#!/usr/bin/env python
# coding: utf-8

# In[1]:


import numpy as np
from sklearn.datasets import load_iris
from sklearn.model_selection import StratifiedKFold as skStratifiedKFold


# In[2]:


class StratifiedKFold():
    def __init__(self, n_splits=5, shuffle=False, random_state=0):
        self.n_splits = n_splits
        self.shuffle = shuffle
        self.random_state = random_state

    def _kfold(self, count, rng):
        indices = np.arange(count)
        if self.shuffle:
            rng.shuffle(indices)
        fold_sizes = np.full(self.n_splits, count // self.n_splits)
        fold_sizes[:count % self.n_splits] += 1
        current = 0
        for fold_size in fold_sizes:
            test_mask = np.zeros(count, dtype=bool)
            test_mask[current:current + fold_size] = True
            yield indices[test_mask]
            current += fold_size

    def _make_test_folds(self, X, y):
        rng = np.random.RandomState(self.random_state)
        unique_y, y_inversed = np.unique(y, return_inverse=True)
        y_counts = np.bincount(y_inversed)
        test_folds = np.zeros(X.shape[0])
        per_cls_cvs = [self._kfold(count, rng) for count in y_counts]
        test_folds = np.zeros(X.shape[0])
        for test_fold_indices, per_cls_splits in enumerate(zip(*per_cls_cvs)):
            for cls, test_split in zip(unique_y, per_cls_splits):
                cls_test_folds = test_folds[y == cls]
                cls_test_folds[test_split] = test_fold_indices
                test_folds[y == cls] = cls_test_folds
        return test_folds

    def _iter_test_masks(self, X, y):
        test_folds = self._make_test_folds(X, y)
        for i in range(self.n_splits):
            yield test_folds == i

    def split(self, X, y):
        indices = np.arange(X.shape[0])
        for test_index in self._iter_test_masks(X, y):
            yield indices[~test_index], indices[test_index]


# In[3]:


X, y = load_iris(return_X_y=True)
cv1 = StratifiedKFold(n_splits=5)
cv2 = skStratifiedKFold(n_splits=5)
for (train1, test1), (train2, test2) in zip(cv1.split(X, y), cv2.split(X, y)):
    assert np.array_equal(train1, train2)
    assert np.array_equal(test1, test2)


# In[4]:


X, y = load_iris(return_X_y=True)
cv1 = StratifiedKFold(n_splits=5, shuffle=True, random_state=0)
cv2 = skStratifiedKFold(n_splits=5, shuffle=True, random_state=0)
for (train1, test1), (train2, test2) in zip(cv1.split(X, y), cv2.split(X, y)):
    assert np.array_equal(train1, train2)
    assert np.array_equal(test1, test2)