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diff --git a/bzrlib/tests/test_estimate_compressed_size.py b/bzrlib/tests/test_estimate_compressed_size.py
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+++ b/bzrlib/tests/test_estimate_compressed_size.py
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+# Copyright (C) 2011 Canonical Ltd
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+"""Tests for our estimation of compressed content."""
+
+import zlib
+import hashlib
+
+from bzrlib import (
+    estimate_compressed_size,
+    tests,
+    )
+
+
+class TestZLibEstimator(tests.TestCase):
+
+    def get_slightly_random_content(self, length, seed=''):
+        """We generate some hex-data that can be seeded.
+
+        The output should be deterministic, but the data stream is effectively
+        random.
+        """
+        h = hashlib.md5(seed)
+        hex_content = []
+        count = 0
+        while count < length:
+            b = h.hexdigest()
+            hex_content.append(b)
+            h.update(b)
+            count += len(b)
+        return ''.join(hex_content)[:length]
+
+    def test_adding_content(self):
+        ze = estimate_compressed_size.ZLibEstimator(32000)
+        raw_data = self.get_slightly_random_content(60000)
+        block_size = 1000
+        for start in xrange(0, len(raw_data), block_size):
+            ze.add_content(raw_data[start:start+block_size])
+            if ze.full():
+                break
+        # Practise showed that 'start' was 56000. However, zlib is a bit
+        # platform dependent, so give it +/- 5%.
+        self.assertTrue(54000 <= start <= 58000,
+            "Unexpected amount of raw data added: %d bytes" % (start,))
+        # The real compression should be 'close' to 32000, real measurement was
+        # 32401
+        raw_comp = zlib.compress(raw_data[:start])
+        self.assertTrue(31000 < len(raw_comp) < 33000,
+            "Unexpected compressed size: %d bytes" % (len(raw_comp),))
+
+    def test_adding_more_content(self):
+        ze = estimate_compressed_size.ZLibEstimator(64000)
+        raw_data = self.get_slightly_random_content(150000)
+        block_size = 1000
+        for start in xrange(0, len(raw_data), block_size):
+            ze.add_content(raw_data[start:start+block_size])
+            if ze.full():
+                break
+        # Practise showed that 'start' was 112000
+        self.assertTrue(110000 <= start <= 114000,
+            "Unexpected amount of raw data added: %d bytes" % (start,))
+        # The real compression should be 'close' to 32000, real measurement was
+        # 32401
+        raw_comp = zlib.compress(raw_data[:start])
+        self.assertTrue(63000 < len(raw_comp) < 65000,
+            "Unexpected compressed size: %d bytes" % (len(raw_comp),))