TinyChan

let mod=(n,m)=>(n%m+m)%m,
    oper={
  "+":(a,b)=>a+b,"-":(a,b)=>a-b,
  "*":(a,b)=>a*b,"/":(a,b)=>a/b,
  "%":mod,"**":(a,b)=>a**b
},fun={ //.length for N of arguments
  sin:Math.sin,rand:Math.random,
  abs:Math.abs,floor:Math.floor
},chooseRand=a=>a[Math.floor(Math.random()*a.length)];
/*
Examples of Reverse Polish Notation (RPN):
  t 220 * 1 % → (t*220)%1
  rand 1 t 1 % - 5 ** * → rand()*((1-(t%1))**5)
  .5 1 10 t * 2 % - abs - → .5-abs(1-((10*t)%2))
*/
function genExpr(){
  const ops=Object.keys(oper),
        unaries=Object.keys(fun).filter(k=>fun[k].length===1),
        terminals=["t",...Object.keys(fun).filter(k=>fun[k].length===0)];
  let stack=[],esd=0,maxLen=10; //eval stack depth
  while(esd<1||Math.random()>.2||stack.length<3){
    let options=["terminal"];
    if(esd>=1)options.push("unary");
    if(esd>=2)options.push("binary");
    const choice=chooseRand(options);
    switch(choice){
      case"terminal":
        if(Math.random()<.2){
          stack.push(chooseRand(terminals));
        }else{
          stack.push(floor(random()*21)*.5-5);
        }
        esd++;
      break;
      case"unary":
        stack.push(chooseRand(unaries));
      break;
      case"binary":
        stack.push(chooseRand(ops));
        esd--;
      break;
    }
    if(stack.length>=maxLen)break;
  }
  //ensure minimum size and balance
  while(esd!==1||stack.length<3){
    if(esd<1){
      stack.push(chooseRand([.5,1])); //push dummy operand
      esd++;
    }else if(esd>1){
      stack.push("+"); //try to reduce stack
      esd--;
    }else{break;}
  }
  return stack;
}
function evalExpr(stack){
  return function(tVal){
    let s=[];
    for(let token of stack){
      try{
        if(typeof token==="number"){
          s.push(token);
        }else if(token==="t"){
          s.push(tVal);
        }else if(oper.hasOwnProperty(token)){
          if(s.length<2)return 0; //not enough
          const b=s.pop(),a=s.pop(),
                result=oper[token](a,b);
          s.push(isFinite(result)?result:0);
        }else if(fun.hasOwnProperty(token)){
          if(fun[token].length===0){s.push(fun[token]());continue;} //e.g. rand
          if(!s.length)return 0; //no args to push
          const result=fun[token](s.pop()); //TODO: multiple args
          s.push(isFinite(result)?result:0);
        }else{return 0;}
      }catch(e){
        console.error("Eval error:",e,"at",token);
        return 0;
      }
    }
    const result=s.pop()||0;
    return isNaN(result)||!isFinite(result)?0:result;
  };
}
/*let timer={
  t:0,l:.1
},expr=genExpr(),eFun=evalExpr(expr);
return(t,SR)=>{
  timer.t+=1/SR;
  if(timer.t>=timer.l){
    timer.t%=timer.l;
    expr=genExpr();eFun=evalExpr(expr);
  }
  return eFun(t);
};*/
let split=str=>str.split(" ").map(e=>{let n=parseFloat(e);return isNaN(n)?e:n;}),
    eFun=[
  evalExpr(split(".5 1 440 t * 2 % - abs -")),
  evalExpr(split("rand .5 - 1 t 1 % - 5 ** *")),
  ...(new Array(5).fill(0).map(()=>evalExpr(genExpr())))
];
return t=>{
  return eFun[floor(t/2)%eFun.length](t);
};