feat: support IfThen translation from Substrait to Calcite

isthmus/src/main/java/io/substrait/isthmus/expression/ExpressionRexConverter.java

@@ -7,15 +7,20 @@
 import io.substrait.type.Type;
 import io.substrait.util.DecimalUtil;
 import java.math.BigDecimal;
+import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
+import java.util.stream.Stream;
 import org.apache.calcite.avatica.util.ByteString;
 import org.apache.calcite.rel.type.RelDataTypeFactory;
-import org.apache.calcite.rex.*;
+import org.apache.calcite.rex.RexBuilder;
+import org.apache.calcite.rex.RexInputRef;
+import org.apache.calcite.rex.RexNode;
 import org.apache.calcite.sql.SqlIntervalQualifier;
 import org.apache.calcite.sql.SqlOperator;
+import org.apache.calcite.sql.fun.SqlStdOperatorTable;
 import org.apache.calcite.sql.parser.SqlParserPos;
 import org.apache.calcite.util.TimeString;
 import org.apache.calcite.util.TimestampString;
@@ -99,6 +104,11 @@ public RexNode visit(Expression.FP64Literal expr) throws RuntimeException {
         expr.value(), typeConverter.toCalcite(typeFactory, expr.getType()));
   }
 
+  @Override
+  public RexNode visit(Expression.FixedCharLiteral expr) throws RuntimeException {
+    return rexBuilder.makeLiteral(expr.value());
+  }
+
   @Override
   public RexNode visit(Expression.StrLiteral expr) throws RuntimeException {
     return rexBuilder.makeLiteral(
@@ -167,6 +177,19 @@ public RexNode visit(Expression.DecimalLiteral expr) throws RuntimeException {
     return rexBuilder.makeLiteral(decimal, typeConverter.toCalcite(typeFactory, expr.getType()));
   }
 
+  @Override
+  public RexNode visit(Expression.IfThen expr) throws RuntimeException {
+    // In Calcite, the arguments to the CASE operator are given as:
+    //   <cond1> <value1> <cond2> <value2> ... <condN> <valueN> ... <else>
+    Stream<RexNode> ifThenArgs =
+        expr.ifClauses().stream()
+            .flatMap(
+                clause -> Stream.of(clause.condition().accept(this), clause.then().accept(this)));
+    Stream<RexNode> elseArg = Stream.of(expr.elseClause().accept(this));
+    List<RexNode> args = Stream.concat(ifThenArgs, elseArg).collect(Collectors.toList());
+    return rexBuilder.makeCall(SqlStdOperatorTable.CASE, args);
+  }
+
   @Override
   public RexNode visit(Expression.ScalarFunctionInvocation expr) throws RuntimeException {
     var eArgs = expr.arguments();

isthmus/src/test/java/io/substrait/isthmus/Substrait2SqlTest.java

@@ -183,4 +183,10 @@ public void simpleStringOpTest() throws Exception {
     assertSqlSubstraitRelRoundTrip(
         "select l_comment from lineitem where upper(l_comment) <> l_comment");
   }
+
+  @Test
+  public void caseWhenTest() throws Exception {
+    assertSqlSubstraitRelRoundTrip(
+        "select case when p_size > 100 then 'large' when p_size > 50 then 'medium' else 'small' end from part");
+  }
 }