OpCode::TIf(2, None), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "call(1) stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push -1 stack len");
        assert_eq!(interp.stack.0[0], -1, "push -1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "tif stack ");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 1 stack len");
        assert_eq!(interp.stack.0[0], 1, "push 1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "ret stack len");
        assert_eq!(interp.stack.0[0], 1, "ret stack val");
        interp.tick()?; // no ret on wordlist[1] since it's ‘tif’
        assert_eq!(interp.stack.0.len(), 2, "push 0 stack len");
        assert_eq!(interp.stack.0[1], 0, "push 0 stack val");

        Ok(())
    }

    #[test]
    fn tail_if_then_false() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Call(1), OpCode::Num(0)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(0), OpCode::TIf(2, None), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "call(1) stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 0 stack len");
        assert_eq!(interp.stack.0[0], 0, "push 0 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "tif stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "ret stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 0 stack len");
        assert_eq!(interp.stack.0[0], 0, "push 0 val");

        Ok(())
    }

    #[test]
    fn tail_if_else_then_true() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Call(1), OpCode::Num(-2)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(-1), OpCode::TIf(2, Some(3)), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(2), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "call(1) stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push -1 stack len");
        assert_eq!(interp.stack.0[0], -1, "push -1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "tif stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 1 stack len");
        assert_eq!(interp.stack.0[0], 1, "push 1 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "ret stack len");
        assert_eq!(interp.stack.0[0], 1, "ret val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 2, "push -2 stack len");
        assert_eq!(interp.stack.0[1], -2, "push -2 val");

        Ok(())
    }

    #[test]
    fn tail_if_else_then_false() -> Result<(), RuntimeError> {
        let mut interp = Interp::new();
        interp.wordlist.0.push(ByteCode(vec![OpCode::Call(1), OpCode::Num(-2)]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(0), OpCode::TIf(2, Some(3)), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(1), OpCode::Ret]));
        interp.wordlist.0.push(ByteCode(vec![OpCode::Num(2), OpCode::Ret]));

        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "call(1) stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 0 stack len");
        assert_eq!(interp.stack.0[0], 0, "push 0 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 0, "tif stack len");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "push 2 stack len");
        assert_eq!(interp.stack.0[0], 2, "push 2 val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 1, "ret stack len");
        assert_eq!(interp.stack.0[0], 2, "ret val");
        interp.tick()?;
        assert_eq!(interp.stack.0.len(), 2, "push -2 stack len");
        assert_eq!(interp.stack.0[1], -2, "push -2 val");

        Ok(())
    }
}