//! An `Instance` contains all the runtime state used by execution of a wasm
//! module.

use cretonne::ir;
use cton_wasm::GlobalIndex;
use module::Module;

const PAGE_SIZE: usize = 65536;

/// An Instance of a WebAssemby module.
#[derive(Debug)]
pub struct Instance {
    /// WebAssembly table data.
    pub tables: Vec<Vec<usize>>,

    /// WebAssembly linear memory data.
    pub memories: Vec<Vec<u8>>,

    /// WebAssembly global variable data.
    pub globals: Vec<u8>,
}

impl Instance {
    /// Create a new `Instance`.
    pub fn new(module: &Module) -> Self {
        let mut result = Self {
            tables: Vec::new(),
            memories: Vec::new(),
            globals: Vec::new(),
        };
        result.instantiate_tables(module);
        result.instantiate_memories(module);
        result.instantiate_globals(module);
        result
    }

    /// Allocate memory in `self` for just the tables of the current module,
    /// without any initializers applied yet.
    fn instantiate_tables(&mut self, module: &Module) {
        debug_assert!(self.tables.is_empty());
        self.tables.reserve(module.tables.len());
        for table in &module.tables {
            let len = table.size;
            let mut v = Vec::with_capacity(len);
            v.resize(len, 0);
            self.tables.push(v);
        }
    }

    /// Allocate memory in `instance` for just the memories of the current module,
    /// without any initializers applied yet.
    fn instantiate_memories(&mut self, module: &Module) {
        debug_assert!(self.memories.is_empty());
        // Allocate the underlying memory and initialize it to all zeros.
        self.memories.reserve(module.memories.len());
        for memory in &module.memories {
            let len = memory.pages_count * PAGE_SIZE;
            let mut v = Vec::with_capacity(len);
            v.resize(len, 0);
            self.memories.push(v);
        }
    }

    /// Allocate memory in `instance` for just the globals of the current module,
    /// without any initializers applied yet.
    fn instantiate_globals(&mut self, module: &Module) {
        debug_assert!(self.globals.is_empty());
        // Allocate the underlying memory and initialize it to all zeros.
        let globals_data_size = module.globals.len() * 8;
        self.globals.resize(globals_data_size, 0);
    }

    /// Returns a slice of the contents of allocated linear memory.
    pub fn inspect_memory(&self, memory_index: usize, address: usize, len: usize) -> &[u8] {
        &self.memories.get(memory_index).expect(
            format!(
                "no memory for index {}",
                memory_index
            ).as_str(),
        )
            [address..address + len]
    }

    /// Shows the value of a global variable.
    pub fn inspect_global(&self, global_index: GlobalIndex, ty: ir::Type) -> &[u8] {
        let offset = global_index * 8;
        let len = ty.bytes() as usize;
        &self.globals[offset..offset + len]
    }
}