TST: Add LzwCodec for encoding (#2883)

This PR adds an abstract Codec class as well as an LzwCodec encoder implementation.

While we might only need decoding for pypdf, having both decoding and encoding in one class massively helps with testing. We can still get it wrong, but it's harder to get both the encoder and the decoder wrong in a consistent way.

pypdf/_codecs/_codecs.py

@@ -0,0 +1,158 @@
+"""
+This module is for codecs only.
+
+While the codec implementation can contain details of the PDF specification,
+the module should not do any PDF parsing.
+"""
+
+from abc import ABC, abstractmethod
+from typing import Dict, List
+
+
+class Codec(ABC):
+    """Abstract base class for all codecs."""
+
+    @abstractmethod
+    def encode(self, data: bytes) -> bytes:
+        """
+        Encode the input data.
+
+        Args:
+            data: Data to encode.
+
+        Returns:
+            Encoded data.
+        """
+
+    @abstractmethod
+    def decode(self, data: bytes) -> bytes:
+        """
+        Decode the input data.
+
+        Args:
+            data: Data to decode.
+
+        Returns:
+            Decoded data.
+        """
+
+
+class LzwCodec(Codec):
+    """Lempel-Ziv-Welch (LZW) adaptive compression codec."""
+
+    CLEAR_TABLE_MARKER = 256  # Special code to indicate table reset
+    EOD_MARKER = 257  # End-of-data marker
+    INITIAL_BITS_PER_CODE = 9  # Initial code bit width
+    MAX_BITS_PER_CODE = 12  # Maximum code bit width
+
+    def __init__(self) -> None:
+        """Initialize codec and reset the compression table."""
+        self.clear_table()
+
+    def clear_table(self) -> None:
+        """Reset the encoding table and coding state to initial conditions."""
+        self.table: Dict[bytes, int] = {bytes([i]): i for i in range(256)}
+        self.next_code = self.EOD_MARKER + 1
+        self.bits_per_code = self.INITIAL_BITS_PER_CODE
+        self.max_code_value = (1 << self.bits_per_code) - 1
+
+    def encode(self, data: bytes) -> bytes:
+        """
+        Encode data using the LZW compression algorithm.
+
+        Taken from PDF 1.7 specs, "7.4.4.2 Details of LZW Encoding".
+        """
+        result_codes: List[int] = []
+
+        # The encoder shall begin by issuing a clear-table code
+        result_codes.append(self.CLEAR_TABLE_MARKER)
+        self.clear_table()
+
+        current_sequence = b""
+        for byte in data:
+            next_sequence = current_sequence + bytes([byte])
+
+            if next_sequence in self.table:
+                # Extend current sequence if already in the table
+                current_sequence = next_sequence
+            else:
+                # Output code for the current sequence
+                result_codes.append(self.table[current_sequence])
+
+                # Add the new sequence to the table if there's room
+                if self.next_code <= (1 << self.MAX_BITS_PER_CODE) - 1:
+                    self.table[next_sequence] = self.next_code
+                    self.next_code += 1
+                    # Increase bits_per_code if necessary
+                    if (
+                        self.next_code > self.max_code_value
+                        and self.bits_per_code < self.MAX_BITS_PER_CODE
+                    ):
+                        self.bits_per_code += 1
+                        self.max_code_value = (1 << self.bits_per_code) - 1
+                else:
+                    # If the table is full, emit a clear-table command
+                    result_codes.append(self.CLEAR_TABLE_MARKER)
+                    self.clear_table()
+
+                # Start new sequence
+                current_sequence = bytes([byte])
+
+        # Ensure everything actually is encoded
+        if current_sequence:
+            result_codes.append(self.table[current_sequence])
+
+        result_codes.append(self.EOD_MARKER)
+
+        return self.pack_codes_into_bytes(result_codes)
+
+    def pack_codes_into_bytes(self, codes: List[int]) -> bytes:
+        """
+        Convert the list of result codes into a continuous byte stream, with codes packed as per the code bit-width.
+        The bit-width starts at 9 bits and expands as needed.
+        """
+        # Reset coding state
+        self.clear_table()
+        buffer = 0
+        bits_in_buffer = 0
+        output = bytearray()
+
+        for code in codes:
+            buffer = (buffer << self.bits_per_code) | code
+            bits_in_buffer += self.bits_per_code
+
+            # Codes shall be packed into a continuous bit stream, high-order bit
+            # first. This stream shall then be divided into bytes, high-order bit
+            # first.
+            while bits_in_buffer >= 8:
+                bits_in_buffer -= 8
+                output.append((buffer >> bits_in_buffer) & 0xFF)
+
+            # After a clear-table marker, reset coding state
+            if code == self.CLEAR_TABLE_MARKER:
+                self.clear_table()
+            elif code == self.EOD_MARKER:
+                # Do not increment next_code for EOD_MARKER
+                pass
+            else:
+                # Increase next_code after processing each code (except special codes)
+                self.next_code += 1
+                # Increase bits_per_code if necessary
+                if (
+                    self.next_code > self.max_code_value
+                    and self.bits_per_code < self.MAX_BITS_PER_CODE
+                ):
+                    self.bits_per_code += 1
+                    self.max_code_value = (1 << self.bits_per_code) - 1
+
+        # Flush any remaining bits in the buffer
+        if bits_in_buffer > 0:
+            output.append((buffer << (8 - bits_in_buffer)) & 0xFF)
+
+        return bytes(output)
+
+    def decode(self, data: bytes) -> bytes:
+        """Decode data using LZW."""
+        from ..filters import LZWDecode
+
+        return LZWDecode.Decoder(data).decode()

tests/test_codecs.py

@@ -0,0 +1,43 @@
+"""Test LZW-related code."""
+
+import pytest
+
+from pypdf._codecs._codecs import LzwCodec
+
+test_cases = [
+    pytest.param(b"", id="Empty input"),
+    pytest.param(b"A", id="Single character"),
+    pytest.param(b"AAAAAA", id="Repeating character"),
+    pytest.param(b"Hello, World!", id="Simple text"),
+    pytest.param(b"ABABABABABAB", id="Repeating pattern"),
+    pytest.param(b"The quick brown fox jumps over the lazy dog", id="Longer text"),
+    pytest.param(b"\x00\xFF\x00\xFF", id="Binary data"),
+    pytest.param(
+        b"BBBCBDBEBFBGBHBIBJBKBLBMBNBOBPBQBRBSBTBUBVBWBXBYBZB[B\\B]B^B_B`BaBbBcBdBeBfBgBhBiBjBkBlBmBnBoBpBqBrBsBtBuBvBwBxByCBCCCDCECFCGCHCICJCKCLCMCNCOCPCQCRCSCTCUCVCWCXCYCZC[C\\C]C^C_C`CaCbCcCdCeCfCgChCiCjCkClCmCnCoCpCqCrCsCtCuCvCwCxCyDBDCDDDEDFDGDHDIDJDKDLDMDNDODPDQDRDSDTDUDVDWDXDYDZD[D\\D]D^D_D`DaDbDcDdDeDfDgDhDiDjDkDlDmDnDoDpDqDrDsDtDuDvDwDxDyEBECEDEEEFEGEHEIEJEKELEMENEOEPEQERESETEUEVEWEXEYEZE[E\\E]E^E_E`EaEbEcEdEeEfEgEhEiEjEkElEmEnEoEpEqErEsEtEuEvEwExEyFBFCFDFEFFFGFHFIFJFKFLFMFNFOFPFQFRFSFTFUFVFWFXFYFZF[F\\F]F^F_F`FaFbFcFdFeFfFgFhFiFjFkFlFmFnFoFpFqFrFsFtFuFvFwFxFyGBGCGDGEGFGGGHGIGJGKGLGMGNGOGPGQGRGSGTGUGVGWGXGYGZG[G\\G]G^G_G`GaGbGcGdGeGfGgGhGiGjGkGlGmGnGoGpGqGrGsGtGuGvGwGxGyHBHCHDHEHFHGHHHIHJHKHLHMHNHOHPHQHRHSHTHUHVHWHXHYHZH[H\\H]H^H_H`HaHbHcHdHeHfHgHhHiHjHkHlHmHnHoHpHqHrHsHtHuHvHwHxHyIBICIDIEIFIGIHIIIJIKILIMINIOIPIQIRISITIUIVIWIXIYIZI[I\\I]I^I_I`IaIbIcIdIeIfIgIhIiIjIkIlImInIoIpIqIrIsItIuIvIwIxIyJBJCJDJEJFJGJHJIJJJKJLJMJNJOJPJQJRJSJTJUJVJWJXJYJZJ[J\\J]J^J_J`JaJbJcJdJeJfJgJhJiJjJkJlJmJnJoJpJqJrJsJtJuJvJwJxJyKBKCKDKEKFKGKHKIKJKKKLKMKNKOKPKQKRKSKTKUKVKWKXKYKZK[K\\K]K^K_K`KaKbKcKdKeKfKgKhKiKjKkKlKmKnKoKpKqKrKsKtKuKvKwKxKyLBLCLDLELFLGLHLILJLKLLLMLNLOLPLQLRLSLTLULVLWLXLYLZL[L\\L]L^L_L`LaLbLcLdLeLfLgLhLiLjLkLlLmLnLoLpLqLrLsLtLuLvLwLxLyMBMCMDMEMFMGMHMIMJMKMLMMMNMOMPMQMRMSMTMUMVMWMXMYMZM[M\\M]M^M_M`MaMbMcMdMeMfMgMhMiMjMkMlMmMnMoMpMqMrMsMtMuMvMwMxMyNBNCNDNENFNGNHNINJNKNLNMNNNONPNQNRNSNTNUNVNWNXNYNZN[N\\N]N^N_N`NaNbNcNdNeNfNgNhNiNjNkNlNmNnNoNpNqNrNsNtNuNvNwNxNyOBOCODOEOFOGOHOIOJOKOLOMONOOOPOQOROSOTOUOVOWOXOYOZO[O\\O]O^O_O`OaObOcOdOeOfOgOhOiOjOkOlOmOnOoOpOqOrOsOtOuOvOwOxOyPBPCPDPEPFPGPHPIPJPKPLPMPNPOPPPQPRPSPTPUPVPWPXPYPZP[P\\P]P^P_P`PaPbPcPdPePfPgPhPiPjPkPlPmPnPoPpPqPrPsPtPuPvPwPxPyQBQCQDQEQFQGQHQIQJQKQLQMQNQOQPQQQRQSQTQUQVQWQXQYQZQ[Q\\Q]Q^Q_Q`QaQbQcQdQeQfQgQhQiQjQkQlQmQnQoQpQqQrQsQtQuQvQwQxQyRBRCRDRERFRGRHRIRJRKRLRMRNRORPRQRRRSRTRURVRWRXRYRZR[R\\R]R^R_R`RaRbRcRdReRfRgRhRiRjRkRlRmRnRoRpRqRrRsRtRuRvRwRxRySBSCSDSESFSGSHSISJSKSLSMSNSOSPSQSRSSSTSUSVSWSXSYSZS[S\\S]S^S_S`SaSbScSdSeSfSgShSiSjSkSlSmSnSoSpSqSrSsStSuSvSwSxSyTBTCTDTETFTGTHTITJTKTLTMTNTOTPTQTRTSTTTUTVTWTXTYTZT[T\\T]T^T_T`TaTbTcTdTeTfTgThTiTjTkTlTmTnToTpTqTrTsTtTuTvTwTxTyUBUCUDUEUFUGUHUIUJUKULUMUNUOUPUQURUSUTUUUVUWUXUYUZU[U\\U]U^U_U`UaUbUcUdUeUfUgUhUiUjUkUlUmUnUoUpUqUrUsUtUuUvUwUxUyVBVCVDVEVFVGVHVIVJVKVLVMVNVOVPVQVRVSVTVUVVVWVXVYVZV[V\\V]V^V_V`VaVbVcVdVeVfVgVhViVjVkVlVmVnVoVpVqVrVsVtVuVvVwVxVyWBWCWDWEWFWGWHWIWJWKWLWMWNWOWPWQWRWSWTWUWVWWWXWYWZW[W\\W]W^W_W`WaWbWcWdWeWfWgWhWiWjWkWlWmWnWoWpWqWrWsWtWuWvWwWxWyXBXCXDXEXFXGXHXIXJXKXLXMXNXOXPXQXRXSXTXUXVXWXXXYXZX[X\\X]X^X_X`XaXbXcXdXeXfXgXhXiXjXkXlXmXnXoXpXqXrXsXtXuXvXwXxXyYBYCYDYEYFYGYHYIYJYKYLYMYNYOYPYQYRYSYTYUYVYWYXYYYZY[Y\\Y]Y^Y_Y`YaYbYcYdYeYfYgYhYiYjYkYlYmYnYoYpYqYrYsYtYuYvYwYxYyZBZCZDZEZFZGZHZIZJZKZLZMZNZOZPZQZRZSZTZUZVZWZXZYZZZ[Z\\Z]Z^Z_Z`ZaZbZcZdZeZfZgZhZiZjZkZlZmZnZoZpZqZrZsZtZuZvZwZxZy[B[C[D[E[F[G[H[I[J[K[L[M[N[O[P[Q[R[S[T[U[V[W[X[Y[Z[[[\\[][^[_[`[a[b[c[d[e[f[g[h[i[j[k[l[m[n[o[p[q[r[s[t[u[v[w[x[y\\B\\C\\D\\E\\F\\G\\H\\I\\J\\K\\L\\M\\N\\O\\P\\Q\\R\\S\\T\\U\\V\\W\\X\\Y\\Z\\[\\\\\\]\\^\\_\\`\\a\\b\\c\\d\\e\\f\\g\\h\\i\\j\\k\\l\\m\\n\\o\\p\\q\\r\\s\\t\\u\\v\\w\\x\\y]B]C]D]E]F]G]H]I]J]K]L]M]N]O]P]Q]R]S]T]U]V]W]X]Y]Z][]\\]]]^]_]`]a]b]c]d]e]f]g]h]i]j]k]l]m]n]o]p]q]r]s]t]u]v]w]x]y^B^C^D^E^F^G^H^I^J^K^L^M^N^O^P^Q^R^S^T^U^V^W^X^Y^Z^[^\\^]^^^_^`^a^b^c^d^e^f^g^h^i^j^k^l^m^n^o^p^q^r^s^t^u^v^w^x^y_B_C_D_E_F_G_H_I_J_K_L_M_N_O_P_Q_R_S_T_U_V_W_X_Y_Z_[_\\_]_^___`_a_b_c_d_e_f_g_h_i_j_k_l_m_n_o_p_q_r_s_t_u_v_w_x_y`B`C`D`E`F`G`H`I`J`K`L`M`N`O`P`Q`R`S`T`U`V`W`X`Y`Z`[`\\`]`^`_```a`b`c`d`e`f`g`h`i`j`k`l`m`n`o`p`q`r`s`t`u`v`w`x`yaBaCaDaEaFaGaHaIaJaKaLaMaNaOaPaQaRaSaTaUaVaWaXaYaZa[a\\a]a^a_a`aaabacadaeafagahaiajakalamanaoapaqarasatauavawaxaybBbCbDbEbFbGbHbIbJbKbLbMbNbObPbQbRbSbTbUbVbWbXbYbZb[b\\b]b^b_b`babbbcbdbebfbgbhbibjbkblbmbnbobpbqbrbsbtbubvbwbxbycBcCcDcEcFcGcHcIcJcKcLcMcNcOcPcQcRcScTcUcVcWcXcYcZc[c\\c]c^c_c`cacbcccdcecfcgchcicjckclcmcncocpcqcrcsctcucvcwcxcydBdCdDdEdFdGdHdIdJdKdLdMdNdOdPdQdRdSdTdUdVdWdXdYdZd[d\\d]d^d_d`dadbdcdddedfdgdhdidjdkdldmdndodpdqdrdsdtdudvdwdxdyeBeCeDeEeFeGeHeIeJeKeLeMeNeOePeQeReSeTeUeVeWeXeYeZe[e\\e]e^e_e`eaebecedeeefegeheiejekelemeneoepeqereseteuevewexeyfBfCfDfEfFfGfHfIfJfKfLfMfNfOfPfQfRfSfTfUfVfWfXfYfZf[f\\f]f^f_f`fafbfcfdfefffgfhfifjfkflfmfnfofpfqfrfsftfufvfwfxfygBgCgDgEgFgGgHgIgJgKgLgMgNgOgPgQgRgSgTgUgVgWgXgYgZg[g\\g]g^g_g`gagbgcgdgegfggghgigjgkglgmgngogpgqgrgsgtgugvgwgxgyhBhChDhEhFhGhHhIhJhKhLhMhNhOhPhQhRhShThUhVhWhXhYhZh[h\\h]h^h_h`hahbhchdhehfhghhhihjhkhlhmhnhohphqhrhshthuhvhwhxhyiBiCiDiEiFiGiHiIiJiKiLiMiNiOiPiQiRiSiTiUiViWiXiYiZi[i\\i]i^i_i`iaibicidieifigihiiijikiliminioipiqirisitiuiviwixiyjBjCjDjEjFjGjHjIjJjKjLjMjNjOjPjQjRjSjTjUjVjWjXjYjZj[j\\j]j^j_j`jajbjcjdjejfjgjhjijjjkjljmjnjojpjqjrjsjtjujvjwjxjykBkCkDkEkFkGkHkIkJkKkLkMkNkOkPkQkRkSkTkUkVkWkXkYkZk[k\\k]k^k_k`kakbkckdkekfkgkhkikjkkklkmknkokpkqkrksktkukvkwkxkylBlClDlElFlGlHlIlJlKlLlMlNlOlPlQlRlSlTlUlVlWlXlYlZl[l\\l]l^l_l`lalblcldlelflglhliljlklllmlnlolplqlrlsltlulvlwlxlymBmCmDmEmFmGmHmImJmKmLmMmNmOmPmQmRmSmTmUmVmWmXmYmZm[m\\m]m^m_m`mambmcmdmemfmgmhmimj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+        id="Table overflow",
+    ),
+]
+
+
+@pytest.mark.parametrize("data", test_cases)
+def test_encode_decode(data):
+    """Decoder and encoder match."""
+    codec = LzwCodec()
+    compressed_data = codec.encode(data)
+    decoded = codec.decode(compressed_data)
+    assert decoded == data
+
+
+@pytest.mark.parametrize(
+    ("plain", "expected_encoded"),
+    [
+        (b"", b"\x80@@"),
+        (b"A", b"\x80\x10` "),
+        (b"AAAAAA", b"\x80\x10`P8\x08"),
+        (b"Hello, World!", b"\x80\x12\x0c\xa6\xc3a\xbcX +\x9b\xceF\xc3 \x86\x02"),
+    ],
+)
+def test_encode_lzw(plain, expected_encoded):
+    codec = LzwCodec()
+    actual_encoded = codec.encode(plain)
+    assert actual_encoded == expected_encoded