feat(compression): add Compressor protocol (#3590)

Define the plugin interface for compression methods. Each compressor
implements the Compressor protocol with a compress() method that returns
encoded data and ancillary data.

BUG=part of #3256

Author: rkuester

tensorflow/lite/micro/compression/BUILD

@@ -261,6 +261,16 @@ tflm_py_test(
     ],
 )
 
+tflm_py_library(
+    name = "compressor",
+    srcs = ["compressor.py"],
+    deps = [
+        ":decode",
+        ":model_editor",
+        ":spec",
+    ],
+)
+
 tflm_py_binary(
     name = "view",
     srcs = [

tensorflow/lite/micro/compression/compressor.py

@@ -0,0 +1,80 @@
+# Copyright 2026 The TensorFlow Authors. All Rights Reserved.
+#
+# 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.
+"""Compression plugin interface."""
+
+from dataclasses import dataclass
+from typing import Protocol
+
+from tflite_micro.tensorflow.lite.micro.compression import decode
+from tflite_micro.tensorflow.lite.micro.compression import model_editor
+from tflite_micro.tensorflow.lite.micro.compression import spec
+
+
+class CompressionError(Exception):
+  """Raised when compression fails for the reason documented in the message."""
+
+  def __init__(self, message, wrapped_exception=None):
+    if wrapped_exception:
+      super().__init__(f"{message}: {str(wrapped_exception)}")
+    else:
+      super().__init__(message)
+    self.original_exception = wrapped_exception
+
+
+@dataclass
+class CompressionResult:
+  """Result of compressing a tensor.
+
+  Attributes:
+    encoded_data: The compressed tensor data (e.g., packed indices for LUT).
+    ancillary_data: The complete ancillary data tensor bytes (DCM + type-specific
+                    data). This is the full buffer contents for the ancillary
+                    tensor.
+  """
+  encoded_data: bytes
+  ancillary_data: bytes
+
+
+class Compressor(Protocol):
+  """Protocol that compression plugins must implement.
+
+  Each compression method (LUT, Huffman, Pruning) provides a class implementing
+  this protocol. The compress() function uses duck typing to call the plugin.
+  """
+
+  @property
+  def decode_type(self) -> decode.DecodeType:
+    """The DecodeType constant for this compression method."""
+    ...
+
+  def compress(
+      self,
+      tensor: model_editor.Tensor,
+      method: spec.CompressionMethod,
+  ) -> CompressionResult:
+    """Compress a tensor according to the specified method.
+
+    Args:
+      tensor: The tensor to compress. Must have data (tensor.array is not None)
+              and quantization parameters for axis inference.
+      method: The compression method spec (e.g., LookUpTableCompression).
+
+    Returns:
+      CompressionResult with encoded tensor data and ancillary data bytes.
+
+    Raises:
+      CompressionError: If compression fails (e.g., too many unique values
+                        for specified bitwidth, missing quantization, etc.).
+    """
+    ...