contrib/python-zstandard/tests/test_train_dictionary.py

rhg: Don’t compare ambiguous files one byte at a time

Even though the use of `BufReader` reduces the number of syscalls to read
the file from disk, `.bytes()` yields a separate `Result` for every byte.
Creating those results and dispatching on them is most likely costly.

Instead, this commit opts for simplicity by reading the entire file into memory
and comparing a single pair of byte strings. Note that memory already needs to
contain the entire previous contents of the file, as read from the filelog.
So with an extremely large file this doubles memory use but does not make it
grow by orders of magnitude.

At first I wrote code that still avoids reading the entire file into memory
and compares one buffer at a time with `BufReader`. Find this code below for
posterity. However its correctness is subtle. I ended up preferring the
simplicity of the obviously-correct single comparison.

```rust
let mut reader = BufReader::new(fobj);
let mut expected = &contents_in_p1[..];
loop {
    let buf = reader.fill_buf().when_reading_file(&fs_path)?;
    if buf.is_empty() {
        // Found EOF
        return Ok(expected.is_empty());
    } else if let Some(rest) = expected.drop_prefix(buf) {
        // What we read so far matches the expected content, continue reading
        let buf_len = buf.len();
        reader.consume(buf_len);
        expected = rest
    } else {
        // Found different content
        return Ok(false);
    }
}
```

Differential Revision: https://phab.mercurial-scm.org/D11412

import struct
import sys
import unittest

import zstandard as zstd

from .common import (
    generate_samples,
    make_cffi,
    random_input_data,
    TestCase,
)

if sys.version_info[0] >= 3:
    int_type = int
else:
    int_type = long


@make_cffi
class TestTrainDictionary(TestCase):
    def test_no_args(self):
        with self.assertRaises(TypeError):
            zstd.train_dictionary()

    def test_bad_args(self):
        with self.assertRaises(TypeError):
            zstd.train_dictionary(8192, u"foo")

        with self.assertRaises(ValueError):
            zstd.train_dictionary(8192, [u"foo"])

    def test_no_params(self):
        d = zstd.train_dictionary(8192, random_input_data())
        self.assertIsInstance(d.dict_id(), int_type)

        # The dictionary ID may be different across platforms.
        expected = b"\x37\xa4\x30\xec" + struct.pack("<I", d.dict_id())

        data = d.as_bytes()
        self.assertEqual(data[0:8], expected)

    def test_basic(self):
        d = zstd.train_dictionary(8192, generate_samples(), k=64, d=16)
        self.assertIsInstance(d.dict_id(), int_type)

        data = d.as_bytes()
        self.assertEqual(data[0:4], b"\x37\xa4\x30\xec")

        self.assertEqual(d.k, 64)
        self.assertEqual(d.d, 16)

    def test_set_dict_id(self):
        d = zstd.train_dictionary(
            8192, generate_samples(), k=64, d=16, dict_id=42
        )
        self.assertEqual(d.dict_id(), 42)

    def test_optimize(self):
        d = zstd.train_dictionary(
            8192, generate_samples(), threads=-1, steps=1, d=16
        )

        # This varies by platform.
        self.assertIn(d.k, (50, 2000))
        self.assertEqual(d.d, 16)


@make_cffi
class TestCompressionDict(TestCase):
    def test_bad_mode(self):
        with self.assertRaisesRegex(ValueError, "invalid dictionary load mode"):
            zstd.ZstdCompressionDict(b"foo", dict_type=42)

    def test_bad_precompute_compress(self):
        d = zstd.train_dictionary(8192, generate_samples(), k=64, d=16)

        with self.assertRaisesRegex(
            ValueError, "must specify one of level or "
        ):
            d.precompute_compress()

        with self.assertRaisesRegex(
            ValueError, "must only specify one of level or "
        ):
            d.precompute_compress(
                level=3, compression_params=zstd.CompressionParameters()
            )

    def test_precompute_compress_rawcontent(self):
        d = zstd.ZstdCompressionDict(
            b"dictcontent" * 64, dict_type=zstd.DICT_TYPE_RAWCONTENT
        )
        d.precompute_compress(level=1)

        d = zstd.ZstdCompressionDict(
            b"dictcontent" * 64, dict_type=zstd.DICT_TYPE_FULLDICT
        )
        with self.assertRaisesRegex(
            zstd.ZstdError, "unable to precompute dictionary"
        ):
            d.precompute_compress(level=1)