docs: Generate expression compatibility docs from code

docs/source/contributor-guide/adding_a_new_expression.md

@@ -70,10 +70,12 @@ object CometUnhex extends CometExpressionSerde[Unhex] {
 }
 ```
 
-The `CometExpressionSerde` trait provides three methods you can override:
+The `CometExpressionSerde` trait provides several methods you can override:
 
 - `convert(expr: T, inputs: Seq[Attribute], binding: Boolean): Option[Expr]` - **Required**. Converts the Spark expression to protobuf. Return `None` if the expression cannot be converted.
-- `getSupportLevel(expr: T): SupportLevel` - Optional. Returns the level of support for the expression. See "Using getSupportLevel" section below for details.
+- `getSupportLevel(expr: T): SupportLevel` - Optional. Returns the level of support for the expression at planning time, based on a specific expression instance. See "Using getSupportLevel" section below for details.
+- `getIncompatibleReasons(): Seq[String]` - Optional. Returns reasons why this expression may produce different results than Spark. Used to generate the Compatibility Guide. See "Documenting Incompatible and Unsupported Reasons" below.
+- `getUnsupportedReasons(): Seq[String]` - Optional. Returns reasons why this expression may not be supported by Comet (for example, unsupported data types or format strings). Used to generate the Compatibility Guide. See "Documenting Incompatible and Unsupported Reasons" below.
 - `getExprConfigName(expr: T): String` - Optional. Returns a short name for configuration keys. Defaults to the Spark class name.
 
 For simple scalar functions that map directly to a DataFusion function, you can use the built-in `CometScalarFunction` implementation:
@@ -208,6 +210,65 @@ When the query planner encounters an expression:
 
 Any notes provided will be logged to help with debugging and understanding why an expression was not used.
 
+#### Documenting Incompatible and Unsupported Reasons
+
+In addition to `getSupportLevel`, which governs runtime planning decisions, the serde trait exposes two static documentation methods:
+
+- `getIncompatibleReasons(): Seq[String]` - Reasons the expression may produce different results than Spark.
+- `getUnsupportedReasons(): Seq[String]` - Reasons the expression, or certain usages of it, may not be supported by Comet.
+
+These methods do not affect runtime behavior. They are called by `GenerateDocs` (`spark/src/main/scala/org/apache/comet/GenerateDocs.scala`) when building the user-facing Compatibility Guide pages under `docs/source/user-guide/latest/compatibility/expressions/` (for example, `math.md`, `datetime.md`, `array.md`, `aggregate.md`, `struct.md`). Each reason is rendered as a bullet in the corresponding page.
+
+Key differences from `getSupportLevel`:
+
+- **No expression instance.** Both methods take no arguments, so they describe the expression in general rather than a specific call site. Use `getSupportLevel` for checks that depend on data types, argument values, or other per-instance details.
+- **Markdown-friendly.** Each returned string is written to a Markdown document, so you can embed backticks, links, and line breaks. Keep each reason self-contained, since they are rendered as separate bullets.
+- **Regenerated by CI.** The lists are collected by `GenerateDocs` and published by CI on every merge to `main`. The generated Markdown is not committed to the repo, so you do not need to regenerate or commit it yourself. The reasons do not have to match the `notes` passed to `Compatible`, `Incompatible`, or `Unsupported`, but keeping them consistent avoids confusing users.
+
+##### Example: Incompatibility note
+
+```scala
+object CometStructsToJson extends CometExpressionSerde[StructsToJson] {
+
+  override def getIncompatibleReasons(): Seq[String] = Seq(
+    "Does not support `+Infinity` and `-Infinity` for numeric types (float, double)." +
+      " (https://github.com/apache/datafusion-comet/issues/3016)")
+
+  override def getSupportLevel(expr: StructsToJson): SupportLevel =
+    Incompatible(
+      Some(
+        "Does not support Infinity/-Infinity for numeric types" +
+          " (https://github.com/apache/datafusion-comet/issues/3016)"))
+
+  // ... convert ...
+}
+```
+
+##### Example: Unsupported note
+
+```scala
+object CometSortArray extends CometExpressionSerde[SortArray] {
+
+  override def getUnsupportedReasons(): Seq[String] = Seq(
+    "Nested arrays with `Struct` or `Null` child values are not supported natively and will" +
+      " fall back to Spark.")
+
+  // ... convert ...
+}
+```
+
+##### Example: Enumerating supported values
+
+When the expression only supports a known set of argument values, build the reason from that set so the documentation stays in sync with the code:
+
+```scala
+override def getUnsupportedReasons(): Seq[String] = Seq(
+  "Only the following formats are supported:" +
+    supportedFormats.keys.toSeq.sorted
+      .map(k => s"`$k`")
+      .mkString("\n  - ", "\n  - ", ""))
+```
+
 #### Adding Spark-side Tests for the New Expression
 
 It is important to verify that the new expression is correctly recognized by the native execution engine and matches the expected Spark behavior. The preferred way to add test coverage is to write a SQL test file using the SQL file test framework. This approach is simpler than writing Scala test code and makes it easy to cover many input combinations and edge cases.

docs/source/user-guide/latest/compatibility/expressions/aggregate.md

@@ -19,16 +19,5 @@ under the License.
 
 # Aggregate Expressions
 
-## Incompatible Aggregates
-
-- **CollectSet**: Comet deduplicates NaN values (treats `NaN == NaN`) while Spark treats each NaN as a distinct value.
-  When `spark.comet.exec.strictFloatingPoint=true`, `collect_set` on floating-point types falls back to Spark unless
-  `spark.comet.expression.CollectSet.allowIncompatible=true` is set.
-
-## ANSI Mode
-
-Comet will fall back to Spark for the following aggregate expressions when ANSI mode is enabled. These can be enabled by setting `spark.comet.expression.EXPRNAME.allowIncompatible=true`, where `EXPRNAME` is the Spark expression class name. See the [Comet Supported Expressions Guide](../../expressions.md) for more information on this configuration setting.
-
-- Average (supports all numeric inputs except decimal types)
-
-There is an [epic](https://github.com/apache/datafusion-comet/issues/313) where we are tracking the work to fully implement ANSI support.
+<!--BEGIN:EXPR_COMPAT[aggregate]-->
+<!--END:EXPR_COMPAT-->

docs/source/user-guide/latest/compatibility/expressions/array.md

@@ -19,4 +19,5 @@ under the License.
 
 # Array Expressions
 
-- **SortArray**: Nested arrays with `Struct` or `Null` child values are not supported natively and will fall back to Spark.
+<!--BEGIN:EXPR_COMPAT[array]-->
+<!--END:EXPR_COMPAT-->

docs/source/user-guide/latest/compatibility/expressions/datetime.md

@@ -19,9 +19,5 @@ under the License.
 
 # Date/Time Expressions
 
-- **Hour, Minute, Second**: Incorrectly apply timezone conversion to TimestampNTZ inputs. TimestampNTZ stores local
-  time without timezone, so no conversion should be applied. These expressions work correctly with Timestamp inputs.
-  [#3180](https://github.com/apache/datafusion-comet/issues/3180)
-- **TruncTimestamp (date_trunc)**: Produces incorrect results when used with non-UTC timezones. Compatible when
-  timezone is UTC.
-  [#2649](https://github.com/apache/datafusion-comet/issues/2649)
+<!--BEGIN:EXPR_COMPAT[datetime]-->
+<!--END:EXPR_COMPAT-->

docs/source/user-guide/latest/compatibility/expressions/index.md

@@ -31,6 +31,7 @@ Compatibility notes are grouped by expression category:
 aggregate
 array
 datetime
+math
 struct
 cast
 ```

docs/source/user-guide/latest/compatibility/expressions/math.md

@@ -0,0 +1,23 @@
+<!---
+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.
+-->
+
+# Math Expressions
+
+<!--BEGIN:EXPR_COMPAT[math]-->
+<!--END:EXPR_COMPAT-->

docs/source/user-guide/latest/compatibility/expressions/struct.md

@@ -19,5 +19,5 @@ under the License.
 
 # Struct Expressions
 
-- **StructsToJson (to_json)**: Does not support `+Infinity` and `-Infinity` for numeric types (float, double).
-  [#3016](https://github.com/apache/datafusion-comet/issues/3016)
+<!--BEGIN:EXPR_COMPAT[struct]-->
+<!--END:EXPR_COMPAT-->

spark/src/main/scala/org/apache/comet/GenerateDocs.scala

@@ -39,10 +39,48 @@ object GenerateDocs {
 
   private val publicConfigs: Set[ConfigEntry[_]] = CometConf.allConfs.filter(_.isPublic).toSet
 
+  /** (expression class simple name, incompatible reasons, unsupported reasons) */
+  private type CategoryNotes = Seq[(String, Seq[String], Seq[String])]
+
+  /**
+   * Mapping from expression category to the compatibility guide page where that category's
+   * auto-generated notes should be written, along with a function that produces the notes for
+   * that category from the serde maps in `QueryPlanSerde`.
+   */
+  private def categoryPages: Map[String, (String, () => CategoryNotes)] = Map(
+    "array" -> ("compatibility/expressions/array.md",
+    () =>
+      QueryPlanSerde.arrayExpressions.toSeq.map { case (cls, serde) =>
+        (cls.getSimpleName, serde.getIncompatibleReasons(), serde.getUnsupportedReasons())
+      }),
+    "datetime" -> ("compatibility/expressions/datetime.md",
+    () =>
+      QueryPlanSerde.temporalExpressions.toSeq.map { case (cls, serde) =>
+        (cls.getSimpleName, serde.getIncompatibleReasons(), serde.getUnsupportedReasons())
+      }),
+    "math" -> ("compatibility/expressions/math.md",
+    () =>
+      QueryPlanSerde.mathExpressions.toSeq.map { case (cls, serde) =>
+        (cls.getSimpleName, serde.getIncompatibleReasons(), serde.getUnsupportedReasons())
+      }),
+    "struct" -> ("compatibility/expressions/struct.md",
+    () =>
+      QueryPlanSerde.structExpressions.toSeq.map { case (cls, serde) =>
+        (cls.getSimpleName, serde.getIncompatibleReasons(), serde.getUnsupportedReasons())
+      }),
+    "aggregate" -> ("compatibility/expressions/aggregate.md",
+    () =>
+      QueryPlanSerde.aggrSerdeMap.toSeq.map { case (cls, serde) =>
+        (cls.getSimpleName, serde.getIncompatibleReasons(), serde.getUnsupportedReasons())
+      }))
+
   def main(args: Array[String]): Unit = {
     val userGuideLocation = args(0)
     generateConfigReference(s"$userGuideLocation/configs.md")
     generateCompatibilityGuide(s"$userGuideLocation/compatibility/expressions/cast.md")
+    for ((category, (page, notesFn)) <- categoryPages) {
+      generateExpressionCompatNotes(s"$userGuideLocation/$page", category, notesFn())
+    }
   }
 
   private def generateConfigReference(filename: String): Unit = {
@@ -121,6 +159,46 @@ object GenerateDocs {
     w.close()
   }
 
+  private def generateExpressionCompatNotes(
+      filename: String,
+      category: String,
+      notes: CategoryNotes): Unit = {
+    val beginTag = s"<!--BEGIN:EXPR_COMPAT[$category]-->"
+    val lines = readFile(filename)
+    val w = new BufferedOutputStream(new FileOutputStream(filename))
+    for (line <- lines) {
+      w.write(s"${line.stripTrailing()}\n".getBytes)
+      if (line.trim == beginTag) {
+        writeExpressionCompatNotes(w, notes)
+      }
+    }
+    w.close()
+  }
+
+  private def writeExpressionCompatNotes(w: BufferedOutputStream, notes: CategoryNotes): Unit = {
+    val sorted = notes.sortBy(_._1).filter { case (_, incompat, unsupported) =>
+      incompat.nonEmpty || unsupported.nonEmpty
+    }
+    for ((name, incompat, unsupported) <- sorted) {
+      w.write(s"\n### $name\n".getBytes)
+      if (incompat.nonEmpty) {
+        w.write(
+          (s"\nThe following incompatibilities cause `$name` to fall back to Spark by default." +
+            s" Set `spark.comet.expression.$name.allowIncompatible=true` to enable Comet" +
+            " acceleration despite these differences.\n\n").getBytes)
+        for (reason <- incompat) {
+          w.write(s"- $reason\n".getBytes)
+        }
+      }
+      if (unsupported.nonEmpty) {
+        w.write("\nThe following cases are not supported by Comet:\n\n".getBytes)
+        for (reason <- unsupported) {
+          w.write(s"- $reason\n".getBytes)
+        }
+      }
+    }
+  }
+
   private def writeCastMatrixForMode(w: BufferedOutputStream, mode: CometEvalMode.Value): Unit = {
     val sortedTypes = CometCast.supportedTypes.sortBy(_.typeName)
     val typeNames = sortedTypes.map(_.typeName.replace("(10,2)", ""))

spark/src/main/scala/org/apache/comet/serde/CometAggregateExpressionSerde.scala

@@ -39,6 +39,26 @@ trait CometAggregateExpressionSerde[T <: AggregateFunction] {
    */
   def getExprConfigName(expr: T): String = expr.getClass.getSimpleName
 
+  /**
+   * Get documentation for usages where this expression may be incompatible with Spark. This is
+   * called from GenerateDocs when generating the Compatibility Guide. Each reason should be
+   * written in Markdown and may span multiple lines.
+   *
+   * @return
+   *   List of reasons, defaulting to an empty list.
+   */
+  def getIncompatibleReasons(): Seq[String] = Seq.empty
+
+  /**
+   * Get documentation for usages where this expression is unsupported with Spark. This is called
+   * from GenerateDocs when generating the Compatibility Guide. Each reason should be written in
+   * Markdown and may span multiple lines.
+   *
+   * @return
+   *   List of reasons, defaulting to an empty list.
+   */
+  def getUnsupportedReasons(): Seq[String] = Seq.empty
+
   /**
    * Determine the support level of the expression based on its attributes.
    *

spark/src/main/scala/org/apache/comet/serde/CometExpressionSerde.scala

@@ -37,6 +37,26 @@ trait CometExpressionSerde[T <: Expression] {
    */
   def getExprConfigName(expr: T): String = expr.getClass.getSimpleName
 
+  /**
+   * Get documentation for usages where this expression may be incompatible with Spark. This is
+   * called from GenerateDocs when generating the Compatibility Guide. Each reason should be
+   * written in Markdown and may span multiple lines.
+   *
+   * @return
+   *   List of reasons, defaulting to an empty list.
+   */
+  def getIncompatibleReasons(): Seq[String] = Seq.empty
+
+  /**
+   * Get documentation for usages where this expression is unsupported with Spark. This is called
+   * from GenerateDocs when generating the Compatibility Guide. Each reason should be written in
+   * Markdown and may span multiple lines.
+   *
+   * @return
+   *   List of reasons, defaulting to an empty list.
+   */
+  def getUnsupportedReasons(): Seq[String] = Seq.empty
+
   /**
    * Determine the support level of the expression based on its attributes.
    *