// Copyright (C) MongoDB, Inc. 2017-present.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

// Package bsoncore contains functions that can be used to encode and decode BSON
// elements and values to or from a slice of bytes. These functions are aimed at
// allowing low level manipulation of BSON and can be used to build a higher
// level BSON library.
//
// The Read* functions within this package return the values of the element and
// a boolean indicating if the values are valid. A boolean was used instead of
// an error because any error that would be returned would be the same: not
// enough bytes. This library attempts to do no validation, it will only return
// false if there are not enough bytes for an item to be read. For example, the
// ReadDocument function checks the length, if that length is larger than the
// number of bytes availble, it will return false, if there are enough bytes, it
// will return those bytes and true. It is the consumers responsibility to
// validate those bytes.
//
// The Append* functions within this package will append the type value to the
// given dst slice. If the slice has enough capacity, it will not grow the
// slice. The Append*Element functions within this package operate in the same
// way, but additionally append the BSON type and the key before the value.
package bsoncore // import "go.mongodb.org/mongo-driver/x/bsonx/bsoncore"

import (
	"bytes"
	"fmt"
	"math"
	"strconv"
	"time"

	"go.mongodb.org/mongo-driver/bson/bsontype"
	"go.mongodb.org/mongo-driver/bson/primitive"
)

// EmptyDocumentLength is the length of a document that has been started/ended but has no elements.
const EmptyDocumentLength = 5

// nullTerminator is a string version of the 0 byte that is appended at the end of cstrings.
const nullTerminator = string(byte(0))

// AppendType will append t to dst and return the extended buffer.
func AppendType(dst []byte, t bsontype.Type) []byte { return append(dst, byte(t)) }

// AppendKey will append key to dst and return the extended buffer.
func AppendKey(dst []byte, key string) []byte { return append(dst, key+nullTerminator...) }

// AppendHeader will append Type t and key to dst and return the extended
// buffer.
func AppendHeader(dst []byte, t bsontype.Type, key string) []byte {
	dst = AppendType(dst, t)
	dst = append(dst, key...)
	return append(dst, 0x00)
	// return append(AppendType(dst, t), key+string(0x00)...)
}

// TODO(skriptble): All of the Read* functions should return src resliced to start just after what
// was read.

// ReadType will return the first byte of the provided []byte as a type. If
// there is no availble byte, false is returned.
func ReadType(src []byte) (bsontype.Type, []byte, bool) {
	if len(src) < 1 {
		return 0, src, false
	}
	return bsontype.Type(src[0]), src[1:], true
}

// ReadKey will read a key from src. The 0x00 byte will not be present
// in the returned string. If there are not enough bytes available, false is
// returned.
func ReadKey(src []byte) (string, []byte, bool) { return readcstring(src) }

// ReadKeyBytes will read a key from src as bytes. The 0x00 byte will
// not be present in the returned string. If there are not enough bytes
// available, false is returned.
func ReadKeyBytes(src []byte) ([]byte, []byte, bool) { return readcstringbytes(src) }

// ReadHeader will read a type byte and a key from src. If both of these
// values cannot be read, false is returned.
func ReadHeader(src []byte) (t bsontype.Type, key string, rem []byte, ok bool) {
	t, rem, ok = ReadType(src)
	if !ok {
		return 0, "", src, false
	}
	key, rem, ok = ReadKey(rem)
	if !ok {
		return 0, "", src, false
	}

	return t, key, rem, true
}

// ReadHeaderBytes will read a type and a key from src and the remainder of the bytes
// are returned as rem. If either the type or key cannot be red, ok will be false.
func ReadHeaderBytes(src []byte) (header []byte, rem []byte, ok bool) {
	if len(src) < 1 {
		return nil, src, false
	}
	idx := bytes.IndexByte(src[1:], 0x00)
	if idx == -1 {
		return nil, src, false
	}
	return src[:idx], src[idx+1:], true
}

// ReadElement reads the next full element from src. It returns the element, the remaining bytes in
// the slice, and a boolean indicating if the read was successful.
func ReadElement(src []byte) (Element, []byte, bool) {
	if len(src) < 1 {
		return nil, src, false
	}
	t := bsontype.Type(src[0])
	idx := bytes.IndexByte(src[1:], 0x00)
	if idx == -1 {
		return nil, src, false
	}
	length, ok := valueLength(src[idx+2:], t) // We add 2 here because we called IndexByte with src[1:]
	if !ok {
		return nil, src, false
	}
	elemLength := 1 + idx + 1 + int(length)
	if elemLength > len(src) {
		return nil, src, false
	}
	if elemLength < 0 {
		return nil, src, false
	}
	return src[:elemLength], src[elemLength:], true
}

// AppendValueElement appends value to dst as an element using key as the element's key.
func AppendValueElement(dst []byte, key string, value Value) []byte {
	dst = AppendHeader(dst, value.Type, key)
	dst = append(dst, value.Data...)
	return dst
}

// ReadValue reads the next value as the provided types and returns a Value, the remaining bytes,
// and a boolean indicating if the read was successful.
func ReadValue(src []byte, t bsontype.Type) (Value, []byte, bool) {
	data, rem, ok := readValue(src, t)
	if !ok {
		return Value{}, src, false
	}
	return Value{Type: t, Data: data}, rem, true
}

// AppendDouble will append f to dst and return the extended buffer.
func AppendDouble(dst []byte, f float64) []byte {
	return appendu64(dst, math.Float64bits(f))
}

// AppendDoubleElement will append a BSON double element using key and f to dst
// and return the extended buffer.
func AppendDoubleElement(dst []byte, key string, f float64) []byte {
	return AppendDouble(AppendHeader(dst, bsontype.Double, key), f)
}

// ReadDouble will read a float64 from src. If there are not enough bytes it
// will return false.
func ReadDouble(src []byte) (float64, []byte, bool) {
	bits, src, ok := readu64(src)
	if !ok {
		return 0, src, false
	}
	return math.Float64frombits(bits), src, true
}

// AppendString will append s to dst and return the extended buffer.
func AppendString(dst []byte, s string) []byte {
	return appendstring(dst, s)
}

// AppendStringElement will append a BSON string element using key and val to dst
// and return the extended buffer.
func AppendStringElement(dst []byte, key, val string) []byte {
	return AppendString(AppendHeader(dst, bsontype.String, key), val)
}

// ReadString will read a string from src. If there are not enough bytes it
// will return false.
func ReadString(src []byte) (string, []byte, bool) {
	return readstring(src)
}

// AppendDocumentStart reserves a document's length and returns the index where the length begins.
// This index can later be used to write the length of the document.
//
// TODO(skriptble): We really need AppendDocumentStart and AppendDocumentEnd.
// AppendDocumentStart would handle calling ReserveLength and providing the index of the start of
// the document. AppendDocumentEnd would handle taking that start index, adding the null byte,
// calculating the length, and filling in the length at the start of the document.
func AppendDocumentStart(dst []byte) (index int32, b []byte) { return ReserveLength(dst) }

// AppendDocumentStartInline functions the same as AppendDocumentStart but takes a pointer to the
// index int32 which allows this function to be used inline.
func AppendDocumentStartInline(dst []byte, index *int32) []byte {
	idx, doc := AppendDocumentStart(dst)
	*index = idx
	return doc
}

// AppendDocumentElementStart writes a document element header and then reserves the length bytes.
func AppendDocumentElementStart(dst []byte, key string) (index int32, b []byte) {
	return AppendDocumentStart(AppendHeader(dst, bsontype.EmbeddedDocument, key))
}

// AppendDocumentEnd writes the null byte for a document and updates the length of the document.
// The index should be the beginning of the document's length bytes.
func AppendDocumentEnd(dst []byte, index int32) ([]byte, error) {
	if int(index) > len(dst)-4 {
		return dst, fmt.Errorf("not enough bytes available after index to write length")
	}
	dst = append(dst, 0x00)
	dst = UpdateLength(dst, index, int32(len(dst[index:])))
	return dst, nil
}

// AppendDocument will append doc to dst and return the extended buffer.
func AppendDocument(dst []byte, doc []byte) []byte { return append(dst, doc...) }

// AppendDocumentElement will append a BSON embeded document element using key
// and doc to dst and return the extended buffer.
func AppendDocumentElement(dst []byte, key string, doc []byte) []byte {
	return AppendDocument(AppendHeader(dst, bsontype.EmbeddedDocument, key), doc)
}

// BuildDocument will create a document with the given slice of elements and will append
// it to dst and return the extended buffer.
func BuildDocument(dst []byte, elems ...[]byte) []byte {
	idx, dst := ReserveLength(dst)
	for _, elem := range elems {
		dst = append(dst, elem...)
	}
	dst = append(dst, 0x00)
	dst = UpdateLength(dst, idx, int32(len(dst[idx:])))
	return dst
}

// BuildDocumentValue creates an Embedded Document value from the given elements.
func BuildDocumentValue(elems ...[]byte) Value {
	return Value{Type: bsontype.EmbeddedDocument, Data: BuildDocument(nil, elems...)}
}

// BuildDocumentElement will append a BSON embedded document elemnt using key and the provided
// elements and return the extended buffer.
func BuildDocumentElement(dst []byte, key string, elems ...[]byte) []byte {
	return BuildDocument(AppendHeader(dst, bsontype.EmbeddedDocument, key), elems...)
}

// BuildDocumentFromElements is an alaias for the BuildDocument function.
var BuildDocumentFromElements = BuildDocument

// ReadDocument will read a document from src. If there are not enough bytes it
// will return false.
func ReadDocument(src []byte) (doc Document, rem []byte, ok bool) { return readLengthBytes(src) }

// AppendArrayStart appends the length bytes to an array and then returns the index of the start
// of those length bytes.
func AppendArrayStart(dst []byte) (index int32, b []byte) { return ReserveLength(dst) }

// AppendArrayElementStart appends an array element header and then the length bytes for an array,
// returning the index where the length starts.
func AppendArrayElementStart(dst []byte, key string) (index int32, b []byte) {
	return AppendArrayStart(AppendHeader(dst, bsontype.Array, key))
}

// AppendArrayEnd appends the null byte to an array and calculates the length, inserting that
// calculated length starting at index.
func AppendArrayEnd(dst []byte, index int32) ([]byte, error) { return AppendDocumentEnd(dst, index) }

// AppendArray will append arr to dst and return the extended buffer.
func AppendArray(dst []byte, arr []byte) []byte { return append(dst, arr...) }

// AppendArrayElement will append a BSON array element using key and arr to dst
// and return the extended buffer.
func AppendArrayElement(dst []byte, key string, arr []byte) []byte {
	return AppendArray(AppendHeader(dst, bsontype.Array, key), arr)
}

// BuildArray will append a BSON array to dst built from values.
func BuildArray(dst []byte, values ...Value) []byte {
	idx, dst := ReserveLength(dst)
	for pos, val := range values {
		dst = AppendValueElement(dst, strconv.Itoa(pos), val)
	}
	dst = append(dst, 0x00)
	dst = UpdateLength(dst, idx, int32(len(dst[idx:])))
	return dst
}

// BuildArrayElement will create an array element using the provided values.
func BuildArrayElement(dst []byte, key string, values ...Value) []byte {
	return BuildArray(AppendHeader(dst, bsontype.Array, key), values...)
}

// ReadArray will read an array from src. If there are not enough bytes it
// will return false.
func ReadArray(src []byte) (arr Document, rem []byte, ok bool) { return readLengthBytes(src) }

// AppendBinary will append subtype and b to dst and return the extended buffer.
func AppendBinary(dst []byte, subtype byte, b []byte) []byte {
	if subtype == 0x02 {
		return appendBinarySubtype2(dst, subtype, b)
	}
	dst = append(appendLength(dst, int32(len(b))), subtype)
	return append(dst, b...)
}

// AppendBinaryElement will append a BSON binary element using key, subtype, and
// b to dst and return the extended buffer.
func AppendBinaryElement(dst []byte, key string, subtype byte, b []byte) []byte {
	return AppendBinary(AppendHeader(dst, bsontype.Binary, key), subtype, b)
}

// ReadBinary will read a subtype and bin from src. If there are not enough bytes it
// will return false.
func ReadBinary(src []byte) (subtype byte, bin []byte, rem []byte, ok bool) {
	length, rem, ok := ReadLength(src)
	if !ok {
		return 0x00, nil, src, false
	}
	if len(rem) < 1 { // subtype
		return 0x00, nil, src, false
	}
	subtype, rem = rem[0], rem[1:]

	if len(rem) < int(length) {
		return 0x00, nil, src, false
	}

	if subtype == 0x02 {
		length, rem, ok = ReadLength(rem)
		if !ok || len(rem) < int(length) {
			return 0x00, nil, src, false
		}
	}

	return subtype, rem[:length], rem[length:], true
}

// AppendUndefinedElement will append a BSON undefined element using key to dst
// and return the extended buffer.
func AppendUndefinedElement(dst []byte, key string) []byte {
	return AppendHeader(dst, bsontype.Undefined, key)
}

// AppendObjectID will append oid to dst and return the extended buffer.
func AppendObjectID(dst []byte, oid primitive.ObjectID) []byte { return append(dst, oid[:]...) }

// AppendObjectIDElement will append a BSON ObjectID element using key and oid to dst
// and return the extended buffer.
func AppendObjectIDElement(dst []byte, key string, oid primitive.ObjectID) []byte {
	return AppendObjectID(AppendHeader(dst, bsontype.ObjectID, key), oid)
}

// ReadObjectID will read an ObjectID from src. If there are not enough bytes it
// will return false.
func ReadObjectID(src []byte) (primitive.ObjectID, []byte, bool) {
	if len(src) < 12 {
		return primitive.ObjectID{}, src, false
	}
	var oid primitive.ObjectID
	copy(oid[:], src[0:12])
	return oid, src[12:], true
}

// AppendBoolean will append b to dst and return the extended buffer.
func AppendBoolean(dst []byte, b bool) []byte {
	if b {
		return append(dst, 0x01)
	}
	return append(dst, 0x00)
}

// AppendBooleanElement will append a BSON boolean element using key and b to dst
// and return the extended buffer.
func AppendBooleanElement(dst []byte, key string, b bool) []byte {
	return AppendBoolean(AppendHeader(dst, bsontype.Boolean, key), b)
}

// ReadBoolean will read a bool from src. If there are not enough bytes it
// will return false.
func ReadBoolean(src []byte) (bool, []byte, bool) {
	if len(src) < 1 {
		return false, src, false
	}

	return src[0] == 0x01, src[1:], true
}

// AppendDateTime will append dt to dst and return the extended buffer.
func AppendDateTime(dst []byte, dt int64) []byte { return appendi64(dst, dt) }

// AppendDateTimeElement will append a BSON datetime element using key and dt to dst
// and return the extended buffer.
func AppendDateTimeElement(dst []byte, key string, dt int64) []byte {
	return AppendDateTime(AppendHeader(dst, bsontype.DateTime, key), dt)
}

// ReadDateTime will read an int64 datetime from src. If there are not enough bytes it
// will return false.
func ReadDateTime(src []byte) (int64, []byte, bool) { return readi64(src) }

// AppendTime will append time as a BSON DateTime to dst and return the extended buffer.
func AppendTime(dst []byte, t time.Time) []byte {
	return AppendDateTime(dst, t.Unix()*1000+int64(t.Nanosecond()/1e6))
}

// AppendTimeElement will append a BSON datetime element using key and dt to dst
// and return the extended buffer.
func AppendTimeElement(dst []byte, key string, t time.Time) []byte {
	return AppendTime(AppendHeader(dst, bsontype.DateTime, key), t)
}

// ReadTime will read an time.Time datetime from src. If there are not enough bytes it
// will return false.
func ReadTime(src []byte) (time.Time, []byte, bool) {
	dt, rem, ok := readi64(src)
	return time.Unix(dt/1e3, dt%1e3*1e6), rem, ok
}

// AppendNullElement will append a BSON null element using key to dst
// and return the extended buffer.
func AppendNullElement(dst []byte, key string) []byte { return AppendHeader(dst, bsontype.Null, key) }

// AppendRegex will append pattern and options to dst and return the extended buffer.
func AppendRegex(dst []byte, pattern, options string) []byte {
	return append(dst, pattern+nullTerminator+options+nullTerminator...)
}

// AppendRegexElement will append a BSON regex element using key, pattern, and
// options to dst and return the extended buffer.
func AppendRegexElement(dst []byte, key, pattern, options string) []byte {
	return AppendRegex(AppendHeader(dst, bsontype.Regex, key), pattern, options)
}

// ReadRegex will read a pattern and options from src. If there are not enough bytes it
// will return false.
func ReadRegex(src []byte) (pattern, options string, rem []byte, ok bool) {
	pattern, rem, ok = readcstring(src)
	if !ok {
		return "", "", src, false
	}
	options, rem, ok = readcstring(rem)
	if !ok {
		return "", "", src, false
	}
	return pattern, options, rem, true
}

// AppendDBPointer will append ns and oid to dst and return the extended buffer.
func AppendDBPointer(dst []byte, ns string, oid primitive.ObjectID) []byte {
	return append(appendstring(dst, ns), oid[:]...)
}

// AppendDBPointerElement will append a BSON DBPointer element using key, ns,
// and oid to dst and return the extended buffer.
func AppendDBPointerElement(dst []byte, key, ns string, oid primitive.ObjectID) []byte {
	return AppendDBPointer(AppendHeader(dst, bsontype.DBPointer, key), ns, oid)
}

// ReadDBPointer will read a ns and oid from src. If there are not enough bytes it
// will return false.
func ReadDBPointer(src []byte) (ns string, oid primitive.ObjectID, rem []byte, ok bool) {
	ns, rem, ok = readstring(src)
	if !ok {
		return "", primitive.ObjectID{}, src, false
	}
	oid, rem, ok = ReadObjectID(rem)
	if !ok {
		return "", primitive.ObjectID{}, src, false
	}
	return ns, oid, rem, true
}

// AppendJavaScript will append js to dst and return the extended buffer.
func AppendJavaScript(dst []byte, js string) []byte { return appendstring(dst, js) }

// AppendJavaScriptElement will append a BSON JavaScript element using key and
// js to dst and return the extended buffer.
func AppendJavaScriptElement(dst []byte, key, js string) []byte {
	return AppendJavaScript(AppendHeader(dst, bsontype.JavaScript, key), js)
}

// ReadJavaScript will read a js string from src. If there are not enough bytes it
// will return false.
func ReadJavaScript(src []byte) (js string, rem []byte, ok bool) { return readstring(src) }

// AppendSymbol will append symbol to dst and return the extended buffer.
func AppendSymbol(dst []byte, symbol string) []byte { return appendstring(dst, symbol) }

// AppendSymbolElement will append a BSON symbol element using key and symbol to dst
// and return the extended buffer.
func AppendSymbolElement(dst []byte, key, symbol string) []byte {
	return AppendSymbol(AppendHeader(dst, bsontype.Symbol, key), symbol)
}

// ReadSymbol will read a symbol string from src. If there are not enough bytes it
// will return false.
func ReadSymbol(src []byte) (symbol string, rem []byte, ok bool) { return readstring(src) }

// AppendCodeWithScope will append code and scope to dst and return the extended buffer.
func AppendCodeWithScope(dst []byte, code string, scope []byte) []byte {
	length := int32(4 + 4 + len(code) + 1 + len(scope)) // length of cws, length of code, code, 0x00, scope
	dst = appendLength(dst, length)

	return append(appendstring(dst, code), scope...)
}

// AppendCodeWithScopeElement will append a BSON code with scope element using
// key, code, and scope to dst
// and return the extended buffer.
func AppendCodeWithScopeElement(dst []byte, key, code string, scope []byte) []byte {
	return AppendCodeWithScope(AppendHeader(dst, bsontype.CodeWithScope, key), code, scope)
}

// ReadCodeWithScope will read code and scope from src. If there are not enough bytes it
// will return false.
func ReadCodeWithScope(src []byte) (code string, scope []byte, rem []byte, ok bool) {
	length, rem, ok := ReadLength(src)
	if !ok || len(src) < int(length) {
		return "", nil, src, false
	}

	code, rem, ok = readstring(rem)
	if !ok {
		return "", nil, src, false
	}

	scope, rem, ok = ReadDocument(rem)
	if !ok {
		return "", nil, src, false
	}
	return code, scope, rem, true
}

// AppendInt32 will append i32 to dst and return the extended buffer.
func AppendInt32(dst []byte, i32 int32) []byte { return appendi32(dst, i32) }

// AppendInt32Element will append a BSON int32 element using key and i32 to dst
// and return the extended buffer.
func AppendInt32Element(dst []byte, key string, i32 int32) []byte {
	return AppendInt32(AppendHeader(dst, bsontype.Int32, key), i32)
}

// ReadInt32 will read an int32 from src. If there are not enough bytes it
// will return false.
func ReadInt32(src []byte) (int32, []byte, bool) { return readi32(src) }

// AppendTimestamp will append t and i to dst and return the extended buffer.
func AppendTimestamp(dst []byte, t, i uint32) []byte {
	return appendu32(appendu32(dst, i), t) // i is the lower 4 bytes, t is the higher 4 bytes
}

// AppendTimestampElement will append a BSON timestamp element using key, t, and
// i to dst and return the extended buffer.
func AppendTimestampElement(dst []byte, key string, t, i uint32) []byte {
	return AppendTimestamp(AppendHeader(dst, bsontype.Timestamp, key), t, i)
}

// ReadTimestamp will read t and i from src. If there are not enough bytes it
// will return false.
func ReadTimestamp(src []byte) (t, i uint32, rem []byte, ok bool) {
	i, rem, ok = readu32(src)
	if !ok {
		return 0, 0, src, false
	}
	t, rem, ok = readu32(rem)
	if !ok {
		return 0, 0, src, false
	}
	return t, i, rem, true
}

// AppendInt64 will append i64 to dst and return the extended buffer.
func AppendInt64(dst []byte, i64 int64) []byte { return appendi64(dst, i64) }

// AppendInt64Element will append a BSON int64 element using key and i64 to dst
// and return the extended buffer.
func AppendInt64Element(dst []byte, key string, i64 int64) []byte {
	return AppendInt64(AppendHeader(dst, bsontype.Int64, key), i64)
}

// ReadInt64 will read an int64 from src. If there are not enough bytes it
// will return false.
func ReadInt64(src []byte) (int64, []byte, bool) { return readi64(src) }

// AppendDecimal128 will append d128 to dst and return the extended buffer.
func AppendDecimal128(dst []byte, d128 primitive.Decimal128) []byte {
	high, low := d128.GetBytes()
	return appendu64(appendu64(dst, low), high)
}

// AppendDecimal128Element will append a BSON primitive.28 element using key and
// d128 to dst and return the extended buffer.
func AppendDecimal128Element(dst []byte, key string, d128 primitive.Decimal128) []byte {
	return AppendDecimal128(AppendHeader(dst, bsontype.Decimal128, key), d128)
}

// ReadDecimal128 will read a primitive.Decimal128 from src. If there are not enough bytes it
// will return false.
func ReadDecimal128(src []byte) (primitive.Decimal128, []byte, bool) {
	l, rem, ok := readu64(src)
	if !ok {
		return primitive.Decimal128{}, src, false
	}

	h, rem, ok := readu64(rem)
	if !ok {
		return primitive.Decimal128{}, src, false
	}

	return primitive.NewDecimal128(h, l), rem, true
}

// AppendMaxKeyElement will append a BSON max key element using key to dst
// and return the extended buffer.
func AppendMaxKeyElement(dst []byte, key string) []byte {
	return AppendHeader(dst, bsontype.MaxKey, key)
}

// AppendMinKeyElement will append a BSON min key element using key to dst
// and return the extended buffer.
func AppendMinKeyElement(dst []byte, key string) []byte {
	return AppendHeader(dst, bsontype.MinKey, key)
}

// EqualValue will return true if the two values are equal.
func EqualValue(t1, t2 bsontype.Type, v1, v2 []byte) bool {
	if t1 != t2 {
		return false
	}
	v1, _, ok := readValue(v1, t1)
	if !ok {
		return false
	}
	v2, _, ok = readValue(v2, t2)
	if !ok {
		return false
	}
	return bytes.Equal(v1, v2)
}

// valueLength will determine the length of the next value contained in src as if it
// is type t. The returned bool will be false if there are not enough bytes in src for
// a value of type t.
func valueLength(src []byte, t bsontype.Type) (int32, bool) {
	var length int32
	ok := true
	switch t {
	case bsontype.Array, bsontype.EmbeddedDocument, bsontype.CodeWithScope:
		length, _, ok = ReadLength(src)
	case bsontype.Binary:
		length, _, ok = ReadLength(src)
		length += 4 + 1 // binary length + subtype byte
	case bsontype.Boolean:
		length = 1
	case bsontype.DBPointer:
		length, _, ok = ReadLength(src)
		length += 4 + 12 // string length + ObjectID length
	case bsontype.DateTime, bsontype.Double, bsontype.Int64, bsontype.Timestamp:
		length = 8
	case bsontype.Decimal128:
		length = 16
	case bsontype.Int32:
		length = 4
	case bsontype.JavaScript, bsontype.String, bsontype.Symbol:
		length, _, ok = ReadLength(src)
		length += 4
	case bsontype.MaxKey, bsontype.MinKey, bsontype.Null, bsontype.Undefined:
		length = 0
	case bsontype.ObjectID:
		length = 12
	case bsontype.Regex:
		regex := bytes.IndexByte(src, 0x00)
		if regex < 0 {
			ok = false
			break
		}
		pattern := bytes.IndexByte(src[regex+1:], 0x00)
		if pattern < 0 {
			ok = false
			break
		}
		length = int32(int64(regex) + 1 + int64(pattern) + 1)
	default:
		ok = false
	}

	return length, ok
}

func readValue(src []byte, t bsontype.Type) ([]byte, []byte, bool) {
	length, ok := valueLength(src, t)
	if !ok || int(length) > len(src) {
		return nil, src, false
	}

	return src[:length], src[length:], true
}

// ReserveLength reserves the space required for length and returns the index where to write the length
// and the []byte with reserved space.
func ReserveLength(dst []byte) (int32, []byte) {
	index := len(dst)
	return int32(index), append(dst, 0x00, 0x00, 0x00, 0x00)
}

// UpdateLength updates the length at index with length and returns the []byte.
func UpdateLength(dst []byte, index, length int32) []byte {
	dst[index] = byte(length)
	dst[index+1] = byte(length >> 8)
	dst[index+2] = byte(length >> 16)
	dst[index+3] = byte(length >> 24)
	return dst
}

func appendLength(dst []byte, l int32) []byte { return appendi32(dst, l) }

func appendi32(dst []byte, i32 int32) []byte {
	return append(dst, byte(i32), byte(i32>>8), byte(i32>>16), byte(i32>>24))
}

// ReadLength reads an int32 length from src and returns the length and the remaining bytes. If
// there aren't enough bytes to read a valid length, src is returned unomdified and the returned
// bool will be false.
func ReadLength(src []byte) (int32, []byte, bool) {
	ln, src, ok := readi32(src)
	if ln < 0 {
		return ln, src, false
	}
	return ln, src, ok
}

func readi32(src []byte) (int32, []byte, bool) {
	if len(src) < 4 {
		return 0, src, false
	}
	return (int32(src[0]) | int32(src[1])<<8 | int32(src[2])<<16 | int32(src[3])<<24), src[4:], true
}

func appendi64(dst []byte, i64 int64) []byte {
	return append(dst,
		byte(i64), byte(i64>>8), byte(i64>>16), byte(i64>>24),
		byte(i64>>32), byte(i64>>40), byte(i64>>48), byte(i64>>56),
	)
}

func readi64(src []byte) (int64, []byte, bool) {
	if len(src) < 8 {
		return 0, src, false
	}
	i64 := (int64(src[0]) | int64(src[1])<<8 | int64(src[2])<<16 | int64(src[3])<<24 |
		int64(src[4])<<32 | int64(src[5])<<40 | int64(src[6])<<48 | int64(src[7])<<56)
	return i64, src[8:], true
}

func appendu32(dst []byte, u32 uint32) []byte {
	return append(dst, byte(u32), byte(u32>>8), byte(u32>>16), byte(u32>>24))
}

func readu32(src []byte) (uint32, []byte, bool) {
	if len(src) < 4 {
		return 0, src, false
	}

	return (uint32(src[0]) | uint32(src[1])<<8 | uint32(src[2])<<16 | uint32(src[3])<<24), src[4:], true
}

func appendu64(dst []byte, u64 uint64) []byte {
	return append(dst,
		byte(u64), byte(u64>>8), byte(u64>>16), byte(u64>>24),
		byte(u64>>32), byte(u64>>40), byte(u64>>48), byte(u64>>56),
	)
}

func readu64(src []byte) (uint64, []byte, bool) {
	if len(src) < 8 {
		return 0, src, false
	}
	u64 := (uint64(src[0]) | uint64(src[1])<<8 | uint64(src[2])<<16 | uint64(src[3])<<24 |
		uint64(src[4])<<32 | uint64(src[5])<<40 | uint64(src[6])<<48 | uint64(src[7])<<56)
	return u64, src[8:], true
}

// keep in sync with readcstringbytes
func readcstring(src []byte) (string, []byte, bool) {
	idx := bytes.IndexByte(src, 0x00)
	if idx < 0 {
		return "", src, false
	}
	return string(src[:idx]), src[idx+1:], true
}

// keep in sync with readcstring
func readcstringbytes(src []byte) ([]byte, []byte, bool) {
	idx := bytes.IndexByte(src, 0x00)
	if idx < 0 {
		return nil, src, false
	}
	return src[:idx], src[idx+1:], true
}

func appendstring(dst []byte, s string) []byte {
	l := int32(len(s) + 1)
	dst = appendLength(dst, l)
	dst = append(dst, s...)
	return append(dst, 0x00)
}

func readstring(src []byte) (string, []byte, bool) {
	l, rem, ok := ReadLength(src)
	if !ok {
		return "", src, false
	}
	if len(src[4:]) < int(l) {
		return "", src, false
	}

	return string(rem[:l-1]), rem[l:], true
}

// readLengthBytes attempts to read a length and that number of bytes. This
// function requires that the length include the four bytes for itself.
func readLengthBytes(src []byte) ([]byte, []byte, bool) {
	l, _, ok := ReadLength(src)
	if !ok {
		return nil, src, false
	}
	if len(src) < int(l) {
		return nil, src, false
	}
	return src[:l], src[l:], true
}

func appendBinarySubtype2(dst []byte, subtype byte, b []byte) []byte {
	dst = appendLength(dst, int32(len(b)+4)) // The bytes we'll encode need to be 4 larger for the length bytes
	dst = append(dst, subtype)
	dst = appendLength(dst, int32(len(b)))
	return append(dst, b...)
}
