// Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: BUSL-1.1

package crypto

import (
	"context"
	"crypto/sha256"
	"fmt"
	"io"

	wrapping "github.com/hashicorp/go-kms-wrapping/v2"
	"github.com/hashicorp/go-kms-wrapping/v2/aead"
	"github.com/hashicorp/go-kms-wrapping/v2/extras/multi"
	"golang.org/x/crypto/hkdf"
)

// DerivedReader returns a reader from which keys can be read, using the
// given wrapper, reader length limit, salt and context info. Salt and info can
// be nil.
//
// Example:
//
//	reader, _ := NewDerivedReader(wrapper, userId, jobId)
//	key := ed25519.GenerateKey(reader)
func NewDerivedReader(ctx context.Context, wrapper wrapping.Wrapper, lenLimit int64, salt, info []byte) (*io.LimitedReader, error) {
	const op = "crypto.NewDerivedReader"
	if wrapper == nil {
		return nil, fmt.Errorf("%s: missing wrapper: %w", op, ErrInvalidParameter)
	}
	if lenLimit < 20 {
		return nil, fmt.Errorf("%s: lenLimit must be >= 20: %w", op, ErrInvalidParameter)
	}
	var aeadWrapper *aead.Wrapper
	switch w := wrapper.(type) {
	case *multi.PooledWrapper:
		raw := w.WrapperForKeyId("__base__")
		var ok bool
		if aeadWrapper, ok = raw.(*aead.Wrapper); !ok {
			return nil, fmt.Errorf("%s: unexpected wrapper type from multiwrapper base: %w", op, ErrInvalidParameter)
		}
	case *aead.Wrapper:
		aeadWrapper = w
	default:
		return nil, fmt.Errorf("%s: unknown wrapper type: %w", op, ErrInvalidParameter)
	}

	keyBytes, err := aeadWrapper.KeyBytes(ctx)
	if err != nil {
		return nil, fmt.Errorf("%s: error reading aead key bytes: %w", op, err)
	}
	if keyBytes == nil {
		return nil, fmt.Errorf("%s: aead wrapper missing bytes: %w", op, ErrInvalidParameter)
	}

	reader := hkdf.New(sha256.New, keyBytes, salt, info)
	return &io.LimitedReader{
		R: reader,
		N: lenLimit,
	}, nil
}