const PAGE_SIZE: usize = 4 * 1024;
const PAGE_MASK: usize = !(PAGE_SIZE - 1);

struct Page {
    virtual_addr: usize,
    real_addr: usize,
    permission: usize,
    allocated: usize,
}
pub struct Memory {
    memory: Box<[u8]>,
    pages: Vec<Page>,
}
impl Memory {
    pub fn new(size: usize) -> Memory {
        let mut pages: Vec<Page> = Vec::with_capacity(size / PAGE_SIZE);
        for i in 0..size / PAGE_SIZE {
            pages[i] = Page {
                virtual_addr: usize::MAX,
                real_addr: i * PAGE_SIZE,
                permission: 0,
                allocated: 0
            }
        }
        Memory {
            memory: vec![0u8; size].into_boxed_slice(),
            pages,
        }
    }

    pub fn create_page(&mut self, virtual_addr: usize, permission: usize) -> Option<usize> {
        // Find first page that isn't allocated and set it to allocated
        for (i, page) in self.pages.iter_mut().enumerate() {
            if page.allocated == 0 {
                page.virtual_addr = virtual_addr;
                page.permission = permission;
                page.allocated = 1;
                return Some(i);
            }
        }
        None
    }

    pub fn lookup_page(&mut self, virtual_addr: usize, permission: usize) -> Option<usize> {
        for (i, page) in self.pages.iter().enumerate() {
            if page.virtual_addr == virtual_addr && permission >= page.permission && page.allocated == 1 {
                return Some(i);
            }
        }
        return self.create_page(virtual_addr, permission);
    }

    pub fn ro_lookup(&self, virtual_addr: usize, permission: usize) -> Option<usize> {
        for (i, page) in self.pages.iter().enumerate() {
            if page.virtual_addr == virtual_addr && page.permission == permission && page.allocated == 1 {
                return Some(i);
            }
        }
        None
    }

    pub fn write(&mut self, address: usize, permission: usize, value: u8) -> Result<(), ()> {
        let page = address & PAGE_MASK;
        let offset = address - page;

        if let Some(page) = self.lookup_page(address, permission) {
            self.memory[self.pages[page].real_addr + offset] = value;
            Ok(())
        } else {
            Err(())
        }
    }

    pub fn read(&self, address: usize, permission: usize) -> Result<u8, ()> {
        let page = address & PAGE_MASK;
        let offset = address - page;

        if let Some(page) = self.ro_lookup(address, permission) {
            Ok(self.memory[self.pages[page].real_addr + offset])
        } else {
            Err(())
        }
    }
    
    pub fn read_multiple_bytes(&self, address: usize, size: usize, permission: usize) -> Result<Box<[u8]>, ()> {
        let mut ret = [];
        for addr in  address..address+size {
            if let Ok(byte) = self.read(addr, permission) {
                ret[addr - address] = byte;
            } else {
                return Err(());
            }
        }
        Ok(Box::new(ret))
    }
    
    pub fn write_multiple_bytes(&mut self, address: usize, bytes: &[u8], permission: usize) -> Result<(), ()> {
        for addr in address..address + bytes.len() {
            if let Err(_) = self.write(addr, permission, bytes[addr - address]) {
                return Err(());
            } 
        }
        Ok(())
    }
}