diff --git a/data/collate.py b/data/collate.py
new file mode 100644
index 0000000..bdd582c
--- /dev/null
+++ b/data/collate.py
@@ -0,0 +1,71 @@
+"""
+functions to make batch for arrays which satisfy some conditions.
+"""
+import numpy as np
+
+def text_collate(minibatch):
+    """
+    minibatch: List[Example]
+    Example: ndarray, shape(T,), dtype: int64
+    """
+    peek_example = minibatch[0]
+    assert len(peek_example.shape) == 1, "text example is an 1D tensor"
+    
+    lengths = [example.shape[0] for example in minibatch] # assume (channel, n_samples) or (n_samples, )
+    max_len = np.max(lengths)
+    
+    batch = []
+    for example in minibatch:
+        pad_len = max_len - example.shape[0]
+        batch.append(np.pad(example, [(0, pad_len)], mode='constant', constant_values=0))
+
+    return np.array(batch, dtype=np.int64)
+
+def wav_collate(minibatch):
+    """
+    minibatch: List[Example]
+    Example: ndarray, shape(C, T) for multi-channel wav, shape(T,) for mono-channel wav, dtype: float32 
+    """
+    peek_example = minibatch[0]
+    if len(peek_example.shape) == 1:
+        mono_channel = True
+    elif len(peek_example.shape) == 2:
+        mono_channel = False
+    
+    lengths = [example.shape[-1] for example in minibatch] # assume (channel, n_samples) or (n_samples, )
+    max_len = np.max(lengths)
+    
+    batch = []
+    for example in minibatch:
+        pad_len = max_len - example.shape[-1]
+        if mono_channel:
+            batch.append(np.pad(example, [(0, pad_len)], mode='constant', constant_values=0.))
+        else:
+            batch.append(np.pad(example, [(0, 0), (0, pad_len)], mode='constant', constant_values=0.)) # what about PCM, no
+    
+    return np.array(batch, dtype=np.float32)
+
+def spec_collate(minibatch):
+    """
+    minibatch: List[Example]
+    Example: ndarray, shape(C, F, T) for multi-channel spectrogram, shape(F, T) for mono-channel spectrogram, dtype: float32 
+    """
+    # assume (F, T) or (C, F, T)
+    peek_example = minibatch[0]
+    if len(peek_example.shape) == 2:
+        mono_channel = True
+    elif len(peek_example.shape) == 3:
+        mono_channel = False
+    
+    lengths = [example.shape[-1] for example in minibatch] # assume (channel, F, n_frame) or (F, n_frame)
+    max_len = np.max(lengths)
+    
+    batch = []
+    for example in minibatch:
+        pad_len = max_len - example.shape[-1]
+        if mono_channel:
+            batch.append(np.pad(example, [(0, 0), (0, pad_len)], mode='constant', constant_values=0.))
+        else:
+            batch.append(np.pad(example, [(0, 0), (0, 0), (0, pad_len)], mode='constant', constant_values=0.)) # what about PCM, no
+    
+    return np.array(batch, dtype=np.float32)   
\ No newline at end of file
diff --git a/data/dataloader.py b/data/dataloader.py
new file mode 100644
index 0000000..231322c
--- /dev/null
+++ b/data/dataloader.py
@@ -0,0 +1,83 @@
+from sampler import SequentialSampler, RandomSampler, BatchSampler
+
+class DataLoader(object):
+    def __init__(self, dataset, batch_size=1, collate_fn = lambda x: x, 
+                 sampler=None, shuffle=False, batch_sampler=None, drop_last=False):
+        self.dataset = dataset
+        self.collate_fn = collate_fn
+        
+        if batch_sampler is not None:
+            # auto_collation with custom batch_sampler
+            if batch_size != 1 or shuffle or sampler is not None or drop_last:
+                raise ValueError('batch_sampler option is mutually exclusive '
+                                 'with batch_size, shuffle, sampler, and '
+                                 'drop_last')
+            batch_size = None
+            drop_last = False
+        elif batch_size is None:
+            # no auto_collation
+            if shuffle or drop_last:
+                raise ValueError('batch_size=None option disables auto-batching '
+                                 'and is mutually exclusive with '
+                                 'shuffle, and drop_last')
+        
+        if sampler is None:  # give default samplers
+            if shuffle:
+                sampler = RandomSampler(dataset)
+            else:
+                sampler = SequentialSampler(dataset)
+
+        if batch_size is not None and batch_sampler is None:
+            # auto_collation without custom batch_sampler
+            batch_sampler = BatchSampler(sampler, batch_size, drop_last)
+
+        self.batch_size = batch_size
+        self.drop_last = drop_last
+        self.sampler = sampler
+        self.batch_sampler = batch_sampler
+    
+    def __iter__(self):
+        return DataIterator(self)
+    
+    @property
+    def _auto_collation(self):
+        # we will auto batching
+        return self.batch_sampler is not None
+
+    @property
+    def _index_sampler(self):
+        # The actual sampler used for generating indices for `_DatasetFetcher`
+        # (see _utils/fetch.py) to read data at each time. This would be
+        # `.batch_sampler` if in auto-collation mode, and `.sampler` otherwise.
+        # We can't change `.sampler` and `.batch_sampler` attributes for BC
+        # reasons.
+        if self._auto_collation:
+            return self.batch_sampler
+        else:
+            return self.sampler
+
+    def __len__(self):
+        return len(self._index_sampler)  # with iterable-style dataset, this will error
+    
+class DataIterator(object):
+    def __init__(self, loader):
+        self.loader = loader
+        self._dataset = loader.dataset
+        
+        self._index_sampler = loader._index_sampler
+        self._sampler_iter = iter(self._index_sampler)
+    
+    def __iter__(self):
+        return self
+    
+    def __next__(self):
+        index = self._next_index()  # may raise StopIteration, TODO(chenfeiyu): use dynamic batch size
+        minibatch = [self._dataset[i] for i in index] # we can abstract it, too to use dynamic batch size
+        minibatch = self.loader.collate_fn(minibatch) # list[Example] -> Batch
+        return minibatch
+    
+    def _next_index(self):
+        return next(self._sampler_iter)
+    
+    def __len__(self):
+        return len(self._index_sampler)
diff --git a/data/dataset.py b/data/dataset.py
new file mode 100644
index 0000000..0c6bb34
--- /dev/null
+++ b/data/dataset.py
@@ -0,0 +1,25 @@
+class Dataset(object):
+    def __init__(self, lazy=True, stream=False):
+        # note that lazy and stream means two different things in our glossary
+        # lazy means to place preprocessing in __getitem__
+        # stram means the data source is itself a stream
+        self.lazy = lazy
+        self.stream = stream
+    
+    def _load_metadata(self):
+        raise NotImplementedError
+    
+    def _get_example(self):
+        """return a Record"""
+        raise NotImplementedError
+    
+    @classmethod
+    def _prepare_metadata(self):
+        raise NotImplementedError
+    
+    def __getitem__(self, index):
+        raise NotImplementedError
+    
+    def __iter__(self):
+        raise NotImplementedError
+
diff --git a/data/ljspeech.py b/data/ljspeech.py
new file mode 100644
index 0000000..b2eb56e
--- /dev/null
+++ b/data/ljspeech.py
@@ -0,0 +1,105 @@
+from pathlib import Path
+import numpy as np
+import pandas as pd
+import librosa
+import g2p
+
+from sampler import SequentialSampler, RandomSampler, BatchSampler
+from dataset import Dataset
+from dataloader import DataLoader
+
+from collate import text_collate, spec_collate
+
+LJSPEECH_ROOT = Path("/Users/chenfeiyu/projects/LJSpeech-1.1")
+class LJSpeech(Dataset):
+    def __init__(self, root=LJSPEECH_ROOT, lazy=True, stream=False):
+        super(LJSpeech, self).__init__(lazy, stream)
+        self.root = root
+        self.metadata = self._prepare_metadata() # we can do this just for luck
+
+        if self.stream:
+            self.examples_generator = self._read() 
+        
+    def _prepare_metadata(self):
+        # if pure-stream case, each _prepare_metadata returns a generator
+        csv_path = self.root.joinpath("metadata.csv")
+        metadata = pd.read_csv(csv_path, sep="|", header=None, quoting=3,
+                               names=["fname", "raw_text", "normalized_text"])
+        return metadata
+    
+    def _read(self):
+        for _, metadatum in self.metadata.iterrows():
+            example = self._get_example(metadatum)
+            yield example
+            
+    def _get_example(self, metadatum):
+        """All the code for generating an Example from a metadatum. If you want a 
+        different preprocessing pipeline, you can override this method. 
+        This method may require several processor, each of which has a lot of options.
+        In this case, you'd better pass a composed transform and pass it to the init
+        method.
+        """
+        
+        fname, raw_text, normalized_text = metadatum
+        wav_path = self.root.joinpath("wavs", fname + ".wav")
+        
+        # load -> trim -> preemphasis -> stft -> magnitude -> mel_scale -> logscale -> normalize
+        wav, sample_rate = librosa.load(wav_path, sr=None) # we would rather use functor to hold its parameters
+        trimed, _ = librosa.effects.trim(wav)
+        preemphasized = librosa.effects.preemphasis(trimed)
+        D = librosa.stft(preemphasized)
+        mag, phase = librosa.magphase(D)
+        mel = librosa.feature.melspectrogram(S=mag)
+        
+        mag = librosa.amplitude_to_db(S=mag)
+        mel = librosa.amplitude_to_db(S=mel)
+        
+        ref_db = 20
+        max_db = 100
+        mel = np.clip((mel - ref_db + max_db) / max_db, 1e-8, 1)
+        mel = np.clip((mag - ref_db + max_db) / max_db, 1e-8, 1)
+
+        phonemes = np.array(g2p.en.text_to_sequence(normalized_text), dtype=np.int64)
+        return (mag, mel, phonemes) # maybe we need to implement it as a map in the future
+
+    def __getitem__(self, index):
+        if self.stream:
+            raise ValueError("__getitem__ is invalid in stream mode")
+        metadatum = self.metadata.iloc[index]
+        example = self._get_example(metadatum)
+        return example
+    
+    def __iter__(self):
+        if self.stream:
+            for example in self.examples_generator:
+                yield example
+        else:
+            for i in range(len(self)):
+                yield self[i]
+    
+    def __len__(self):
+        if self.stream:
+            raise ValueError("__len__ is invalid in stream mode")
+        return len(self.metadata)
+
+
+def fn(minibatch):
+    mag_batch = []
+    mel_batch = []
+    phoneme_batch = []
+    for example in minibatch:
+        mag, mel, phoneme = example
+        mag_batch.append(mag)
+        mel_batch.append(mel)
+        phoneme_batch.append(phoneme)
+    mag_batch = spec_collate(mag_batch)
+    mel_batch = spec_collate(mel_batch)
+    phoneme_batch = text_collate(phoneme_batch)
+    return (mag_batch, mel_batch, phoneme_batch)
+
+if __name__ == "__main__":
+    ljspeech = LJSpeech(LJSPEECH_ROOT)
+    ljspeech_loader = DataLoader(ljspeech, batch_size=16, shuffle=True, collate_fn=fn)
+    for i, batch in enumerate(ljspeech_loader):
+        print(i)
+
diff --git a/data/sampler.py b/data/sampler.py
new file mode 100644
index 0000000..ff6d5d7
--- /dev/null
+++ b/data/sampler.py
@@ -0,0 +1,209 @@
+"""
+At most cases, we have non-stream dataset, which means we can random access it with __getitem__, and we can get the length of the dataset with __len__.
+
+This suffices for a sampler. We implemente sampler as iterable of valid indices. By valid, we mean 0 <= index < N, where N is the length of the dataset. We then collect several indices within a batch and use it to collect examples from the dataset with __getitem__. Then collate this examples to form a batch.
+
+So the sampler is only responsible for generating valid indices.
+"""
+
+
+import numpy as np
+import random
+
+class Sampler(object):
+    def __init__(self, data_source):
+        pass
+
+    def __iter__(self):
+        # return a iterator of indices
+        # or a iterator of list[int], for BatchSampler
+        raise NotImplementedError
+
+
+class SequentialSampler(Sampler):
+    def __init__(self, data_source):
+        self.data_source = data_source
+    
+    def __iter__(self):
+        return iter(range(len(self.data_source)))
+
+    def __len__(self):
+        return len(self.data_source)
+
+
+class RandomSampler(Sampler):
+    def __init__(self, data_source, replacement=False, num_samples=None):
+        self.data_source = data_source
+        self.replacement = replacement
+        self._num_samples = num_samples
+
+        if not isinstance(self.replacement, bool):
+            raise ValueError("replacement should be a boolean value, but got "
+                             "replacement={}".format(self.replacement))
+
+        if self._num_samples is not None and not replacement:
+            raise ValueError("With replacement=False, num_samples should not be specified, "
+                             "since a random permutation will be performed.")
+
+        if not isinstance(self.num_samples, int) or self.num_samples <= 0:
+            raise ValueError("num_samples should be a positive integer "
+                             "value, but got num_samples={}".format(self.num_samples))
+
+    @property
+    def num_samples(self):
+        # dataset size might change at runtime
+        if self._num_samples is None:
+            return len(self.data_source)
+        return self._num_samples
+
+    def __iter__(self):
+        n = len(self.data_source)
+        if self.replacement:
+            return iter(np.random.randint(0, n, size=(self.num_samples,), dtype=np.int64).tolist())
+        return iter(np.random.permutation(n).tolist())
+
+    def __len__(self):
+        return len(self.data_source)
+
+
+class SubsetRandomSampler(Sampler):
+    r"""Samples elements randomly from a given list of indices, without replacement.
+    Arguments:
+        indices (sequence): a sequence of indices
+    """
+
+    def __init__(self, indices):
+        self.indices = indices
+
+    def __iter__(self):
+        return (self.indices[i] for i in np.random.permutation(len(self.indices)))
+
+    def __len__(self):
+        return len(self.indices)
+
+
+class PartialyRandomizedSimilarTimeLengthSampler(Sampler):
+    """Partially randmoized sampler, implemented as a example sampler
+    1. Sort by lengths
+    2. Pick a small patch and randomize it
+    3. Permutate mini-batchs
+    """
+
+    def __init__(self, lengths, batch_size=4, batch_group_size=None,
+                 permutate=True):
+        _lengths = np.array(lengths, dtype=np.int64) # maybe better implement length as a sort key
+        self.lengths = np.sort(_lengths)
+        self.sorted_indices = np.argsort(_lengths)
+
+        self.batch_size = batch_size
+        if batch_group_size is None:
+            batch_group_size = min(batch_size * 32, len(self.lengths))
+            if batch_group_size % batch_size != 0:
+                batch_group_size -= batch_group_size % batch_size
+
+        self.batch_group_size = batch_group_size
+        assert batch_group_size % batch_size == 0
+        self.permutate = permutate
+
+    def __iter__(self):
+        indices = np.copy(self.sorted_indices)
+        batch_group_size = self.batch_group_size
+        s, e = 0, 0
+        for i in range(len(indices) // batch_group_size):
+            s = i * batch_group_size
+            e = s + batch_group_size
+            random.shuffle(indices[s: e]) # inplace
+
+        # Permutate batches
+        if self.permutate:
+            perm = np.arange(len(indices[:e]) // self.batch_size)
+            random.shuffle(perm)
+            indices[:e] = indices[:e].reshape(-1, self.batch_size)[perm, :].reshape(-1)
+
+        # Handle last elements
+        s += batch_group_size
+        #print(indices)
+        if s < len(indices):
+            random.shuffle(indices[s:])
+        
+        return iter(indices)
+
+    def __len__(self):
+        return len(self.sorted_indices)
+
+
+class WeightedRandomSampler(Sampler):
+    r"""Samples elements from ``[0,..,len(weights)-1]`` with given probabilities (weights).
+    Args:
+        weights (sequence)   : a sequence of weights, not necessary summing up to one
+        num_samples (int): number of samples to draw
+        replacement (bool): if ``True``, samples are drawn with replacement.
+            If not, they are drawn without replacement, which means that when a
+            sample index is drawn for a row, it cannot be drawn again for that row.
+    Example:
+        >>> list(WeightedRandomSampler([0.1, 0.9, 0.4, 0.7, 3.0, 0.6], 5, replacement=True))
+        [0, 0, 0, 1, 0]
+        >>> list(WeightedRandomSampler([0.9, 0.4, 0.05, 0.2, 0.3, 0.1], 5, replacement=False))
+        [0, 1, 4, 3, 2]
+    """
+
+    def __init__(self, weights, num_samples, replacement):
+        if not isinstance(num_samples, int) or num_samples <= 0:
+            raise ValueError("num_samples should be a positive integer "
+                             "value, but got num_samples={}".format(num_samples))
+        self.weights = np.array(weights, dtype=np.float64)
+        self.num_samples = num_samples
+        self.replacement = replacement
+
+    def __iter__(self):
+        return iter(np.random.choice(len(self.weights), size=(self.num_samples, ),  
+                                     replace=self.replacement, p=self.weights).tolist())
+
+    def __len__(self):
+        return self.num_samples
+
+
+class BatchSampler(Sampler):
+    r"""Wraps another sampler to yield a mini-batch of indices.
+    Args:
+        sampler (Sampler): Base sampler.
+        batch_size (int): Size of mini-batch.
+        drop_last (bool): If ``True``, the sampler will drop the last batch if
+            its size would be less than ``batch_size``
+    Example:
+        >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=False))
+        [[0, 1, 2], [3, 4, 5], [6, 7, 8], [9]]
+        >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=True))
+        [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
+    """
+
+    def __init__(self, sampler, batch_size, drop_last):
+        if not isinstance(sampler, Sampler):
+            raise ValueError("sampler should be an instance of "
+                             "Sampler, but got sampler={}"
+                             .format(sampler))
+        if not isinstance(batch_size, int) or batch_size <= 0:
+            raise ValueError("batch_size should be a positive integer value, "
+                             "but got batch_size={}".format(batch_size))
+        if not isinstance(drop_last, bool):
+            raise ValueError("drop_last should be a boolean value, but got "
+                             "drop_last={}".format(drop_last))
+        self.sampler = sampler
+        self.batch_size = batch_size
+        self.drop_last = drop_last
+
+    def __iter__(self):
+        batch = []
+        for idx in self.sampler:
+            batch.append(idx)
+            if len(batch) == self.batch_size:
+                yield batch
+                batch = []
+        if len(batch) > 0 and not self.drop_last:
+            yield batch
+
+    def __len__(self):
+        if self.drop_last:
+            return len(self.sampler) // self.batch_size
+        else:
+            return (len(self.sampler) + self.batch_size - 1) // self.batch_size
\ No newline at end of file