TST: Add LzwCodec for encoding

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