From 79af2d01b189eff70723cc74c8e773d58c39c074 Mon Sep 17 00:00:00 2001
From: Yassine Ennebati <4570448+yaziine@users.noreply.github.com>
Date: Thu, 23 Nov 2023 11:07:00 +0400
Subject: [PATCH] Add coalesce function (#513)

---
 .gitignore                            |  3 ++-
 cmd/genji/doc/functions.go            | 33 +++++++++++++-------------
 internal/expr/functions/builtins.go   | 34 ++++++++++++++++++++++++++-
 internal/expr/functions/definition.go | 10 ++++----
 internal/expr/functions/strings.go    |  6 ++---
 internal/stream/operator.go           |  2 +-
 sqltests/expr/coalesce.sql            | 19 +++++++++++++++
 7 files changed, 81 insertions(+), 26 deletions(-)
 create mode 100644 sqltests/expr/coalesce.sql

diff --git a/.gitignore b/.gitignore
index 2f8dc2a9..9e763227 100644
--- a/.gitignore
+++ b/.gitignore
@@ -29,8 +29,9 @@ profile.*
 # Builds
 cmd/genji/genji
 
-# VS Code config
+# IDE config
 .vscode/
+.idea/
 
 # gopls log
 gopls.log
diff --git a/cmd/genji/doc/functions.go b/cmd/genji/doc/functions.go
index 23c49aec..2115014f 100644
--- a/cmd/genji/doc/functions.go
+++ b/cmd/genji/doc/functions.go
@@ -4,19 +4,20 @@ type functionDocs map[string]string
 
 var packageDocs = map[string]functionDocs{
 	"strings": stringsDocs,
-	"math": mathDocs,
-	"":     builtinDocs,
+	"math":    mathDocs,
+	"":        builtinDocs,
 }
 
 var builtinDocs = functionDocs{
-	"pk":     "The pk() function returns the primary key for the current document",
-	"count":  "Returns a count of the number of times that arg1 is not NULL in a group. The count(*) function (with no arguments) returns the total number of rows in the group.",
-	"min":    "Returns the minimum value of the arg1 expression in a group.",
-	"max":    "Returns the maximum value of the arg1 expressein in a group.",
-	"sum":    "The sum function returns the sum of all values taken by the arg1 expression in a group.",
-	"avg":    "The avg function returns the average of all values taken by the arg1 expression in a group.",
-	"typeof": "The typeof function returns the type of arg1.",
-	"len":    "The len function returns length of the arg1 expression if arg1 evals to string, array or document, either returns NULL.",
+	"pk":       "The pk() function returns the primary key for the current document",
+	"count":    "Returns a count of the number of times that arg1 is not NULL in a group. The count(*) function (with no arguments) returns the total number of rows in the group.",
+	"min":      "Returns the minimum value of the arg1 expression in a group.",
+	"max":      "Returns the maximum value of the arg1 expressein in a group.",
+	"sum":      "The sum function returns the sum of all values taken by the arg1 expression in a group.",
+	"avg":      "The avg function returns the average of all values taken by the arg1 expression in a group.",
+	"typeof":   "The typeof function returns the type of arg1.",
+	"len":      "The len function returns length of the arg1 expression if arg1 evals to string, array or document, either returns NULL.",
+	"coalesce": "The coalesce function returns the first non-null argument. NULL is returned if all arguments are null.",
 }
 
 var mathDocs = functionDocs{
@@ -31,9 +32,9 @@ var mathDocs = functionDocs{
 }
 
 var stringsDocs = functionDocs{
-	"lower":  "The lower function returns arg1 to lower-case if arg1 evals to string",
-	"upper":  "The upper function returns arg1 to upper-case if arg1 evals to string",
-	"trim":   "The trim function returns arg1 with leading and trailing characters removed. space by default or arg2",
-	"ltrim":   "The ltrim function returns arg1 with leading characters removed. space by default or arg2",
-	"rtrim":   "The rtrim function returns arg1 with trailing characters removed. space by default or arg2",
-}
\ No newline at end of file
+	"lower": "The lower function returns arg1 to lower-case if arg1 evals to string",
+	"upper": "The upper function returns arg1 to upper-case if arg1 evals to string",
+	"trim":  "The trim function returns arg1 with leading and trailing characters removed. space by default or arg2",
+	"ltrim": "The ltrim function returns arg1 with leading characters removed. space by default or arg2",
+	"rtrim": "The rtrim function returns arg1 with trailing characters removed. space by default or arg2",
+}
diff --git a/internal/expr/functions/builtins.go b/internal/expr/functions/builtins.go
index 3cfad0e4..b7fb8921 100644
--- a/internal/expr/functions/builtins.go
+++ b/internal/expr/functions/builtins.go
@@ -67,6 +67,13 @@ var builtinFunctions = Definitions{
 			return &Len{Expr: args[0]}, nil
 		},
 	},
+	"coalesce": &definition{
+		name:  "coalesce",
+		arity: variadicArity,
+		constructorFn: func(args ...expr.Expr) (expr.Function, error) {
+			return &Coalesce{Exprs: args}, nil
+		},
+	},
 }
 
 // BuiltinDefinitions returns a map of builtin functions.
@@ -716,4 +723,29 @@ func (s *Len) Params() []expr.Expr { return []expr.Expr{s.Expr} }
 // String returns the literal representation of len.
 func (s *Len) String() string {
 	return fmt.Sprintf("LEN(%v)", s.Expr)
-}
\ No newline at end of file
+}
+
+type Coalesce struct {
+	Exprs []expr.Expr
+}
+
+func (c *Coalesce) Eval(e *environment.Environment) (types.Value, error) {
+	for _, exp := range c.Exprs {
+		v, err := exp.Eval(e)
+		if err != nil {
+			return nil, err
+		}
+		if v.Type() != types.NullValue {
+			return v, nil
+		}
+	}
+	return nil, nil
+}
+
+func (c *Coalesce) String() string {
+	return fmt.Sprintf("COALESCE(%v)", c.Exprs)
+}
+
+func (c *Coalesce) Params() []expr.Expr {
+	return c.Exprs
+}
diff --git a/internal/expr/functions/definition.go b/internal/expr/functions/definition.go
index 45c2232c..6abba956 100644
--- a/internal/expr/functions/definition.go
+++ b/internal/expr/functions/definition.go
@@ -7,6 +7,9 @@ import (
 	"github.com/genjidb/genji/internal/expr"
 )
 
+// variadicArity represents an unlimited number of arguments.
+const variadicArity = -1
+
 // A Definition transforms a list of expressions into a Function.
 type Definition interface {
 	Name() string
@@ -57,11 +60,10 @@ func (fd *definition) Name() string {
 }
 
 func (fd *definition) Function(args ...expr.Expr) (expr.Function, error) {
-	if fd.arity == -1 {
-		return fd.constructorFn(args...)
+	if fd.arity == variadicArity && len(args) == 0 {
+		return nil, fmt.Errorf("%s() requires at least one argument", fd.name)
 	}
-
-	if len(args) != fd.arity {
+	if fd.arity != variadicArity && (len(args) != fd.arity) {
 		return nil, fmt.Errorf("%s() takes %d argument(s), not %d", fd.name, fd.arity, len(args))
 	}
 	return fd.constructorFn(args...)
diff --git a/internal/expr/functions/strings.go b/internal/expr/functions/strings.go
index 6074a8af..b309ecf1 100644
--- a/internal/expr/functions/strings.go
+++ b/internal/expr/functions/strings.go
@@ -26,21 +26,21 @@ var stringsFunctions = Definitions{
 	},
 	"trim": &definition{
 		name:  "trim",
-		arity: -1,
+		arity: variadicArity,
 		constructorFn: func(args ...expr.Expr) (expr.Function, error) {
 			return &Trim{Expr: args, TrimFunc: strings.Trim, Name: "TRIM"}, nil
 		},
 	},
 	"ltrim": &definition{
 		name:  "ltrim",
-		arity: -1,
+		arity: variadicArity,
 		constructorFn: func(args ...expr.Expr) (expr.Function, error) {
 			return &Trim{Expr: args, TrimFunc: strings.TrimLeft, Name: "LTRIM"}, nil
 		},
 	},
 	"rtrim": &definition{
 		name:  "rtrim",
-		arity: -1,
+		arity: variadicArity,
 		constructorFn: func(args ...expr.Expr) (expr.Function, error) {
 			return &Trim{Expr: args, TrimFunc: strings.TrimRight, Name: "RTRIM"}, nil
 		},
diff --git a/internal/stream/operator.go b/internal/stream/operator.go
index 5d623626..99c690e0 100644
--- a/internal/stream/operator.go
+++ b/internal/stream/operator.go
@@ -11,7 +11,7 @@ var ErrInvalidResult = errors.New("expression must evaluate to a document")
 
 // An Operator is used to modify a stream.
 // It takes an environment containing the current value as well as any other metadata
-// created by other operatorsand returns a new environment which will be passed to the next operator.
+// created by other operators and returns a new environment which will be passed to the next operator.
 // If it returns a nil environment, the env will be ignored.
 // If it returns an error, the stream will be interrupted and that error will bubble up
 // and returned by this function, unless that error is ErrStreamClosed, in which case
diff --git a/sqltests/expr/coalesce.sql b/sqltests/expr/coalesce.sql
new file mode 100644
index 00000000..44d0114b
--- /dev/null
+++ b/sqltests/expr/coalesce.sql
@@ -0,0 +1,19 @@
+-- test: simple case
+> COALESCE(1,2,3)
+1
+
+-- test: with null
+> COALESCE(null,2,3)
+2
+
+-- test: with different values type
+> COALESCE('hey',2,3)
+'hey'
+
+-- test: with more than one null value with integer
+> COALESCE(null, null, null,3)
+3
+
+-- test: with more than one null value with text
+> COALESCE(null, null, null, 'hey')
+'hey'