const std = @import("std");
const mem = std.mem;
const fmt = std.fmt;
const debug = std.debug;


// read n bytes or return err
fn readExactly(reader: anytype, buffer: []u8) !void{
  var have: usize = 0;
  while(have < buffer.len){
    const amt = try reader.read(buffer[have..]);
    if(amt == 0){ return error.EndOfStream; }
    have += amt;
  }
}


pub fn main() !void{
  const stdin = std.io.getStdIn().reader();
  const stdout = std.io.getStdOut().writer();
  
  var buffer: [64]u8 = [_]u8{0} ** 64;
  var len_bytes_buf: [2]u8 = [_]u8{0} ** 2;
  var reply_buffer: [64]u8 = [_]u8{0} ** 64;
  
  while(true){
    // read msg length (2 bytes, big-endian)
    readExactly(stdin, &len_bytes_buf) catch |err|{
      if(err == error.EndOfStream){ break; }
      return err;
    };
    
    const msg_len = @as(u16, len_bytes_buf[0]) << 8 | len_bytes_buf[1]; // convert to u16, big-endian
    
    //std.debug.print("Full buffer: ", .{});
    //for(buffer[0..msg_len]) |b|{
    //  std.debug.print("{c}", .{b});
    //}
    //std.debug.print("\n", .{});
    
    if(msg_len == 0 or msg_len > buffer.len){
      try stdout.writeAll(&[2]u8{ 0, 0 });
      continue;
    }
    
    readExactly(stdin, buffer[0..msg_len]) catch |err|{ // read msg
      if(err == error.EndOfStream){ break; }
      return err;
    };
    //debug.print("RECV: '{s}'\n", .{buffer[0..msg_len]});
    
    var fbs = std.io.fixedBufferStream(&reply_buffer);
    var writer = fbs.writer();
    
    
    if(mem.eql(u8, buffer[0..msg_len], "hello")){
      try writer.writeAll("Hello world!");
    
    }else if(mem.startsWith(u8, buffer[0..msg_len], "factorial ")){
      const prefix_len = "factorial ".len;
      const n_str = buffer[prefix_len..msg_len];
      //std.debug.print("n_str = '{s}' ({} chars)\n", .{ n_str, n_str.len });
      
      const n = fmt.parseInt(isize, n_str, 10) catch {
        //std.debug.print("error:PARSE ERROR for '{s}'\n", .{n_str});
        try writer.writeAll("error:badarg");
        //try writer.writeAll("0");
        try sendResponse(stdout, fbs.getWritten());
        continue;
      };
      
      if(n < 0){
        //try writer.writeAll("error:badarg");
        //std.debug.print("n < 0 -> badarg\n", .{});
        //try writer.writeAll("0");
        try writer.writeAll("error:badarg: n < 0");
      }else if(n > 34){
        //std.debug.print("n > 34 -> too large\n", .{});
        //try writer.writeAll("0");
        try writer.writeAll("error:input too large, max is 34");
      }else{
        var result: u128 = 1;
        var i = @as(u128, @intCast(n));
        while (i > 1) : (i -= 1) result *= i;
        //std.debug.print("factorial({d}) = {d}\n", .{n, result});
        try fmt.formatInt(result, 10, .lower, .{}, writer);
      }
    
    }else{
      try writer.writeAll("error:unknown command");
    }
    
    try sendResponse(stdout, fbs.getWritten());
  }
}


fn sendResponse(writer: anytype, response: []const u8) !void{
  const len: u16 = @intCast(response.len);
  
  //std.debug.print("SEND: len={d} (0x{X:0>4}) data='{s}'\n", .{ len, len, response });
  
  try writer.writeByte(@intCast((len >> 8) & 0xFF)); // high byte
  try writer.writeByte(@intCast(len & 0xFF));        // low byte
  try writer.writeAll(response);
}