Add functions packages support (#419)

* Add packaged functions support

* Add tests for math.floor func

* Export FunctionsTable

* Extract func stuff into its own package

* Rename stuff

* Fix tests

* Move doc package to cmd/genji

* Adjust naming, typos

internal/expr/functions/scalar_definition_test.go

@@ -0,0 +1,59 @@
+package functions_test
+
+import (
+	"testing"
+
+	"github.com/genjidb/genji/document"
+	"github.com/genjidb/genji/internal/environment"
+	"github.com/genjidb/genji/internal/expr"
+	"github.com/genjidb/genji/internal/expr/functions"
+	"github.com/stretchr/testify/require"
+)
+
+func TestScalarFunctionDef(t *testing.T) {
+	def := functions.NewScalarDefinition(
+		"foo",
+		3,
+		func(args ...document.Value) (document.Value, error) {
+			arg1 := args[0].V.(int64)
+			arg2 := args[1].V.(int64)
+			arg3 := args[2].V.(int64)
+
+			return document.NewIntegerValue(arg1 + arg2 + arg3), nil
+		},
+	)
+
+	t.Run("Name()", func(t *testing.T) {
+		require.Equal(t, "foo", def.Name())
+	})
+
+	t.Run("Arity()", func(t *testing.T) {
+		require.Equal(t, 3, def.Arity())
+	})
+
+	t.Run("String()", func(t *testing.T) {
+		require.Equal(t, "foo(arg1, arg2, arg3)", def.String())
+	})
+
+	t.Run("Function()", func(t *testing.T) {
+		fb := document.NewFieldBuffer()
+		fb = fb.Add("a", document.NewIntegerValue(2))
+		env := environment.New(fb)
+		expr1 := expr.Add(expr.LiteralValue(document.NewIntegerValue(1)), expr.LiteralValue(document.NewIntegerValue(0)))
+		expr2 := expr.Path(document.NewPath("a"))
+		expr3 := expr.Div(expr.LiteralValue(document.NewIntegerValue(6)), expr.LiteralValue(document.NewIntegerValue(2)))
+
+		t.Run("OK", func(t *testing.T) {
+			fexpr, err := def.Function(expr1, expr2, expr3)
+			require.NoError(t, err)
+			v, err := fexpr.Eval(env)
+			require.NoError(t, err)
+			require.Equal(t, document.NewIntegerValue(1+2+3), v)
+		})
+
+		t.Run("NOK", func(t *testing.T) {
+			_, err := def.Function(expr1, expr2)
+			require.Error(t, err)
+		})
+	})
+}