// Package credential generates and verifies DWH installation credentials.
package credential

import (
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"io"
	"strings"

	"github.com/aritmolab/thothii/tools/dwh-auth/internal/record"
)

const Prefix = "thtdwh_v1"
const KeyIDEncodedLength = 16
const SecretEncodedLength = 43
const MaxHeaderBytes = 128

type Material struct {
	Value  []byte
	KeyID  string
	Digest record.Digest
}

// Generate creates one canonical version 1 credential from random.
func Generate(random io.Reader) (Material, error) {
	keyIDBytes := make([]byte, 12)
	secretBytes := make([]byte, 32)
	if _, err := io.ReadFull(random, keyIDBytes); err != nil {
		return Material{}, err
	}
	if _, err := io.ReadFull(random, secretBytes); err != nil {
		return Material{}, err
	}
	keyID := base64.RawURLEncoding.EncodeToString(keyIDBytes)
	secret := base64.RawURLEncoding.EncodeToString(secretBytes)
	value := []byte(Prefix + "." + keyID + "." + secret)
	sum := sha256.Sum256(value)
	return Material{Value: value, KeyID: keyID, Digest: record.Digest(sum)}, nil
}

// VerifyV1 authenticates one canonical version 1 credential and returns its key ID.
func VerifyV1(value []byte, expected record.Digest) (string, bool) {
	if len(value) > MaxHeaderBytes || len(value) != len(Prefix)+1+KeyIDEncodedLength+1+SecretEncodedLength {
		return "", false
	}
	parts := strings.Split(string(value), ".")
	if len(parts) != 3 || parts[0] != Prefix || !canonicalBase64(parts[1], 12, KeyIDEncodedLength) || !canonicalBase64(parts[2], 32, SecretEncodedLength) {
		return "", false
	}
	sum := sha256.Sum256(value)
	if subtle.ConstantTimeCompare(sum[:], expected[:]) != 1 {
		return "", false
	}
	return parts[1], true
}

// VerifyLegacy authenticates an opaque legacy credential without interpreting its syntax.
func VerifyLegacy(value []byte, expected record.Digest) bool {
	if len(value) < 1 || len(value) > MaxHeaderBytes {
		return false
	}
	for _, b := range value {
		if b <= 0x1f || b == 0x7f {
			return false
		}
	}
	sum := sha256.Sum256(value)
	return subtle.ConstantTimeCompare(sum[:], expected[:]) == 1
}

func canonicalBase64(value string, decodedLength, encodedLength int) bool {
	if len(value) != encodedLength {
		return false
	}
	decoded, err := base64.RawURLEncoding.DecodeString(value)
	return err == nil && len(decoded) == decodedLength && base64.RawURLEncoding.EncodeToString(decoded) == value
}