From e33426d0cf6f8f3503ca5ac7f117b749ec4f9acf Mon Sep 17 00:00:00 2001
From: rachithrr <rachith.ramaswamy@molecula.com>
Date: Thu, 8 Dec 2022 04:01:30 +0530
Subject: [PATCH] FB-1795: Implement REVERSE() scalar string function (#2335)

(cherry picked from commit d33bf4811f9803ae626f88ecbd183fd804b4b76a)
---
 sql3/planner/expression.go              |  2 ++
 sql3/planner/expressionanalyzercall.go  |  2 ++
 sql3/planner/inbuiltfunctionsstring.go  | 41 +++++++++++++++++++++++++
 sql3/test/defs/defs.go                  |  1 +
 sql3/test/defs/defs_string_functions.go | 32 +++++++++++++++++++
 5 files changed, 78 insertions(+)
 create mode 100644 sql3/planner/inbuiltfunctionsstring.go
 create mode 100644 sql3/test/defs/defs_string_functions.go

diff --git a/sql3/planner/expression.go b/sql3/planner/expression.go
index 5ebbd3360..d4ee5a042 100644
--- a/sql3/planner/expression.go
+++ b/sql3/planner/expression.go
@@ -1476,6 +1476,8 @@ func (n *callPlanExpression) Evaluate(currentRow []interface{}) (interface{}, er
 		return n.EvaluateSetContainsAll(currentRow)
 	case "DATEPART":
 		return n.EvaluateDatepart(currentRow)
+	case "REVERSE":
+		return n.EvaluateReverse(currentRow)
 	default:
 		return nil, sql3.NewErrInternalf("unhandled function name '%s'", n.name)
 	}
diff --git a/sql3/planner/expressionanalyzercall.go b/sql3/planner/expressionanalyzercall.go
index 8581df641..6e8a41169 100644
--- a/sql3/planner/expressionanalyzercall.go
+++ b/sql3/planner/expressionanalyzercall.go
@@ -241,6 +241,8 @@ func (p *ExecutionPlanner) analyzeCallExpression(call *parser.Call, scope parser
 
 	case "SUBTABLE":
 		return p.analyzeFunctionSubtable(call, scope)
+	case "REVERSE":
+		return p.analyseFunctionReverse(call, scope)
 
 	default:
 		return nil, sql3.NewErrCallUnknownFunction(call.Name.NamePos.Line, call.Name.NamePos.Column, call.Name.Name)
diff --git a/sql3/planner/inbuiltfunctionsstring.go b/sql3/planner/inbuiltfunctionsstring.go
new file mode 100644
index 000000000..71143a96c
--- /dev/null
+++ b/sql3/planner/inbuiltfunctionsstring.go
@@ -0,0 +1,41 @@
+package planner
+
+import (
+	"github.com/molecula/featurebase/v3/sql3"
+	"github.com/molecula/featurebase/v3/sql3/parser"
+)
+
+func (p *ExecutionPlanner) analyseFunctionReverse(call *parser.Call, scope parser.Statement) (parser.Expr, error) {
+	//one argument
+	if len(call.Args) != 1 {
+		return nil, sql3.NewErrCallParameterCountMismatch(call.Rparen.Line, call.Rparen.Column, call.Name.Name, 1, len(call.Args))
+	}
+
+	if !typeIsString(call.Args[0].DataType()) {
+		return nil, sql3.NewErrStringExpressionExpected(call.Args[0].Pos().Line, call.Args[0].Pos().Column)
+	}
+
+	call.ResultDataType = parser.NewDataTypeString()
+
+	return call, nil
+}
+
+// reverses the string
+func (n *callPlanExpression) EvaluateReverse(currentRow []interface{}) (interface{}, error) {
+	argOneEval, err := n.args[0].Evaluate(currentRow)
+	if err != nil {
+		return nil, err
+	}
+
+	stringArgOne, ok := argOneEval.(string)
+	if !ok {
+		return nil, sql3.NewErrInternalf("unexpected type converion %T", argOneEval)
+	}
+
+	// reverse the string
+	runes := []rune(stringArgOne)
+	for i, j := 0, len(runes)-1; i < j; i, j = i+1, j-1 {
+		runes[i], runes[j] = runes[j], runes[i]
+	}
+	return string(runes), nil
+}
diff --git a/sql3/test/defs/defs.go b/sql3/test/defs/defs.go
index c4a7f5fd5..c6c8d0d69 100644
--- a/sql3/test/defs/defs.go
+++ b/sql3/test/defs/defs.go
@@ -23,6 +23,7 @@ var TableTests []TableTest = []TableTest{
 	setFunctionTests,
 	setParameterTests,
 	datePartTests,
+	stringScalarFunctionsTests,
 
 	insertTest,
 	keyedInsertTest,
diff --git a/sql3/test/defs/defs_string_functions.go b/sql3/test/defs/defs_string_functions.go
new file mode 100644
index 000000000..4e4fc560c
--- /dev/null
+++ b/sql3/test/defs/defs_string_functions.go
@@ -0,0 +1,32 @@
+package defs
+
+// string function tests
+var stringScalarFunctionsTests = TableTest{
+
+	Table: tbl(
+		"stringscalarfunctions",
+		srcHdrs(
+			srcHdr("_id", fldTypeID),
+			srcHdr("a", fldTypeInt, "min 0", "max 1000"),
+			srcHdr("b", fldTypeInt, "min 0", "max 1000"),
+			srcHdr("ts", fldTypeTimestamp),
+		),
+		srcRows(
+			srcRow(int64(1), int64(10), int64(100), knownTimestamp()),
+		),
+	),
+	SQLTests: []SQLTest{
+		{
+			SQLs: sqls(
+				"select reverse('this')",
+			),
+			ExpHdrs: hdrs(
+				hdr("", fldTypeString),
+			),
+			ExpRows: rows(
+				row(string("siht")),
+			),
+			Compare: CompareExactUnordered,
+		},
+	},
+}