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'''
Created on Mar 3, 2019

@author: gcampagn
'''

import torch
import ujson as json
import os
import logging

logger = logging.getLogger(__name__)

class Saver(object):
    '''
    Wrap pytorch's save functionality into an interface similar to tensorflow.train.Saver
    
    In particular, this class takes care of automatically cleaning up old checkpoints,
    and creating checkpoint files to keep track of which saves are valid and which are not.
    '''

    def __init__(self, savedir, max_to_keep=5):
        self._savedir = savedir
        self._max_to_keep = max_to_keep
        assert max_to_keep >= 1
        
        self._loaded_last_checkpoints = False
        self._latest_checkpoint = None
        self._all_checkpoints = None

    def _maybe_load_last_checkpoints(self):
        if self._loaded_last_checkpoints:
            return
        
        try:
            with open(os.path.join(self._savedir, 'checkpoint.json')) as fp:
                data = json.load(fp)
                self._loaded_last_checkpoints = True
                self._all_checkpoints = data['all']
                self._latest_checkpoint = data['latest']
        except FileNotFoundError:
            self._loaded_last_checkpoints = True
            self._all_checkpoints = []
            self._latest_checkpoint = None
        
    def save(self, save_dict, global_step):
        self._maybe_load_last_checkpoints()
        
        filename = 'iteration_' + str(global_step) + '.pth'
        abspath = os.path.join(self._savedir, filename)
        
        self._latest_checkpoint = filename
        self._all_checkpoints.append(filename)
        if len(self._all_checkpoints) > self._max_to_keep:
            try:
                todelete = self._all_checkpoints.pop(0)
                os.unlink(os.path.join(self._savedir, todelete))
            except (OSError, IOError) as e:
                logging.warn('Failed to delete old checkpoint: %s', e)
        torch.save(save_dict, abspath)
        with open(os.path.join(self._savedir, 'checkpoint.json'), 'w') as fp:
            json.dump(dict(all=self._all_checkpoints, latest=self._latest_checkpoint), fp)