package backup

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"net/url"
	"slices"
	"strings"
	"time"
)

// emptySHA256 is the payload hash of a request with no body.
const emptySHA256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"

// sign adds AWS Signature Version 4 headers for S3 to req: X-Amz-Date,
// X-Amz-Content-Sha256 and Authorization. Every header already on the
// request is signed, along with the host.
func sign(req *http.Request, payloadSHA256, accessKey, secretKey, region string, at time.Time) {
	at = at.UTC()
	stamp := at.Format("20060102T150405Z")
	date := at.Format("20060102")
	req.Header.Set("X-Amz-Date", stamp)
	req.Header.Set("X-Amz-Content-Sha256", payloadSHA256)

	host := req.Host
	if host == "" {
		host = req.URL.Host
	}
	headers := map[string]string{"host": host}
	for name, values := range req.Header {
		headers[strings.ToLower(name)] = strings.TrimSpace(strings.Join(values, ","))
	}
	names := make([]string, 0, len(headers))
	for name := range headers {
		names = append(names, name)
	}
	slices.Sort(names)
	var canonHeaders strings.Builder
	for _, name := range names {
		canonHeaders.WriteString(name + ":" + headers[name] + "\n")
	}
	signed := strings.Join(names, ";")

	canonical := strings.Join([]string{
		req.Method,
		canonicalPath(req.URL),
		canonicalQuery(req.URL.Query()),
		canonHeaders.String(),
		signed,
		payloadSHA256,
	}, "\n")
	scope := date + "/" + region + "/s3/aws4_request"
	toSign := "AWS4-HMAC-SHA256\n" + stamp + "\n" + scope + "\n" + hexSHA256([]byte(canonical))

	key := hmacSHA256([]byte("AWS4"+secretKey), date)
	key = hmacSHA256(key, region)
	key = hmacSHA256(key, "s3")
	key = hmacSHA256(key, "aws4_request")
	signature := hex.EncodeToString(hmacSHA256(key, toSign))

	req.Header.Set("Authorization", "AWS4-HMAC-SHA256 Credential="+accessKey+"/"+scope+
		", SignedHeaders="+signed+", Signature="+signature)
}

// canonicalPath is the request path with each segment URI-encoded once.
func canonicalPath(u *url.URL) string {
	p := u.EscapedPath()
	if p == "" {
		return "/"
	}
	if unescaped, err := url.PathUnescape(p); err == nil {
		p = unescaped
	}
	segments := strings.Split(p, "/")
	for i, s := range segments {
		segments[i] = uriEncode(s)
	}
	return strings.Join(segments, "/")
}

// canonicalQuery is the query sorted by name, names and values URI-encoded.
func canonicalQuery(q url.Values) string {
	pairs := make([]string, 0, len(q))
	for name, values := range q {
		for _, v := range values {
			pairs = append(pairs, uriEncode(name)+"="+uriEncode(v))
		}
	}
	slices.Sort(pairs)
	return strings.Join(pairs, "&")
}

// uriEncode percent-encodes everything but the unreserved characters, as
// SigV4 requires; url.QueryEscape differs on space and '~'.
func uriEncode(s string) string {
	var b strings.Builder
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z' || c >= '0' && c <= '9' || c == '-' || c == '_' || c == '.' || c == '~' {
			b.WriteByte(c)
			continue
		}
		b.WriteString("%" + strings.ToUpper(hex.EncodeToString([]byte{c})))
	}
	return b.String()
}

func hmacSHA256(key []byte, msg string) []byte {
	m := hmac.New(sha256.New, key)
	m.Write([]byte(msg))
	return m.Sum(nil)
}

func hexSHA256(b []byte) string {
	sum := sha256.Sum256(b)
	return hex.EncodeToString(sum[:])
}