// Mgmt
// Copyright (C) James Shubin and the project contributors
// Written by James Shubin <james@shubin.ca> and the project contributors
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.
//
// Additional permission under GNU GPL version 3 section 7
//
// If you modify this program, or any covered work, by linking or combining it
// with embedded mcl code and modules (and that the embedded mcl code and
// modules which link with this program, contain a copy of their source code in
// the authoritative form) containing parts covered by the terms of any other
// license, the licensors of this program grant you additional permission to
// convey the resulting work. Furthermore, the licensors of this program grant
// the original author, James Shubin, additional permission to update this
// additional permission if he deems it necessary to achieve the goals of this
// additional permission.

package core // TODO: should this be in its own individual package?

import (
	"context"
	"crypto/rand"
	"fmt"
	"math"
	"math/big"

	"github.com/purpleidea/mgmt/lang/funcs"
	"github.com/purpleidea/mgmt/lang/interfaces"
	"github.com/purpleidea/mgmt/lang/types"
	"github.com/purpleidea/mgmt/util/errwrap"
)

const (
	// Random1FuncName is the name this function is registered as.
	Random1FuncName = "random1"

	// arg names...
	random1ArgNameLength = "length"

	alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
)

func init() {
	funcs.Register(Random1FuncName, func() interfaces.Func { return &Random1Func{} })
}

// Random1Func returns one random string of a certain length. If you change the
// length, then it will produce a new random value.
type Random1Func struct {
	// XXX: To produce a stream of random values every N seconds, make a
	// built-in function or use the dual <|> hack below?
	// XXX: Maybe it should be returning a stream of floats [0,1] as well,
	// which someone can later map to the alphabet that they want. Should
	// random() take an interval to know how often to spit out values? It
	// could also just do it once per second, and we could filter for less.
	// If we want something high precision, we could add that in the future.
	// We could name that "random" and this one can be "random1" until we
	// deprecate it.
	init *interfaces.Init

	length uint16 // last length
	result string // last random
}

// String returns a simple name for this function. This is needed so this struct
// can satisfy the pgraph.Vertex interface.
func (obj *Random1Func) String() string {
	return Random1FuncName
}

// ArgGen returns the Nth arg name for this function.
func (obj *Random1Func) ArgGen(index int) (string, error) {
	seq := []string{random1ArgNameLength}
	if l := len(seq); index >= l {
		return "", fmt.Errorf("index %d exceeds arg length of %d", index, l)
	}
	return seq[index], nil
}

// Validate makes sure we've built our struct properly. It is usually unused for
// normal functions that users can use directly.
func (obj *Random1Func) Validate() error {
	return nil
}

// Info returns some static info about itself.
func (obj *Random1Func) Info() *interfaces.Info {
	return &interfaces.Info{
		Pure: false,
		Memo: false,
		Fast: false,
		Spec: false,
		Sig:  types.NewType(fmt.Sprintf("func(%s int) str", random1ArgNameLength)),
		Err:  obj.Validate(),
	}
}

// generate generates a random string.
func generate(length uint16) (string, error) {
	max := len(alphabet) - 1 // last index
	output := ""

	// FIXME: have someone verify this is cryptographically secure & correct
	for i := uint16(0); i < length; i++ {
		big, err := rand.Int(rand.Reader, big.NewInt(int64(max)))
		if err != nil {
			return "", errwrap.Wrapf(err, "could not generate random string")
		}
		ix := big.Int64()
		output += string(alphabet[ix])
	}

	if length != 0 && output == "" { // safety against empty strings
		return "", fmt.Errorf("string is empty")
	}

	if uint16(len(output)) != length { // safety against weird bugs
		return "", fmt.Errorf("random string is too short") // bug!
	}

	return output, nil
}

// Init runs some startup code for this function.
func (obj *Random1Func) Init(init *interfaces.Init) error {
	obj.init = init
	return nil
}

// Call this function with the input args and return the value if it is possible
// to do so at this time.
func (obj *Random1Func) Call(ctx context.Context, args []types.Value) (types.Value, error) {
	if len(args) < 1 {
		return nil, fmt.Errorf("not enough args")
	}
	length := args[0].Int()

	if length < 0 || length > math.MaxUint16 {
		// On error, reset the cached values. This *may* be useful if we
		// want to use the future "except" operator to produce an stream
		// of random values-- we could flip flop between two "random1()"
		// functions to successively get a val from one, while resetting
		// the other one. Which happens right here... Here's an example:
		//
		// $now = datetime.now()
		// $len = 8 # length of rand
		// # alternate every second
		// $out = if math.mod($now, 2) == 0 {
		// 	random1($len) <|> random1(-1)
		// } else {
		// 	random1(-1) <|> random1($len)
		// }
		//
		// Perhaps it's just better to have a core rand stream function?
		obj.length = 0
		obj.result = ""
		return nil, fmt.Errorf("can't generate an invalid length")
	}

	if uint16(length) == obj.length { // same, so use cached value
		return &types.StrValue{
			V: obj.result,
		}, nil
	}
	obj.length = uint16(length) // cache

	result, err := generate(uint16(length))
	if err != nil {
		return nil, err // no errwrap needed b/c helper func
	}
	obj.result = result // cache

	return &types.StrValue{
		V: result,
	}, nil
}