strings: fix type hints and uncovered bugs (#2555)

* strings: fix type hints and uncovered bugs

changelog

add strings tests

strings: fix buf_filled_with

fix strings tests

refactor: optimize and document buf_filled_with function in strings.py

docs: add docstring to buf_filled_with function

doc

strings: add typing

* strings: more validation and testing

thanks @fariss

* copyright

tests/test_strings.py

@@ -0,0 +1,105 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from capa.features.extractors.strings import (
+    String,
+    buf_filled_with,
+    is_printable_str,
+    extract_ascii_strings,
+    extract_unicode_strings,
+)
+
+
+def test_buf_filled_with():
+    # single repeating byte
+    assert buf_filled_with(b"\x00" * 8, 0x00) is True
+    assert buf_filled_with(b"\xFF" * 8, 0xFF) is True
+
+    # mixed bytes
+    assert buf_filled_with(b"\x00\x01" * 8, 0x00) is False
+    assert buf_filled_with(b"ABCD" * 8, ord("A")) is False
+
+    # edge cases
+    assert buf_filled_with(b"", 0x00) is False  # Empty buffer
+    assert buf_filled_with(b"\x00", 0x00) is True  # Single byte
+
+
+def test_extract_ascii_strings():
+    # test empty buffer
+    assert list(extract_ascii_strings(b"")) == []
+
+    buf = b"Hello World\x00This is a test\x00"
+    strings = list(extract_ascii_strings(buf))
+    assert len(strings) == 2
+    assert strings[0] == String("Hello World", 0)
+    assert strings[1] == String("This is a test", 12)
+
+    # min length
+    buf = b"Hi\x00Test\x00"
+    strings = list(extract_ascii_strings(buf, n=4))
+    assert len(strings) == 1
+    assert strings[0] == String("Test", 3)
+
+    # non-ASCII strings
+    buf = b"Hello\xFFWorld\x00"
+    strings = list(extract_ascii_strings(buf))
+    assert len(strings) == 2
+    assert strings[0] == String("Hello", 0)
+    assert strings[1] == String("World", 6)
+
+    # only non-ASCII
+    assert list(extract_ascii_strings(b"\xFF\xFF\xFF")) == []
+
+    buf = b"\x00" * 8 + b"ValidString\x00"
+    strings = list(extract_ascii_strings(buf))
+    assert len(strings) == 1
+    assert strings[0] == String("ValidString", 8)
+
+
+def test_extract_unicode_strings():
+    buf = b"H\x00e\x00l\x00l\x00o\x00\x00\x00"
+    strings = list(extract_unicode_strings(buf))
+    assert len(strings) == 1
+    assert strings[0] == String("Hello", 0)
+
+    # min length
+    buf = b"H\x00i\x00\x00\x00T\x00e\x00s\x00t\x00\x00\x00"
+    strings = list(extract_unicode_strings(buf, n=4))
+    assert len(strings) == 1
+    assert strings[0] == String("Test", 6)
+
+    # invalid Unicode sequences
+    buf = b"H\x00\xFF\x00l\x00l\x00o\x00\x00\x00"
+    strings = list(extract_unicode_strings(buf))
+    assert len(strings) == 0
+
+    # repeating bytes (should be skipped)
+    buf = b"\x00" * 8 + b"V\x00a\x00l\x00i\x00d\x00\x00\x00"
+    strings = list(extract_unicode_strings(buf))
+    assert len(strings) == 1
+    assert strings[0] == String("Valid", 8)
+
+
+def test_is_printable_str():
+    assert is_printable_str("Hello World") is True
+    assert is_printable_str("123!@#") is True
+    assert is_printable_str("\t\n\r") is True  # whitespace is printable
+
+    assert is_printable_str("\x00\x01\x02") is False
+    assert is_printable_str("Hello\x07World") is False
+    assert is_printable_str("\x1b[31m") is False  # ANSI escape codes
+
+    assert is_printable_str("") is True  # empty string
+    assert is_printable_str(" ") is True  # single space
+    assert is_printable_str("\x7f") is False  # DEL character