[V1][Spec Decode] Ngram Spec Decode (#12193)

Signed-off-by: LiuXiaoxuanPKU <lilyliupku@gmail.com>

tests/v1/spec_decode/test_ngram.py

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: Apache-2.0
+import pytest
+
+from vllm.v1.spec_decode.ngram_proposer import NgramProposer
+from vllm.v1.utils import ConstantList
+
+
+@pytest.fixture
+def proposer():
+    return NgramProposer()
+
+
+def test_kmp_lps_array(proposer):
+    assert proposer._kmp_lps_array([]) == []
+    assert proposer._kmp_lps_array([1]) == [0]
+    assert proposer._kmp_lps_array([1, 1, 1]) == [0, 1, 2]
+    assert proposer._kmp_lps_array([1, 2, 3, 4]) == [0, 0, 0, 0]
+    assert proposer._kmp_lps_array([1, 2, 1, 2, 3]) == [0, 0, 1, 2, 0]
+
+
+def test_find_subarray_kmp(proposer):
+    X = ConstantList([1, 2, 3, 4, 1, 2, 3, 5, 6])
+    assert proposer._find_subarray_kmp(X, 2, 2) is None
+    X = ConstantList([1, 2, 3, 4, 1, 2, 3])
+    assert proposer._find_subarray_kmp(X, 2, 3) == [4, 1, 2]
+    assert proposer._find_subarray_kmp(X, 2, 2) == [4, 1]
+    assert proposer._find_subarray_kmp(X, 1, 3) == [4, 1, 2]
+    assert proposer._find_subarray_kmp(X, 1, 2) == [4, 1]
+    X = ConstantList([1, 3, 6, 2, 3, 4, 1, 2, 3])
+    assert proposer._find_subarray_kmp(X, 2, 3) == [4, 1, 2]
+    # Return on the first match
+    assert proposer._find_subarray_kmp(X, 1, 3) == [6, 2, 3]