main.rs

use anyhow::Result;
use wasmtime::*;
use std::ptr::copy;

fn main() -> Result<()> {
    // Create our `Store` context and then compile a module and create an
    // instance from the compiled module all in one go.
    let wasmtime_store = Store::default();
    // Load Wasm module
    let module = Module::from_file(wasmtime_store.engine(), "../modules/memory.wasm")?;
    let instance = Instance::new(&wasmtime_store, &module, &[])?;
    // Expose alloc function from Wasm module
    let alloc = instance
        .get_func("alloc")
        .ok_or(anyhow::format_err!("failed to find `alloc` export"))?
        .get1::<i32, i32>()?;

    // Get linear memory
    let memory = instance
        .get_memory("memory")
        .ok_or(anyhow::format_err!("failed to find `memory` export"))?;

    // Input string
    let text = String::from("The input string");
    let size = text.len() as i32;

    // Allocate memory and visualize it.
    let mem_ptr = alloc(size+30)?;
    println!("Pointer received {:#x}, {}", mem_ptr, size);
    println!("Host memory pointer {:#x?}", memory.data_ptr());
    println!("wasm allocated memory {:#x?}", 
    unsafe{ memory.data_unchecked_mut().as_ptr() });
    let pointer = unsafe { memory.data_ptr().add(mem_ptr as usize) };
    println!("address for wasm object in rust: {:#x?}", pointer);
    // Copy input data in linear memory-
    unsafe {
        let bytes = text.as_bytes();
        copy(bytes.as_ptr(), 
        pointer, 
        bytes.len());
    }

    // Expose append function from wasm module
    let append = instance
    .get_func("append")
    .ok_or(anyhow::format_err!("failed to find `append` export"))?
    .get2::<i32, i32, i32>()?;
    // Execute function
    let new_size = append(mem_ptr, size)?;
    println!("New Size received: {}", new_size);
    // Get output data
    let byte3 = unsafe { 
        String::from_utf8(memory.data_unchecked()[mem_ptr as usize..][..new_size as usize]
        .to_vec()).unwrap()
    };
    println!("Result: {}", byte3);

    Ok(())
}