const std = @import("std");

const Bus = @import("Bus.zig");
const Scheduler = @import("scheduler.zig").Scheduler;
const Arm7tdmi = @import("cpu.zig").Arm7tdmi;

const Timer = std.time.Timer;
const Thread = std.Thread;
const Atomic = std.atomic.Atomic;

// One frame operates at 59.7275005696Hz
const cycles_per_frame: u64 = 228 * (308 * 4);
const clock_rate: u64 = 1 << 24;
const clock_period: u64 = std.time.ns_per_s / clock_rate;
const frame_period = (clock_period * cycles_per_frame);

const log = std.log.scoped(.Emulation);

const RunKind = enum {
    Unlimited,
    UnlimitedFPS,
    Limited,
    LimitedFPS,
    LimitedBusy,
};

pub fn run(kind: RunKind, quit: *Atomic(bool), pause: *Atomic(bool), fps: *Atomic(u64), sched: *Scheduler, cpu: *Arm7tdmi, bus: *Bus) void {
    switch (kind) {
        .Unlimited => runUnSync(quit, pause, sched, cpu, bus),
        .Limited => runSync(quit, pause, sched, cpu, bus),
        .UnlimitedFPS => runUnSyncFps(quit, pause, fps, sched, cpu, bus),
        .LimitedFPS => runSyncFps(quit, pause, fps, sched, cpu, bus),
        .LimitedBusy => runBusyLoop(quit, pause, sched, cpu, bus),
    }
}

pub fn runFrame(sched: *Scheduler, cpu: *Arm7tdmi, bus: *Bus) void {
    var cycles: u64 = 0;
    while (cycles < cycles_per_frame) : (cycles += 1) {
        sched.tick += 1;
        _ = cpu.step();

        while (sched.tick >= sched.nextTimestamp()) {
            sched.handleEvent(cpu, bus);
        }
    }
}

pub fn runUnSync(quit: *Atomic(bool), pause: *Atomic(bool), sched: *Scheduler, cpu: *Arm7tdmi, bus: *Bus) void {
    log.info("Unsynchronized EmuThread has begun", .{});
    while (!quit.load(.Unordered)) if (!pause.load(.Unordered)) runFrame(sched, cpu, bus);
}

pub fn runSync(quit: *Atomic(bool), pause: *Atomic(bool), sched: *Scheduler, cpu: *Arm7tdmi, bus: *Bus) void {
    log.info("Synchronized EmuThread has begun", .{});
    var timer = Timer.start() catch unreachable;
    var wake_time: u64 = frame_period;

    while (!quit.load(.Unordered)) if (!pause.load(.Unordered)) {
        runFrame(sched, cpu, bus);

        // Put the Thread to Sleep + Backup Spin Loop
        // This saves on resource usage when frame limiting
        sleep(&timer, &wake_time);

        // Update to the new wake time
        wake_time += frame_period;
    };
}

pub fn runUnSyncFps(quit: *Atomic(bool), pause: *Atomic(bool), fps: *Atomic(u64), sched: *Scheduler, cpu: *Arm7tdmi, bus: *Bus) void {
    log.info("Unsynchronized EmuThread with FPS Tracking has begun", .{});
    var fps_timer = Timer.start() catch unreachable;

    while (!quit.load(.Unordered)) if (!pause.load(.Unordered)) {
        runFrame(sched, cpu, bus);
        fps.store(emuFps(fps_timer.lap()), .Unordered);
    };
}

pub fn runSyncFps(quit: *Atomic(bool), pause: *Atomic(bool), fps: *Atomic(u64), sched: *Scheduler, cpu: *Arm7tdmi, bus: *Bus) void {
    log.info("Synchronized EmuThread has begun", .{});
    var timer = Timer.start() catch unreachable;
    var fps_timer = Timer.start() catch unreachable;
    var wake_time: u64 = frame_period;

    while (!quit.load(.Unordered)) if (!pause.load(.Unordered)) {
        runFrame(sched, cpu, bus);

        // Put the Thread to Sleep + Backup Spin Loop
        // This saves on resource usage when frame limiting
        sleep(&timer, &wake_time);

        // Determine FPS
        fps.store(emuFps(fps_timer.lap()), .Unordered);

        // Update to the new wake time
        wake_time += frame_period;
    };
}

pub fn runBusyLoop(quit: *Atomic(bool), pause: *Atomic(bool), sched: *Scheduler, cpu: *Arm7tdmi, bus: *Bus) void {
    log.info("Run EmuThread with spin-loop sync", .{});
    var timer = Timer.start() catch unreachable;
    var wake_time: u64 = frame_period;

    while (!quit.load(.Unordered)) if (!pause.load(.Unordered)) {
        runFrame(sched, cpu, bus);
        spinLoop(&timer, wake_time);

        // Update to the new wake time
        wake_time += frame_period;
    };
}

fn sleep(timer: *Timer, wake_time: *u64) void {
    const step = std.time.ns_per_ms * 10;
    const timestamp = timer.read();

    // ns_late is non zero if we are late.
    const ns_late = timestamp -| wake_time.*;

    // If we're more than a frame late, skip the rest of this loop
    // Recalculate what our new wake time should be so that we can
    // get "back on track"
    if (ns_late > frame_period) {
        wake_time.* = timestamp + frame_period;
        return;
    }

    const sleep_for = frame_period - ns_late;

    // Employ several sleep calls in periods of 10ms
    // By doing this the behaviour should average out to be
    // more consistent
    const loop_count = sleep_for / step; // How many groups of 10ms

    var i: usize = 0;
    while (i < loop_count) : (i += 1) std.time.sleep(step);

    // Spin to make up the difference if there is a need
    // Make sure that we're using the old wake time and not the onne we recalculated
    spinLoop(timer, wake_time.*);
}

fn spinLoop(timer: *Timer, wake_time: u64) void {
    while (true) if (timer.read() > wake_time) break;
}

inline fn emuFps(left: u64) u64 {
    @setRuntimeSafety(false);
    return @floatToInt(u64, @intToFloat(f64, std.time.ns_per_s) / @intToFloat(f64, left));
}