Factorial

window.onload = function() {
    window.exports = {};

    let n = document.querySelector('#n'),
        f = document.querySelector('#f'),
        fact_str;

    let calc = function() {
        f.value = fact_str(n.value);
    };
    
    n.oninput = calc;

    fetchAndInstantiate("build/main.wasm")
    .then(mod => {
        exports = mod.exports;
        fact_str = function(n) {
            let outptr = exports.fact_str(n);
            let result = copyCStr(exports, outptr);
            return result;
        };
    });
};

use std::mem;
use std::ffi::CString;
use std::os::raw::{c_char, c_void};

fn main() {}

#[no_mangle]
pub extern "C" fn alloc(size: usize) -> *mut c_void {
    let mut buf = Vec::with_capacity(size);
    let ptr = buf.as_mut_ptr();
    mem::forget(buf);
    return ptr as *mut c_void;
}

#[no_mangle]
pub extern "C" fn dealloc_str(ptr: *mut c_char) {
    unsafe {
        let _ = CString::from_raw(ptr);
    }
}

#[no_mangle]
pub fn fact(n: u32) -> u64 {
    let mut n = n as u64;
    let mut result = 1;
    while n > 0 {
        result = result * n;
        n = n - 1;
    }
    result
}

#[no_mangle]
pub fn fact_str(n: u32) -> *mut c_char {
    let res = fact(n);
    let s = format!("{}", res);
    let s = CString::new(s).unwrap();
    s.into_raw()
}