Add document Id generator

- Clean uneccessary classes/interfaces
 - straighten code style.
 - Clean tests.

 Resolves #113

spring-ai-core/src/test/java/org/springframework/ai/document/id/IdGeneratorProviderTest.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2024-2024 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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.
+ */
+package org.springframework.ai.document.id;
+
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class IdGeneratorProviderTest {
+
+	@Test
+	void hashGeneratorGenerateSimilarIdsForSimilarContent() {
+
+		var idGenerator1 = new JdkSha256HexIdGenerator();
+		var idGenerator2 = new JdkSha256HexIdGenerator();
+
+		final String content = "Content";
+		final Map<String, Object> metadata = Map.of("metadata", Set.of("META_DATA"));
+
+		String actualHashes1 = idGenerator1.generateId(content, metadata);
+		String actualHashes2 = idGenerator2.generateId(content, metadata);
+
+		Assertions.assertEquals(actualHashes1, actualHashes2);
+
+		// Assert (other expected behaviors)
+		Assertions.assertDoesNotThrow(() -> UUID.fromString(actualHashes1));
+		Assertions.assertDoesNotThrow(() -> UUID.fromString(actualHashes2));
+	}
+
+	@Test
+	void hashGeneratorGenerateDifferentIdsForDifferentContent() {
+
+		var idGenerator1 = new JdkSha256HexIdGenerator();
+		var idGenerator2 = new JdkSha256HexIdGenerator();
+
+		final String content1 = "Content";
+		final Map<String, Object> metadata1 = Map.of("metadata", Set.of("META_DATA"));
+		final String content2 = content1 + " ";
+		final Map<String, Object> metadata2 = metadata1;
+
+		String actualHashes1 = idGenerator1.generateId(content1, metadata1);
+		String actualHashes2 = idGenerator2.generateId(content2, metadata2);
+
+		Assertions.assertNotEquals(actualHashes1, actualHashes2);
+
+		// Assert (other expected behaviors)
+		Assertions.assertDoesNotThrow(() -> UUID.fromString(actualHashes1));
+		Assertions.assertDoesNotThrow(() -> UUID.fromString(actualHashes2));
+	}
+
+}

spring-ai-core/src/test/java/org/springframework/ai/document/id/JdkSha256HexIdGeneratorTest.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2024-2024 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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.
+ */
+package org.springframework.ai.document.id;
+
+import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+
+import org.assertj.core.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class JdkSha256HexIdGeneratorTest {
+
+	private final JdkSha256HexIdGenerator testee = new JdkSha256HexIdGenerator();
+
+	@Test
+	void messageDigestReturnsDistinctInstances() {
+		final MessageDigest md1 = testee.getMessageDigest();
+		final MessageDigest md2 = testee.getMessageDigest();
+
+		Assertions.assertThat(md1 != md2).isTrue();
+
+		Assertions.assertThat(md1.getAlgorithm()).isEqualTo(md2.getAlgorithm());
+		Assertions.assertThat(md1.getDigestLength()).isEqualTo(md2.getDigestLength());
+		Assertions.assertThat(md1.getProvider()).isEqualTo(md2.getProvider());
+		Assertions.assertThat(md1.toString()).isEqualTo(md2.toString());
+	}
+
+	@Test
+	void messageDigestReturnsInstancesWithIndependentAndReproducibleDigests() {
+		final String updateString1 = "md1_update";
+		final String updateString2 = "md2_update";
+		final Charset charset = StandardCharsets.UTF_8;
+
+		final byte[] md1BytesFirstTry = testee.getMessageDigest().digest(updateString1.getBytes(charset));
+		final byte[] md2BytesFirstTry = testee.getMessageDigest().digest(updateString2.getBytes(charset));
+		final byte[] md1BytesSecondTry = testee.getMessageDigest().digest(updateString1.getBytes(charset));
+		final byte[] md2BytesSecondTry = testee.getMessageDigest().digest(updateString2.getBytes(charset));
+
+		Assertions.assertThat(md1BytesFirstTry).isNotEqualTo(md2BytesFirstTry);
+
+		Assertions.assertThat(md1BytesFirstTry).isEqualTo(md1BytesSecondTry);
+		Assertions.assertThat(md2BytesFirstTry).isEqualTo(md2BytesSecondTry);
+	}
+
+}