Enhance ConstantFoldingOptimizer to fold arithmetics involving timestamps and durations

Note: `evaluateCanonicalTypesToNativeValues` must be enabled for optimization to work
PiperOrigin-RevId: 813922645

Author: l46kok

checker/src/main/java/dev/cel/checker/CelStandardDeclarations.java

@@ -1488,7 +1488,7 @@ public CelFunctionDecl functionDecl() {
       return celFunctionDecl;
     }
 
-    String functionName() {
+    public String functionName() {
       return functionName;
     }
 

optimizer/src/main/java/dev/cel/optimizer/optimizers/BUILD.bazel

@@ -19,12 +19,14 @@ java_library(
         ":default_optimizer_constants",
         "//:auto_value",
         "//bundle:cel",
+        "//checker:standard_decl",
         "//common:cel_ast",
         "//common:cel_source",
         "//common:compiler_common",
         "//common:mutable_ast",
         "//common/ast",
         "//common/ast:mutable_expr",
+        "//common/internal:date_time_helpers",
         "//common/navigation:mutable_navigation",
         "//common/types",
         "//extensions:optional_library",

optimizer/src/main/java/dev/cel/optimizer/optimizers/ConstantFoldingOptimizer.java

@@ -16,6 +16,8 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.collect.ImmutableList.toImmutableList;
 import static com.google.common.collect.MoreCollectors.onlyElement;
+import static dev.cel.checker.CelStandardDeclarations.StandardFunction.DURATION;
+import static dev.cel.checker.CelStandardDeclarations.StandardFunction.TIMESTAMP;
 
 import com.google.auto.value.AutoValue;
 import com.google.common.collect.ImmutableList;
@@ -36,6 +38,7 @@
 import dev.cel.common.ast.CelMutableExpr.CelMutableMap;
 import dev.cel.common.ast.CelMutableExpr.CelMutableStruct;
 import dev.cel.common.ast.CelMutableExprConverter;
+import dev.cel.common.internal.DateTimeHelpers;
 import dev.cel.common.navigation.CelNavigableMutableAst;
 import dev.cel.common.navigation.CelNavigableMutableExpr;
 import dev.cel.common.types.SimpleType;
@@ -45,6 +48,8 @@
 import dev.cel.optimizer.CelOptimizationException;
 import dev.cel.parser.Operator;
 import dev.cel.runtime.CelEvaluationException;
+import java.time.Duration;
+import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -142,6 +147,14 @@ private boolean canFold(CelNavigableMutableExpr navigableExpr) {
           return false;
         }
 
+        // Timestamps/durations in CEL are calls, but they are effectively treated as literals.
+        // Expressions like timestamp(123) cannot be folded directly, but arithmetics involving
+        // timestamps can be optimized.
+        // Ex: timestamp(123) - timestamp(100) = duration("23s")
+        if (isCallTimestampOrDuration(navigableExpr.expr().call())) {
+          return false;
+        }
+
         CelMutableCall mutableCall = navigableExpr.expr().call();
         String functionName = mutableCall.function();
 
@@ -197,14 +210,14 @@ private boolean canFold(CelNavigableMutableExpr navigableExpr) {
   private boolean containsFoldableFunctionOnly(CelNavigableMutableExpr navigableExpr) {
     return navigableExpr
         .allNodes()
-        .allMatch(
-            node -> {
-              if (node.getKind().equals(Kind.CALL)) {
-                return foldableFunctions.contains(node.expr().call().function());
-              }
-
-              return true;
-            });
+        .filter(node -> node.getKind().equals(Kind.CALL))
+        .map(node -> node.expr().call())
+        .allMatch(call -> foldableFunctions.contains(call.function()));
+  }
+
+  private static boolean isCallTimestampOrDuration(CelMutableCall call) {
+    return call.function().equals(TIMESTAMP.functionName())
+        || call.function().equals(DURATION.functionName());
   }
 
   private static boolean canFoldInOperator(CelNavigableMutableExpr navigableExpr) {
@@ -318,6 +331,22 @@ private Optional<CelMutableExpr> maybeAdaptEvaluatedResult(Object result) {
       }
 
       return Optional.of(CelMutableExpr.ofMap(CelMutableMap.create(mapEntries)));
+    } else if (result instanceof Duration) {
+      String durationStrArg = DateTimeHelpers.toString((Duration) result);
+      CelMutableCall durationCall =
+          CelMutableCall.create(
+              DURATION.functionName(),
+              CelMutableExpr.ofConstant(CelConstant.ofValue(durationStrArg)));
+
+      return Optional.of(CelMutableExpr.ofCall(durationCall));
+    } else if (result instanceof Instant) {
+      String timestampStrArg = result.toString();
+      CelMutableCall timestampCall =
+          CelMutableCall.create(
+              TIMESTAMP.functionName(),
+              CelMutableExpr.ofConstant(CelConstant.ofValue(timestampStrArg)));
+
+      return Optional.of(CelMutableExpr.ofCall(timestampCall));
     }
 
     // Evaluated result cannot be folded (e.g: unknowns)

optimizer/src/test/java/dev/cel/optimizer/optimizers/ConstantFoldingOptimizerTest.java

@@ -46,6 +46,11 @@
 
 @RunWith(TestParameterInjector.class)
 public class ConstantFoldingOptimizerTest {
+  private static final CelOptions CEL_OPTIONS =
+      CelOptions.current()
+          .enableTimestampEpoch(true)
+          .evaluateCanonicalTypesToNativeValues(true)
+          .build();
   private static final Cel CEL =
       CelFactory.standardCelBuilder()
           .addVar("x", SimpleType.DYN)
@@ -60,19 +65,20 @@ public class ConstantFoldingOptimizerTest {
               CelFunctionBinding.from("get_true_overload", ImmutableList.of(), unused -> true))
           .addMessageTypes(TestAllTypes.getDescriptor())
           .setContainer(CelContainer.ofName("cel.expr.conformance.proto3"))
+          .setOptions(CEL_OPTIONS)
           .addCompilerLibraries(
               CelExtensions.bindings(),
               CelOptionalLibrary.INSTANCE,
-              CelExtensions.math(CelOptions.DEFAULT),
+              CelExtensions.math(CEL_OPTIONS),
               CelExtensions.strings(),
-              CelExtensions.sets(CelOptions.DEFAULT),
-              CelExtensions.encoders(CelOptions.DEFAULT))
+              CelExtensions.sets(CEL_OPTIONS),
+              CelExtensions.encoders(CEL_OPTIONS))
           .addRuntimeLibraries(
               CelOptionalLibrary.INSTANCE,
-              CelExtensions.math(CelOptions.DEFAULT),
+              CelExtensions.math(CEL_OPTIONS),
               CelExtensions.strings(),
-              CelExtensions.sets(CelOptions.DEFAULT),
-              CelExtensions.encoders(CelOptions.DEFAULT))
+              CelExtensions.sets(CEL_OPTIONS),
+              CelExtensions.encoders(CEL_OPTIONS))
           .build();
 
   private static final CelOptimizer CEL_OPTIMIZER =
@@ -211,6 +217,15 @@ public class ConstantFoldingOptimizerTest {
   @TestParameters("{source: '42 != 42', expected: 'false'}")
   @TestParameters("{source: '[\"foo\",\"bar\"] == [\"foo\",\"bar\"]', expected: 'true'}")
   @TestParameters("{source: '[\"bar\",\"foo\"] == [\"foo\",\"bar\"]', expected: 'false'}")
+  @TestParameters("{source: 'duration(\"1h\") - duration(\"60m\")', expected: 'duration(\"0s\")'}")
+  @TestParameters(
+      "{source: 'duration(\"2h23m42s12ms42us92ns\") + duration(\"129481231298125ns\")', expected:"
+          + " 'duration(\"138103.243340217s\")'}")
+  @TestParameters(
+      "{source: 'timestamp(900000) - timestamp(100)', expected: 'duration(\"899900s\")'}")
+  @TestParameters(
+      "{source: 'timestamp(\"2000-01-01T00:02:03.2123Z\") + duration(\"25h2m32s42ms53us29ns\")',"
+          + " expected: 'timestamp(\"2000-01-02T01:04:35.254353029Z\")'}")
   // TODO: Support folding lists with mixed types. This requires mutable lists.
   // @TestParameters("{source: 'dyn([1]) + [1.0]'}")
   public void constantFold_success(String source, String expected) throws Exception {
@@ -348,6 +363,8 @@ public void constantFold_macros_withoutMacroCallMetadata(String source) throws E
   @TestParameters("{source: 'get_true() == true'}")
   @TestParameters("{source: 'x == x'}")
   @TestParameters("{source: 'x == 42'}")
+  @TestParameters("{source: 'timestamp(100)'}")
+  @TestParameters("{source: 'duration(\"1h\")'}")
   public void constantFold_noOp(String source) throws Exception {
     CelAbstractSyntaxTree ast = CEL.compile(source).getAst();