C0urante commented on code in PR #15379:
URL: https://github.com/apache/kafka/pull/15379#discussion_r1570960525


##########
connect/transforms/src/main/java/org/apache/kafka/connect/transforms/field/SingleFieldPath.java:
##########
@@ -0,0 +1,258 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+package org.apache.kafka.connect.transforms.field;
+
+import org.apache.kafka.common.config.ConfigException;
+import org.apache.kafka.connect.data.Field;
+import org.apache.kafka.connect.data.Schema;
+import org.apache.kafka.connect.data.Struct;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * A SingleFieldPath is composed of one or more field names, known as path 
steps,
+ * to access values within a data object (either {@code Struct} or {@code 
Map<String, Object>}).
+ *
+ * <p>If the SMT requires accessing multiple fields on the same data object, 
use {@link MultiFieldPaths} instead.
+ *
+ * <p>The field path semantics are defined by the {@link FieldSyntaxVersion 
syntax version}.
+ *
+ * @see <a 
href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-821%3A+Connect+Transforms+support+for+nested+structures";>KIP-821</a>
+ * @see FieldSyntaxVersion
+ * @see MultiFieldPaths
+ */
+public class SingleFieldPath {
+    // Invariants:
+    // - A field path can contain one or more steps
+    private static final char BACKTICK = '`';
+    private static final char DOT = '.';
+    private static final char BACKSLASH = '\\';
+
+    private final String originalPath;
+    private final FieldSyntaxVersion version;
+    private final List<String> steps;
+
+    public SingleFieldPath(String pathText, FieldSyntaxVersion version) {
+        this.originalPath = Objects.requireNonNull(pathText, "Field path 
cannot be null");
+        this.version = version;
+        switch (version) {
+            case V1: // backward compatibility
+                this.steps = Collections.singletonList(pathText);
+                break;
+            case V2:
+                this.steps = buildFieldPathV2(pathText);
+                break;
+            default:
+                throw new IllegalArgumentException("Unknown syntax version: " 
+ version);
+        }
+    }
+
+    private static List<String> buildFieldPathV2(String path) {
+        final List<String> steps = new ArrayList<>();
+        // path character index to track backticks and dots and break path 
into steps
+        int idx = 0;
+        while (idx < path.length() && idx >= 0) {
+            if (path.charAt(idx) != BACKTICK) {
+                final int start = idx;
+                idx = path.indexOf(String.valueOf(DOT), idx);
+                if (idx >= 0) { // get path step and move forward
+                    String field = path.substring(start, idx);
+                    steps.add(field);
+                    idx++;
+                } else { // add all
+                    String field = path.substring(start);
+                    steps.add(field);
+                }
+            } else { // has backtick
+                int backtickAt = idx;
+                idx++;
+                StringBuilder field = new StringBuilder();
+                int start = idx;
+                while (true) {
+                    // find closing backtick
+                    idx = path.indexOf(String.valueOf(BACKTICK), idx);
+                    if (idx == -1) { // if not found, then fail
+                        failWhenIncompleteBacktickPair(path, backtickAt);
+                    }
+
+                    // backtick escaped if right after backslash
+                    boolean escaped = path.charAt(idx - 1) == BACKSLASH;
+
+                    if (idx >= path.length() - 1) { // at the end of path
+                        if (escaped) { // but escaped, then fail
+                            failWhenIncompleteBacktickPair(path, backtickAt);
+                        }
+                        field.append(path, start, idx);
+                        // we've reached the end of the path, and the last 
character is the backtick
+                        steps.add(field.toString());
+                        idx++;
+                        break;
+                    }
+
+                    if (path.charAt(idx + 1) != DOT) { // not followed by a dot
+                        // this backtick isn't followed by a dot; include it 
in the field name, but continue
+                        // looking for a matching backtick that is followed by 
a dot
+                        idx++;
+                        continue;
+                    }
+
+                    if (escaped) {
+                        // this backtick was escaped; include it in the field 
name, but continue
+                        // looking for an unescaped matching backtick
+                        field.append(path, start, idx - 1)
+                            .append(BACKTICK);
+
+                        idx++;
+                        start = idx;
+                        continue;
+                    }
+
+                    // we've found our matching backtick
+                    field.append(path, start, idx);
+                    steps.add(field.toString());
+                    idx += 2; // increment by two to include the backtick and 
the dot after it
+                    break;
+                }
+            }
+        }
+        // add last step if last char is a dot
+        if (!path.isEmpty() && path.charAt(path.length() - 1) == DOT)
+            steps.add("");
+        return Collections.unmodifiableList(steps);
+    }
+
+    private static void failWhenIncompleteBacktickPair(String path, int 
backtickAt) {
+        throw new ConfigException("Incomplete backtick pair in path: [" + path 
+ "],"
+                + " consider adding a backslash before backtick at position " 
+ backtickAt
+                + " to escape it");
+    }
+
+    public String originalPath() {
+        return this.originalPath;
+    }
+
+
+    /**
+     * Access a {@code Field} at the current path within a schema {@code 
Schema}
+     * If field is not found, then {@code null} is returned.
+     */
+    public Field fieldFrom(Schema schema) {
+        if (schema == null) return null;
+
+        Schema current = schema;
+        for (String pathSegment : stepsWithoutLast()) {
+            final Field field = current.field(pathSegment);
+            if (field != null) {
+                current = field.schema();
+            } else {
+                return null;
+            }
+        }
+        return current.field(lastStep());
+    }
+
+    /**
+     * Access a value at the current path within a schema-based {@code Struct}
+     * If object is not found, then {@code null} is returned.
+     */
+    public Object valueFrom(Struct struct) {
+        if (struct == null) return null;
+
+        Struct current = struct;
+        for (String pathSegment : stepsWithoutLast()) {
+            // Check to see if the field actually exists
+            if (current.schema().field(pathSegment) == null) {
+                return null;
+            }
+            current = current.getStruct(pathSegment);
+        }
+
+        if (current.schema().field(lastStep()) != null) {
+            return current.get(lastStep());
+        } else {
+            return null;
+        }
+    }
+
+    List<String> stepsWithoutLast() {
+        return steps.subList(0, lastStepIndex());
+    }
+
+    /**
+     * Access a value at the current path within a schemaless {@code 
Map<String, Object>}.
+     * If object is not found, then {@code null} is returned.
+     */
+    @SuppressWarnings("unchecked")
+    public Object valueFrom(Map<String, Object> map) {
+        if (map == null) return null;
+
+        if (steps.size() == 1) {
+            return map.get(steps.get(0));
+        } else {
+            Map<String, Object> current = map;
+            for (int i = 0; i < steps.size(); i++) {
+                if (current == null) {
+                    return null;
+                }
+                if (i == lastStepIndex()) {
+                    return current.get(steps.get(i));
+                } else {
+                    current = (Map<String, Object>) current.get(steps.get(i));
+                }
+            }
+        }
+        return null;

Review Comment:
   We can clean this up a bit:
   ```suggestion
           if (map == null) return null;
   
           Map<String, Object> current = map;
           for (String step : stepsWithoutLast()) {
               current = requireMapOrNull(current.get(step), "nested field 
access");
   
               if (current == null)
                   return null;
           }
           return current.get(lastStep());
   ```
   
   Using `requireMapOrNull` (statically imported from 
`org.apache.kafka.connect.transforms.util.Requirements`) makes this more 
type-safe, and leveraging `stepsWithoutLast` and `lastStep` helps simplify the 
logic.



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