// god is dead and we have killed him and consequently std::time::Instant doesn't work on wasm

use chrono::prelude::*;
use wasm_bindgen::prelude::*;

struct Reading {
    time: DateTime<Local>,
    queue_size: u32,
}

#[derive(Default)]
pub struct History {
    data: Vec<Reading>,
}

pub enum NoETA {
    Pending,
    Fuck,
}

impl History {
    pub fn record(&mut self, queue_size: u32) {
        self.data.push(Reading {
            time: Local::now(),
            queue_size,
        });
    }

    pub fn completion_time(&self) -> Result<js_sys::Date, NoETA> {
        // TODO make this not suck
        let &Reading {
            time: first_time,
            queue_size: first_size,
            ..
        } = self.data.first().ok_or(NoETA::Pending)?;
        let &Reading {
            time: last_time,
            queue_size: last_size,
        } = self.data.last().ok_or(NoETA::Pending)?;
        let overall_time_elapsed = last_time - first_time;
        if overall_time_elapsed.is_zero() {
            return Err(NoETA::Pending);
        }
        let overall_queue_motion = match first_size.checked_sub(last_size) {
            Some(0) => return Err(NoETA::Pending),
            Some(x) => x,
            None => return Err(NoETA::Fuck),
        };
        let duration_per_step = overall_time_elapsed / overall_queue_motion as i32;
        let remaining_duration = duration_per_step * last_size as i32;
        let completion_time = last_time + remaining_duration;
        let completion_time_gmt = completion_time.timestamp_millis();
        Ok(js_sys::Date::new(&JsValue::from(
            completion_time_gmt as f64,
        )))
    }
}