variant.go

package variant

/*

Variant is implemented as a struct with 3 fields: `ptr`, `lenAndType`, `capOrVal`.

`lenAndType` is an int field that is split into 2 parts: `Len` and `Type`. `Type` is
in the least significant 3 bits and contains the numeric value of the Variant type
`Len` use the rest of the bits (61 bits on 64 bit platforms and 29 bits on 32 bit
platforms) and contains the numeric value of the length of the slice that `ptr` points to.

`capOrVal` either contains the capacity of the slice that `ptr` points to or the value
for non-slice types. `capOrVal` is always 64 bits regardless of the GOARCH.

What exactly is stored in the struct fields depends on the type of the Variant. The
diagrams below show the content of the fields for each Variant type.

TypeEmpty:

            +------------------------------+
 ptr        | nil                          |
            +------------------------------+
 lenAndType | 0                            |
            +------------------------------+
 capOrVal   | 0                            |
            +------------------------------+

TypeInt:

            +------------------------------+
 ptr        | nil                          |
            +-----------------------+------+
 lenAndType | Len=0                 |Type=1|
            +-----------------------+------+
 capOrVal   | int value                    |
            +------------------------------+

TypeFloat64:

            +------------------------------+
 ptr        | nil                          |
            +-----------------------+------+
 lenAndType | Len=0                 |Type=2|
            +-----------------------+------+
 capOrVal   | float64 bits stored as int64 |
            +------------------------------+

TypeString:
                                              variable number
                                              of string bytes
            +------------------------------+       +---+
 ptr        | Pointer to string bytes      |------>|   | first byte
            +-----------------------+------+       +---+
 lenAndType | Len of string in bytes|Type=3|       |   |
            +-----------------------+------+       +---+
 capOrVal   | 0                            |        ...
            +------------------------------+       +---+
                                                   |   | last byte
                                                   +---+


TypeBytes:
                                              variable number
                                                 of bytes
            +------------------------------+       +---+
 ptr        | Pointer to byte slice        |------>|   | first byte
            +-----------------------+------+       +---+
 lenAndType | Len of slice          |Type=4|       |   |
            +-----------------------+------+       +---+
 capOrVal   | Capacity of      slice       |        ...
            +------------------------------+       +---+
                                                   |   | last byte
                                                   +---+

TypeValueList:
                                                    variable number of
                                                     Variant elements
            +------------------------------+       +------------------+
 ptr        | Pointer to Variant slice     |------>|                  | first element
            +-----------------------+------+       +------------------+
 lenAndType | Len of slice          |Type=5|       |                  |
            +-----------------------+------+       +------------------+
 capOrVal   | Capacity of slice            |               ...
            +------------------------------+       +------------------+
                                                   |                  | last element
                                                   +------------------+

TypeKeyValueList:
                                                    variable number of
                                                     KeyValue elements
            +------------------------------+       +---+--------------+
 ptr        | Pointer to KeyValue slice    |------>|Key| Value        | first element
            +-----------------------+------+       +---+--------------+
 lenAndType | Len of slice          |Type=6|       |Key| Value        |
            +-----------------------+------+       +---+--------------+
 capOrVal   | Capacity of slice            |               ...
            +------------------------------+       +---+--------------+
                                                   |Key| Value        | last element
                                                   +---+--------------+

*/

import (
	"fmt"
	"reflect"
	"strconv"
	"strings"
	"unsafe"
)

// Type represents the type of a value stored in Variant.
type Type int

// Possible value types that can be stored in Variant.
const (
	// Empty or no value. The default state of zero-initialized Variant.
	TypeEmpty Type = iota

	// An int number.
	TypeInt

	// A float64 number.
	TypeFloat64

	// A string.
	TypeString

	// A []byte slice.
	TypeBytes

	// A list of Variant.
	TypeValueList

	// A list of KeyValue.
	TypeKeyValueList
)

// Number of bits to use for Type field. This should be wide enough to fit all Type values.
const typeFieldBitCount = 3

// Bit mask for Type part of lenAndType field.
const typeFieldMask = (1 << typeFieldBitCount) - 1

// Maximum length of a slice-type that can be stored in Variant. The length of Go slices
// can be at most maxint, however Variant is not able to store lengths of maxint. Len field
// in Variant uses typeFieldBitCount bits less than int, i.e. the maximum length of a slice
// stored in Variant is maxint / (2^typeFieldBitCount), which we calculate below.
const maxSliceLen = int((^uint(0))>>1) >> typeFieldBitCount

// KeyValue is an element that is used for TypeKeyValueList storage.
type KeyValue struct {
	Key   string
	Value Variant
}

// Type returns the type of the currently stored value.
func (v *Variant) Type() Type {
	return Type(v.lenAndType & typeFieldMask)
}

// NewEmpty creates a Variant of TypeEmpty type. Equivalent to Variant{}.
func NewEmpty() Variant {
	return Variant{}
}

// NewString creates a Variant of TypeString type.
func NewString(v string) Variant {
	hdr := (*reflect.StringHeader)(unsafe.Pointer(&v))
	if hdr.Len > maxSliceLen {
		panic("maximum len exceeded")
	}

	return Variant{
		ptr:        unsafe.Pointer(hdr.Data),
		lenAndType: (hdr.Len << typeFieldBitCount) | int(TypeString),
	}
}

// NewStringFromBytes creates a Variant of TypeString type from a slice of bytes
// that represent the string.
//
// WARNING: the string stored inside this Variant will be aliased in the memory and will
// share its storage with the byte slice provided. This means any changes to the bytes
// in the slice will also modify the string in this Variant.
//
// This function should be only used when it is guaranteed that the bytes
// in the slice will not be modified or when the immutability of the string
// stored inside this Variant is not required. In such cases NewStringFromBytes(v)
// provides significant performance advantage over NewString(string(v)) call,
// which will create a copy of byte slice 'v'.
func NewStringFromBytes(v []byte) (r Variant) {
	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&v))
	if hdr.Len > maxSliceLen {
		panic("maximum len exceeded")
	}

	return Variant{
		ptr:        unsafe.Pointer(hdr.Data),
		lenAndType: (hdr.Len << typeFieldBitCount) | int(TypeString),
	}
}

// IntVal returns the stored int value.
// The returned value is undefined if the Variant type is not TypeInt.
func (v *Variant) IntVal() int {
	return int(v.capOrVal)
}

// Float64Val returns the stored float64 value.
// The returned value is undefined if the Variant type is not TypeFloat64.
func (v *Variant) Float64Val() float64 {
	return *(*float64)(unsafe.Pointer(&v.capOrVal))
}

// StringVal returns the stored string value.
// Will panic if the Variant type is not TypeString.
func (v *Variant) StringVal() (s string) {
	if v.Type() != TypeString {
		panic("Variant is not a TypeString")
	}
	dest := (*reflect.StringHeader)(unsafe.Pointer(&s))
	dest.Data = uintptr(v.ptr)
	dest.Len = v.lenAndType >> typeFieldBitCount
	return s
}

// Bytes returns the stored byte slice.
// Will panic if the Variant type is not TypeBytes.
func (v *Variant) Bytes() (b []byte) {
	if v.Type() != TypeBytes {
		panic("Variant is not a TypeBytes")
	}
	dest := (*reflect.SliceHeader)(unsafe.Pointer(&b))
	dest.Data = uintptr(v.ptr)
	dest.Len = v.lenAndType >> typeFieldBitCount
	dest.Cap = int(v.capOrVal)
	return b
}

// ValueList returns the slice of stored Variant values.
//
// Elements in the returned slice are allowed to be modified after this call returns.
// Will panic if the Variant type is not TypeValueList.
//
// It is recommended to use this function instead of ValueAt()/Len() pair to
// iterate over the entire list.
func (v *Variant) ValueList() (s []Variant) {
	if v.Type() != TypeValueList {
		panic("Variant is not a slice")
	}
	dest := (*reflect.SliceHeader)(unsafe.Pointer(&s))
	dest.Data = uintptr(v.ptr)
	dest.Len = v.lenAndType >> typeFieldBitCount
	dest.Cap = int(v.capOrVal)
	return s
}

// ValueAt returns the value at the specified index.
//
// Valid to call only if Variant type is TypeValueList otherwise will panic.
// Will panic if index is negative or is greater or equal the current length.
//
// ValueAt() and Len() can be used to iterate over the list using a for loop,
// however instead it is recommended to call ValueList() and use for-range
// loop over the returned value (the later approach is faster and safer). See
// ValueList() for an example.
func (v *Variant) ValueAt(i int) Variant {
	if v.Type() != TypeValueList {
		panic("Variant is not a TypeValueList")
	}
	if v.ptr == nil {
		panic("index of empty TypeValueList")
	}
	if i < 0 || i >= v.Len() {
		panic("index out of bounds")
	}
	return *(*Variant)(unsafe.Pointer(uintptr(v.ptr) + uintptr(i)*unsafe.Sizeof(Variant{})))
}

// Len returns the length of contained slice-based type.
//
// Valid to call for TypeString, TypeBytes, TypeValueList, TypeKeyValueList types.
// For other types the returned value is undefined.
func (v *Variant) Len() int {
	return v.lenAndType >> typeFieldBitCount
}

// Resize the length of contained slice-based type.
//
// Valid to call for TypeString, TypeBytes, TypeValueList, TypeKeyValueList types.
// Will panic for other types.
// Will panic if len is negative or exceeds the current capacity of the slice or if
// len exceeds maxSliceLen.
func (v *Variant) Resize(len int) {
	switch v.Type() {
	case TypeEmpty, TypeInt, TypeFloat64:
		panic(fmt.Sprintf("Cannot resize Variant type %d", v.Type()))
	}

	if len < 0 {
		panic("negative len is not allowed")
	}
	if len > int(v.capOrVal) {
		panic("cannot resize beyond capacity")
	}
	if len > maxSliceLen {
		panic("maximum len exceeded")
	}
	v.lenAndType = (v.lenAndType & typeFieldMask) | (len << typeFieldBitCount)
}

// KeyValueList return the slice of stored KeyValue.
//
// Valid to call only if Type==TypeKeyValueList otherwise will panic.
// Elements in the returned slice are allowed to be modified after this call returns.
// Such modification will affect the KeyValue stored in this Variant since returned
// slice is a reference type.
//
// It is recommended to use this function instead of KeyValueAt()/Len() pair to
// iterate over the entire list.
func (v *Variant) KeyValueList() (s []KeyValue) {
	if v.Type() != TypeKeyValueList {
		panic("Variant is not a TypeKeyValueList")
	}
	dest := (*reflect.SliceHeader)(unsafe.Pointer(&s))
	dest.Data = uintptr(v.ptr)
	dest.Len = v.lenAndType >> typeFieldBitCount
	dest.Cap = int(v.capOrVal)
	return s
}

// KeyValueAt returns the KeyValue at the specified index.
//
// Valid to call only if Variant type is TypeKeyValueList otherwise will panic.
// The element is returned by pointer to allow the caller to modify the element
// by assigning to it if needed.
// Will panic if index is negative or is greater or equal the current length.
//
// KeyValueAt() and Len() can be used to iterate over the list using a for loop,
// however instead it is recommended to call KeyValueList() and use for-range
// loop over the returned value (the later approach is faster and safer). See
// KeyValueList() for an example.
func (v *Variant) KeyValueAt(index int) *KeyValue {
	if v.Type() != TypeKeyValueList {
		panic("Variant is not a TypeKeyValueList")
	}
	if v.ptr == nil {
		panic("index of empty TypeKeyValueList")
	}
	if index < 0 || index >= v.Len() {
		panic("index out of bounds")
	}
	return (*KeyValue)(unsafe.Pointer(uintptr(v.ptr) + uintptr(index)*unsafe.Sizeof(KeyValue{})))
}

// String returns a human readable string representation of the stored value.
//
// This function is for diagnostic purposes (e.g. to print the value in a log file).
// The format of the returned string is not part of the contract and may change any
// time without warning.
func (v Variant) String() string {
	switch v.Type() {
	case TypeEmpty:
		return ""
	case TypeInt:
		return strconv.Itoa(v.IntVal())
	case TypeFloat64:
		return strconv.FormatFloat(v.Float64Val(), 'g', -1, 64)
	case TypeString:
		return fmt.Sprintf("%q", v.StringVal())
	case TypeBytes:
		return fmt.Sprintf("0x%X", v.Bytes())
	case TypeValueList:
		var strs []string
		for _, e := range v.ValueList() {
			strs = append(strs, e.String())
		}
		return "[" + strings.Join(strs, ",") + "]"
	case TypeKeyValueList:
		var strs []string
		for _, e := range v.KeyValueList() {
			strs = append(strs, fmt.Sprintf("%q:%s", e.Key, e.Value.String()))
		}
		return "{" + strings.Join(strs, ",") + "}"
	}
	panic("invalid Variant type")
}