const STACK_SIZE: usize = 1024;
pub const MEMORY_SIZE: usize = 1<<20;
pub const SCREEN_WIDTH: usize = 512;
pub const SCREEN_HEIGHT: usize = 512;
pub const SCREEN_UPDATE_ADDRESS: usize = 0x7FFFF

#[derive(Debug)]
pub struct Stack {
    buffer: [u16;STACK_SIZE],
    index: usize
}

impl Stack {
    fn push(&mut self, value: u16) {
        self.buffer[self.index] = value;
        self.index = (self.index + 1) % STACK_SIZE;
    }

    fn pop(&mut self) -> u16 {
        self.index = (self.index + STACK_SIZE - 1) % STACK_SIZE;
        self.buffer[self.index]
    }
}

pub fn new_stack() -> Stack {
    Stack{
        buffer: [0u16;STACK_SIZE],
        index: 0
    }
}

pub struct Processor {
    pub stack_a: Stack,
    pub stack_b: Stack,
    pub memory: [u16;MEMORY_SIZE],
    pub carry: bool,
    pub pc: usize
}

impl Processor {
    pub fn run(&mut self) -> bool {
        let instruction = self.memory[self.pc] & 0x3FF;
        let (mut stack, off_stack)  = {
            if (instruction & 0x200) == 0 {
                (&mut self.stack_a, &mut self.stack_b)
            }
            else {(&mut self.stack_b, &mut self.stack_a)}
        };
        let mut is_jump = false;

        if instruction & 0x100 == 0 { //calculation
            self.carry = calc(&mut stack, instruction, self.carry);
        }
        else if instruction & 0x80 == 0 { //jump
            let push_pc = (instruction & 0x40) != 0;
            let conditional = (instruction & 0x20) != 0;
            if !conditional || self.carry {
                let address = (stack.pop() as usize) | ((stack.pop() as usize) << 10);
                is_jump = true;
                if push_pc {
                    let a = self.pc + 1;
                    let upper = a & 0b11111111110000000000 >> 10;
                    let lower = a & 0b1111111111;
                    stack.push(upper as u16);
                    stack.push(lower as u16);
                }
                self.pc = address;
            }
        }
        else {
            match instruction & 0x7F {
                0 => { //immediate value
                    self.pc += 1;
                    stack.push(self.memory[self.pc] & 0x3FF);
                }
                1 => {
                    let upper = self.pc & 0b11111111110000000000 >> 10;
                    let lower = self.pc & 0b1111111111;
                    stack.push(upper as u16);
                    stack.push(lower as u16);
                }
                2 => { //swap
                    let a = stack.pop();
                    let b = stack.pop();
                    stack.push(a);
                    stack.push(b);
                }
                3 => off_stack.push(stack.pop()), //move between stacks
                4 => { //push from memory
                    let address = (stack.pop() as usize) | ((stack.pop() as usize) << 10);
                    stack.push(self.memory[address]);
                }
                5 => { //pop to memory
                    let address = (stack.pop() as usize) | ((stack.pop() as usize) << 10);
                    self.memory[address] = stack.pop();
                }
                6 => { //dup
                    let a = stack.pop();
                    stack.push(a);
                    stack.push(a);
                }
                7 => { //over
                    let a = stack.pop();
                    let b = stack.pop();
                    stack.push(b);
                    stack.push(a);
                    stack.push(b);
                }
                0x7F => return false,
                _ => ()
            }
        }
        if !is_jump {self.pc += 1};
        true
    }
}

fn calc(stack: &mut Stack, instruction: u16, carry_in: bool) -> bool {
    let operand_a = stack.pop();
    let operand_b = stack.pop();
    let mut carry = carry_in;
    let mut result = 0;
    for i in 0..10 {
        let count = ((operand_a >> i) & 1) + ((operand_b >> i) & 1) + (carry as u16);
        result += ((instruction >> (count+4)) & 1) << i;
        carry = ((instruction >> count) & 1) != 0;
    }
    stack.push(result);
    carry
}

pub fn mem_to_buf(memory: &[u16]) -> [u32;SCREEN_WIDTH*SCREEN_HEIGHT] {
    let mut result = [0u32;SCREEN_WIDTH*SCREEN_HEIGHT];
    let mut i = 0;
    for byte in &memory[(MEMORY_SIZE-SCREEN_WIDTH*SCREEN_HEIGHT)..MEMORY_SIZE] {
        result[i] = (((byte & 0b111000000) as u32) << 15) | (((byte & 0b111000) as u32) << 10) | (((byte & 0b111) as u32) << 5); //shifts 3 bits of each 10 bit value into the rgb channels of the buffer, from the last region of memory
        i += 1;
    }
    result
}