# Copyright 2022 Big Vision Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Training loop for distillation as in https://arxiv.org/abs/2106.05237.

It works by having a (set of) teacher model(s) defined the same way as student
in the config and, for now, only distilling logits with one of many loss
functions.

We explored distilling intermediate feature maps, extra data, and other tricks
in depth in two interships in a separate prototype codebase but eventually they
are not necessary, and thus not (yet?) implemented in this codebase.

Thus, for now, there are no extra learnable parameters besides the student.
This keeps code relatively simple.
"""
# pylint: disable=consider-using-from-import
import functools
import importlib
import multiprocessing.pool
import os

from absl import app
from absl import flags
from absl import logging
import big_vision.evaluators.common as eval_common
import big_vision.evaluators.proj.distill.distance as dd
import big_vision.input_pipeline as input_pipeline
import big_vision.optax as bv_optax
import big_vision.utils as u
from clu import parameter_overview
import flax
import jax
import jax.numpy as jnp
from ml_collections import config_flags
import numpy as np
import optax
import tensorflow as tf

from tensorflow.io import gfile

# pylint: disable=logging-fstring-interpolation


config_flags.DEFINE_config_file(
    "config", None, "Training configuration.", lock_config=True)

flags.DEFINE_string("workdir", default=None, help="Work unit directory.")
flags.DEFINE_boolean("cleanup", default=False,
                     help="Delete workdir (only) after successful completion.")

# Adds jax flags to the program.
jax.config.parse_flags_with_absl()


def getfirst(d, *keys):
  """Returns the first of `keys` that's present in mapping `d`."""
  result, found = None, False
  for k in reversed(keys):
    if k in d:
      result, found = d[k], True
  if found:
    return result
  else:
    raise KeyError(f"None of {keys} is in {d.keys()}")


def main(argv):
  del argv
  tf.config.experimental.set_visible_devices([], "GPU")

  config = flags.FLAGS.config
  workdir = flags.FLAGS.workdir
  logging.info(
      f"\u001b[33mHello from process {jax.process_index()} holding "
      f"{jax.local_device_count()}/{jax.device_count()} devices and "
      f"writing to workdir {workdir}.\u001b[0m")

  save_ckpt_path = None
  if workdir:  # Always create if requested, even if we may not write into it.
    gfile.makedirs(workdir)
    save_ckpt_path = os.path.join(workdir, "checkpoint.npz")

  # The pool is used to perform misc operations such as logging in async way.
  pool = multiprocessing.pool.ThreadPool()

  # Here we register preprocessing ops from modules listed on `pp_modules`.
  for m in config.get("pp_modules", ["ops_general", "ops_image", "ops_text"]):
    importlib.import_module(f"big_vision.pp.{m}")

  # This seed makes the Jax part of things (like model init) deterministic.
  # However, full training still won't be deterministic, for example due to the
  # tf.data pipeline not being deterministic even if we would set TF seed.
  # See (internal link) for a fun read on what it takes.
  rng = jax.random.PRNGKey(config.get("seed", 0))

  # These functions do more stuff internally, for OSS release we mock them by
  # trivial alternatives in order to minize disruptions in the code.
  xid, wid = -1, -1
  fillin = lambda s: s
  def info(s, *a):
    logging.info("\u001b[33mNOTE\u001b[0m: " + s, *a)
  def write_note(note):
    if jax.process_index() == 0:
      info("%s", note)

  write_note("Initializing...")

  batch_size = config.input.batch_size
  if batch_size % jax.device_count() != 0:
    raise ValueError(f"Batch size ({batch_size}) must "
                     f"be divisible by device number ({jax.device_count()})")
  info("Global batch size %d on %d hosts results in %d local batch size. With "
       "%d dev per host (%d dev total), that's a %d per-device batch size.",
       batch_size, jax.process_count(), batch_size // jax.process_count(),
       jax.local_device_count(), jax.device_count(),
       batch_size // jax.device_count())

  # First thing after above sanity checks, so we can log "start" ticks.
  mw = u.BigVisionMetricWriter(xid, wid, workdir, config)

  write_note("Initializing train dataset...")
  train_ds, ntrain_img = input_pipeline.training(config.input)

  # Start prefetching already.
  n_prefetch = config.get("prefetch_to_device", 1)
  train_iter = input_pipeline.start_input_pipeline(train_ds, n_prefetch)

  total_steps = u.steps("total", config, ntrain_img, batch_size)
  def get_steps(name, default=ValueError, cfg=config):
    return u.steps(name, cfg, ntrain_img, batch_size, total_steps, default)

  u.chrono.inform(total_steps=total_steps, global_bs=batch_size,
                  steps_per_epoch=ntrain_img / batch_size,
                  measure=mw.measure, write_note=write_note)

  info("Running for %d steps, that means %f epochs",
       total_steps, total_steps * batch_size / ntrain_img)

  # Create student and teacher models
  def get_model_mod(name):  # Used many times.
    mod_name = config[f"{name}_name"]
    return importlib.import_module(f"big_vision.models.{mod_name}")

  write_note("Initializing models...")
  def make_model(name):
    return get_model_mod(name).Model(
        num_classes=config.num_classes, **config.get(name, {}))

  models = {
      "student": make_model("student"),
      **{t: make_model(t) for t in config.teachers}
  }

  # We want all parameters to be created in host RAM, not on any device, they'll
  # be sent there later as needed, otherwise we already encountered two
  # situations where we allocate them twice.
  def get_init(model, name):
    @functools.partial(jax.jit, backend="cpu")
    def _init(rng):
      bs = batch_size // jax.device_count()
      img_size = tuple(getfirst(train_ds.element_spec, name, "image").shape[1:])
      no_image = jnp.zeros((bs,) + img_size, jnp.float32)
      params = flax.core.unfreeze(model.init(rng, no_image))["params"]

      # Set bias in the head to a low value, such that loss is small initially.
      if "init_head_bias" in config:
        params["head"]["bias"] = jnp.full_like(params["head"]["bias"],
                                               config["init_head_bias"])
      return params
    return _init

  rng, *rng_inits = jax.random.split(rng, len(models) + 1)
  with u.chrono.log_timing("z/secs/init"):
    params_cpu = {
        name: get_init(models[name], name=name)(r)
        for name, r in zip(models, rng_inits)}

  if jax.process_index() == 0:
    for name, params in params_cpu.items():
      parameter_overview.log_parameter_overview(params, msg=f"{name} params")
      mw.measure(f"num_params_{name}",
                 sum(p.size for p in jax.tree_leaves(params)))

  write_note(f"Initializing {config.optax_name} optimizer...")
  # For now, we explicitly only optimize the student parameters as there's
  # nothing else to be optimized. If we ever want to add learnable projections
  # or similar for good (we explored but ditched), need to refactor this a bit.
  tx, sched_fns = bv_optax.make(
      config, params_cpu["student"], sched_kw=dict(
          total_steps=total_steps, batch_size=batch_size, data_size=ntrain_img))

  # We jit this, such that the arrays are created on the CPU, not device[0].
  opt_cpu = jax.jit(tx.init, backend="cpu")(params_cpu["student"])
  sched_fns_cpu = [jax.jit(sched_fn, backend="cpu") for sched_fn in sched_fns]

  @jax.named_call
  def loss_fn(student_params, params, data, rngs):
    # Note: need to extract and use `student_params` out of `params` because the
    # first argument of `loss_fn` is what's differentiated wrt.
    params["student"] = student_params

    def fwd(name, params):
      return jax.named_call(models[name].apply, name=name)(
          {"params": params}, getfirst(data, name, "image"),
          train=name == "student", rngs=rngs.get(name)
      )[0]  # logits, unused_outputs
    logits = {name: fwd(name, w) for name, w in params.items()}

    measurements = {}
    for name, lg in logits.items():
      measurements[f"entropy_{name}"] = -jnp.sum(
          jax.nn.log_softmax(lg) * jax.nn.softmax(lg), axis=-1)
      if "labels" in data:
        measurements[f"task_loss_{name}"] = u.softmax_xent(
            logits=lg, labels=data["labels"], reduction=False)

    # NOTE: xent is linear in labels, so for KL, this is actually the same as
    # using a teacher-ensemble in probs-space!
    measurements["distill_loss"] = 0.0
    for name in config.teachers:
      l = dd.dist(logits["student"], logits[name], config.get("distance", "kl"),
                  **config.get("distance_kw", {}))
      measurements[f"distill_loss_{name}"] = l
      measurements["distill_loss"] += l

    outputs = (measurements["distill_loss"], measurements)
    return jax.tree_map(jnp.mean, outputs)

  @functools.partial(jax.pmap, axis_name="batch", donate_argnums=(0, 1))
  def update_fn(params, opt, rng, data):
    """Update step."""

    # Mixup. Note: overwrites the `data` entries (that's intended).
    if config.get("mixup") and config.mixup.p:
      to_mix = {name: data[name]
                for name in ("image", "labels") + tuple(models) if name in data}
      rng, _, to_mix = u.mixup(rng, **config.mixup, **to_mix)
      data = {**data, **to_mix}

    # Get device-specific loss rng.
    rng, *rng_models = jax.random.split(rng, len(models) + 1)
    rngs_models_local = {
        name: {"dropout": jax.random.fold_in(rngi, jax.lax.axis_index("batch"))}
        for name, rngi in zip(models, rng_models)
    }

    w = params["student"]  # Need to explicitly pull out the optimized ones.
    (l, measurements), grads = jax.lax.pmean(
        jax.value_and_grad(loss_fn, has_aux=True)(
            w, params, data, rngs=rngs_models_local),
        axis_name="batch")
    updates, opt = tx.update(grads, opt, w)
    w = optax.apply_updates(w, updates)
    params["student"] = w

    # Take some logging measurements
    gs = jax.tree_leaves(bv_optax.replace_frozen(config.schedule, grads, 0.))
    measurements["l2_grads"] = jnp.sqrt(sum([jnp.vdot(g, g) for g in gs]))
    ps = jax.tree_leaves(w)
    measurements["l2_params"] = jnp.sqrt(sum([jnp.vdot(p, p) for p in ps]))
    us = jax.tree_leaves(updates)
    measurements["l2_updates"] = jnp.sqrt(sum([jnp.vdot(u, u) for u in us]))

    return params, opt, rng, l, measurements

  # We always load the teachers first, because they NEED to be initialized
  # and since we don't ever modify them, we don't store them in checkpoints.
  for name in config.teachers:
    init_def = config[f"{name}_init"]
    write_note(f"Initializing {name} from {init_def}…")
    params_cpu[name] = get_model_mod(name).load(
        params_cpu[name], init_def, config[name],
        **config.get(f"{name}_load", {}))

  # Decide how to initialize training. The order is important.
  # 1. Always resumes from the existing checkpoint, e.g. resumes a finetune job.
  # 2. Resume from a previous checkpoint, e.g. start a cooldown training job.
  # 3. Initialize student from something, e.g. start a fine-tuning job.
  # 4. Train from scratch.
  resume_ckpt_path = None
  if save_ckpt_path and gfile.exists(save_ckpt_path):
    resume_ckpt_path = save_ckpt_path
  elif config.get("resume"):
    resume_ckpt_path = fillin(config.resume)
  if resume_ckpt_path:
    write_note("Resume training from checkpoint...")
    # NOTE: we never change the teachers, so only checkpoint student here.
    checkpoint = {
        "params": params_cpu["student"],
        "opt": opt_cpu,
        "chrono": u.chrono.save(),
    }
    checkpoint_tree = jax.tree_structure(checkpoint)
    loaded = u.load_checkpoint(checkpoint_tree, resume_ckpt_path)
    # bfloat16 type gets lost when data is saved to disk, so we recover it.
    checkpoint = jax.tree_map(u.recover_dtype, loaded)
    params_cpu["student"], opt_cpu = checkpoint["params"], checkpoint["opt"]
    u.chrono.load(checkpoint["chrono"])
  elif config.get("student_init"):
    write_note(f"Initialize student from {config.student_init}...")
    params_cpu["student"] = get_model_mod("student").load(
        params_cpu["student"], config.student_init, config.get("student"),
        **config.get("student_load", {}))
    if jax.process_index() == 0:
      parameter_overview.log_parameter_overview(
          params_cpu["student"], msg="restored (student) params")

  write_note("Kicking off misc stuff...")
  first_step = bv_optax.get_count(opt_cpu)
  u.chrono.inform(first_step=first_step)
  prof = None  # Keeps track of start/stop of profiler state.

  write_note(f"Replicating...\n{u.chrono.note}")
  params_repl = flax.jax_utils.replicate(params_cpu)
  opt_repl = flax.jax_utils.replicate(opt_cpu)

  # Define predict functions that the evaluators can use:
  # 1. One per model
  predict_fns = {}
  for name, model in models.items():
    def fwd(params, image, n=name, m=model):
      return m.apply({"params": params[n]}, image)
    predict_fns[f"{name}_fwd"] = fwd
  # 2. One for the ensemble of all teachers.
  def teacher_ensemble_fwd(params, image):
    all_teacher_logits = [
        models[name].apply(params[name], image)[0]  # return is `logits, out`
        for name in config.teachers
    ]
    return jnp.mean([jax.nn.softmax(l) for l in all_teacher_logits], axis=0), {}
  predict_fns["teacher_ensemble_fwd"] = teacher_ensemble_fwd
  # 3.One for each (student, teacher) pair, eg for distance eval.
  for name in [*config.teachers, "teacher_ensemble"]:
    def fwd(params, image, n=name):  # pylint: disable=function-redefined
      student_ret = predict_fns["student_fwd"](params, image)
      teacher_ret = predict_fns[f"{n}_fwd"](params, image)
      return student_ret, teacher_ret
    predict_fns[f"student_{name}_fwd"] = fwd

  # Only initialize evaluators when they are first needed.
  @functools.lru_cache(maxsize=None)
  def evaluators():
    return eval_common.from_config(
        config, predict_fns,
        lambda s: write_note(f"Init evaluator: {s}…\n{u.chrono.note}"),
        lambda key, cfg: get_steps(key, default=None, cfg=cfg),
    )

  rng, rng_loop = jax.random.split(rng, 2)
  rngs_loop = flax.jax_utils.replicate(rng_loop)
  ckpt_writer = None

  write_note(f"First step compilations...\n{u.chrono.note}")

  # Using a python integer for step here, because opt.state.step is allocated
  # on TPU during replication.
  for step, batch in zip(range(first_step + 1, total_steps + 1), train_iter):
    mw.step_start(step)

    with jax.profiler.StepTraceAnnotation("train_step", step_num=step):
      with u.chrono.log_timing("z/secs/update0", noop=step > first_step + 1):
        params_repl, opt_repl, rngs_loop, loss_value, measurements = update_fn(
            params_repl, opt_repl, rngs_loop, batch)

    # On the first host, let's always profile a handful of early steps.
    if jax.process_index() == 0:
      prof = u.startstop_prof(prof, step, first_step, get_steps("log_training"))

    # Report training progress
    if (u.itstime(step, get_steps("log_training"), total_steps, host=0)
        or u.chrono.warmup and jax.process_index() == 0):
      for i, sched_fn_cpu in enumerate(sched_fns_cpu):
        mw.measure(f"global_schedule{i if i else ''}", sched_fn_cpu(step - 1))
      l = mw.measure("training_loss", loss_value[0])
      for name, value in measurements.items():
        mw.measure(name, value[0])
      u.chrono.tick(step)
      if not np.isfinite(l):
        raise RuntimeError(f"The loss became nan or inf somewhere within steps "
                           f"[{step - get_steps('log_training')}, {step}]")

    # Checkpoint saving
    if (save_ckpt_path and
        (u.itstime(step, get_steps("ckpt", None), total_steps, host=0) or
         u.itstime(step, get_steps("keep_ckpt", None), total_steps, host=0))):
      u.chrono.pause(wait_for=(params_repl["student"], opt_repl))
      u.checkpointing_timeout(ckpt_writer, config.get("ckpt_timeout", 1))
      # We need to transfer the weights over now or else we risk keeping them
      # alive while they'll be updated in a future step, creating hard to debug
      # memory errors (see (internal link)). Also, takes device 0's params only.
      params_cpu["student"], opt_cpu = jax.tree_map(
          lambda x: np.array(x[0]), (params_repl["student"], opt_repl))

      # Check whether we want to keep a copy of the current checkpoint.
      copy_step = None
      if u.itstime(step, get_steps("keep_ckpt", None), total_steps):
        copy_step = step

      ckpt = {"params": params_cpu["student"],
              "opt": opt_cpu,
              "chrono": u.chrono.save()}
      ckpt_writer = pool.apply_async(
          u.save_checkpoint, (ckpt, save_ckpt_path, copy_step))
      u.chrono.resume()

    for (name, evaluator, log_steps, prefix) in evaluators():
      if u.itstime(step, log_steps, total_steps, last=False):
        u.chrono.pause(wait_for=params_repl)
        u.chrono.tick(step)  # Record things like epoch number, core hours etc.
        write_note(f"{name} evaluation...\n{u.chrono.note}")
        with u.chrono.log_timing(f"z/secs/eval/{name}"):
          for key, value in evaluator.run(params_repl):
            mw.measure(f"{prefix}{key}", value)
        u.chrono.resume()
    mw.step_end()

  # Run evals after done with training. Running them here guarantees evals
  # will run if job is restarted after writting the last checkpoint and
  # also supports eval only runs (when total_steps or num_epochs is 0).
  mw.step_start(total_steps)
  for (name, evaluator, _, prefix) in evaluators():
    write_note(f"{name} evaluation...\n{u.chrono.note}")
    with u.chrono.log_timing(f"z/secs/eval/{name}"):
      for key, value in evaluator.run(params_repl):
        mw.measure(f"{prefix}{key}", value)

  # Always give a chance to stop the profiler, no matter how things ended.
  # TODO: can we also do this when dying of an exception like OOM?
  if jax.process_index() == 0 and prof is not None:
    u.startstop_prof(prof)

  # Last note needs to happen before the pool's closed =)
  write_note(f"Done!\n{u.chrono.note}")

  pool.close()
  pool.join()
  mw.close()

  # Make sure all hosts stay up until the end of main.
  u.sync()

  u.maybe_cleanup_workdir(workdir, flags.FLAGS.cleanup, info)


if __name__ == "__main__":
  app.run(main)
