src-d
diff --git a/‎sql/expression/function/logarithm.go
Lines changed: 203 additions & 0 deletions b/‎sql/expression/function/logarithm.go
Lines changed: 203 additions & 0 deletions
@@ -0,0 +1,203 @@
+package function
+
+import (
+	"math"
+	"reflect"
+	"fmt"
+
+	"gopkg.in/src-d/go-mysql-server.v0/sql/expression"
+	"gopkg.in/src-d/go-mysql-server.v0/sql"
+	"gopkg.in/src-d/go-errors.v1"
+)
+
+// ErrInvalidArgumentForLogarithm is returned when an invalid argument value is passed to a
+// logarithm function
+var ErrInvalidArgumentForLogarithm = errors.NewKind("invalid argument value for logarithm: %v")
+
+// LogBaseMaker returns LogBase creator functions with a specific base.
+func LogBaseMaker(base float64) func(e sql.Expression) sql.Expression {
+	return func(e sql.Expression) sql.Expression {
+		return NewLogBase(base, e)
+	}
+}
+
+// LogBase is a function that returns the logarithm of a value with a specific base.
+type LogBase struct {
+	expression.UnaryExpression
+	base float64
+}
+
+// NewLogBase creates a new LogBase expression.
+func NewLogBase(base float64, e sql.Expression) sql.Expression {
+	return &LogBase{UnaryExpression: expression.UnaryExpression{Child: e}, base: base}
+}
+
+func (l *LogBase) String() string {
+	switch l.base {
+	case float64(math.E):
+		return fmt.Sprintf("ln(%s)", l.Child)
+	case float64(10):
+		return fmt.Sprintf("log10(%s)", l.Child)
+	case float64(2):
+		return fmt.Sprintf("log2(%s)", l.Child)
+	default:
+		return fmt.Sprintf("log(%v, %s)", l.base, l.Child)
+	}
+}
+
+// TransformUp implements the Expression interface.
+func (l *LogBase) TransformUp(f sql.TransformExprFunc) (sql.Expression, error) {
+	child, err := l.Child.TransformUp(f)
+	if err != nil {
+		return nil, err
+	}
+	return f(NewLogBase(l.base, child))
+}
+
+// Type returns the resultant type of the function.
+func (l *LogBase) Type() sql.Type {
+	return sql.Float64
+}
+
+// IsNullable implements the sql.Expression interface.
+func (l *LogBase) IsNullable() bool {
+	return l.base == float64(1) || l.base <= float64(0) || l.Child.IsNullable()
+}
+
+// Eval implements the Expression interface.
+func (l *LogBase) Eval(
+	ctx *sql.Context,
+	row sql.Row,
+) (interface{}, error) {
+	v, err := l.Child.Eval(ctx, row)
+	if err != nil {
+		return nil, err
+	}
+
+	if v == nil {
+		return nil, nil
+	}
+
+	val, err := sql.Float64.Convert(v)
+	if err != nil {
+		return nil, sql.ErrInvalidType.New(reflect.TypeOf(v))
+	}
+	return computeLog(val.(float64), l.base)
+}
+
+// Log is a function that returns the natural logarithm of a value.
+type Log struct {
+	expression.BinaryExpression
+}
+
+// NewLn creates a new Log expression.
+func NewLog(args ...sql.Expression) (sql.Expression, error) {
+	argLen := len(args)
+	if argLen == 0 || argLen > 2 {
+		return nil, sql.ErrInvalidArgumentNumber.New("1 or 2", argLen)
+	}
+
+	if argLen == 1 {
+		return &Log{expression.BinaryExpression{Left: expression.NewLiteral(math.E, sql.Float64), Right: args[0]}}, nil
+	} else {
+		return &Log{expression.BinaryExpression{Left: args[0], Right: args[1]}}, nil
+	}
+}
+
+func (l *Log) String() string {
+	return fmt.Sprintf("log(%s, %s)", l.Left, l.Right)
+}
+
+// TransformUp implements the Expression interface.
+func (l *Log) TransformUp(f sql.TransformExprFunc) (sql.Expression, error) {
+	var args = make([]sql.Expression, 2)
+	arg, err := l.Left.TransformUp(f)
+	if err != nil {
+		return nil, err
+	}
+	args[0] = arg
+
+	arg, err = l.Right.TransformUp(f)
+	if err != nil {
+		return nil, err
+	}
+	args[1] = arg
+	expr, err := NewLog(args...)
+	if err != nil {
+		return nil, err
+	}
+
+	return f(expr)
+}
+
+// Children implements the Expression interface.
+func (l *Log) Children() []sql.Expression {
+	return []sql.Expression{l.Left, l.Right}
+}
+
+// Type returns the resultant type of the function.
+func (l *Log) Type() sql.Type {
+	return sql.Float64
+}
+
+// IsNullable implements the Expression interface.
+func (l *Log) IsNullable() bool {
+	return l.Left.IsNullable() || l.Right.IsNullable()
+}
+
+// Eval implements the Expression interface.
+func (l *Log) Eval(
+	ctx *sql.Context,
+	row sql.Row,
+) (interface{}, error) {
+	left, err := l.Left.Eval(ctx, row)
+	if err != nil {
+		return nil, err
+	}
+
+	if left == nil {
+		return nil, nil
+	}
+
+	lhs, err := sql.Float64.Convert(left)
+	if err != nil {
+		return nil, sql.ErrInvalidType.New(reflect.TypeOf(left))
+	}
+
+	right, err := l.Right.Eval(ctx, row)
+	if err != nil {
+		return nil, err
+	}
+
+	if right == nil {
+		return nil, nil
+	}
+
+	rhs, err := sql.Float64.Convert(right)
+	if err != nil {
+		return nil, sql.ErrInvalidType.New(reflect.TypeOf(right))
+	}
+
+	// rhs becomes value, lhs becomes base
+	return computeLog(rhs.(float64), lhs.(float64))
+}
+
+func computeLog(v float64, base float64) (float64, error) {
+	if v <= 0 {
+		return float64(0), ErrInvalidArgumentForLogarithm.New(v)
+	}
+	if base == float64(1) || base <= float64(0) {
+		return float64(0), ErrInvalidArgumentForLogarithm.New(base)
+	}
+	switch base {
+	case float64(2):
+		return math.Log2(v), nil
+	case float64(10):
+		return math.Log10(v), nil
+	case math.E:
+		return math.Log(v), nil
+	default:
+		// LOG(BASE,V) is equivalent to LOG(V) / LOG(BASE).
+		return float64(math.Log(v) / math.Log(base)), nil
+	}
+}