// Package objectlink signs and streams a link to one object in the mesh's object store (novox/hq ADR 0468 §3).
//
// Bytes move over HTTP between a party and the store, on a signed link of at most ten minutes for one object and one
// method, signed for the name the `s3-bucket` binding gives; a document over 100 MiB is refused. Presign makes the
// link (Signature Version 4, query form, path style, payload unsigned, `host` the only signed header); Put and Get
// move the bytes without holding them in memory and count and hash them as they go.
//
// Standard library only: the SDK requires nothing of the mesh's forge.
package objectlink

import (
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/xml"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"sort"
	"strings"
	"time"
)

const (
	// MaxDocument is the largest document a link moves (ADR 0468 §3): the default limit of Put and Get.
	MaxDocument int64 = 100 << 20
	// MaxExpires is the longest a link lives (ADR 0468 §3).
	MaxExpires = 10 * time.Minute

	defaultRegion = "us-east-1"
	algorithm     = "AWS4-HMAC-SHA256"
)

// Request is one link to sign.
type Request struct {
	// Method is GET or PUT.
	Method string
	// Endpoint is the store as the party reaches it, "http://host:port". Its host is signed exactly as written.
	Endpoint string
	Bucket   string
	Key      string
	// AccessKey and SecretKey are the credential the s3-bucket binding gives.
	AccessKey string
	SecretKey string
	// Region defaults to us-east-1.
	Region string
	// Expires is how long the link lives: at least one second, at most ten minutes.
	Expires time.Duration
	// Now is the signing time; the clock's when zero.
	Now time.Time
}

// Presign returns the signed link for one object and one method.
func Presign(r Request) (string, error) {
	if r.Method != http.MethodGet && r.Method != http.MethodPut {
		return "", fmt.Errorf("objectlink: method %q: a link is for GET or PUT (ADR 0468 §3)", r.Method)
	}
	if r.Expires < time.Second || r.Expires > MaxExpires {
		return "", fmt.Errorf("objectlink: a link lives at least 1s and at most %s, not %s (ADR 0468 §3)", MaxExpires, r.Expires)
	}
	if r.Bucket == "" || r.Key == "" || r.AccessKey == "" || r.SecretKey == "" {
		return "", errors.New("objectlink: a bucket, a key and a credential are needed to sign a link")
	}
	u, err := url.Parse(r.Endpoint)
	if err != nil || u.Host == "" || (u.Scheme != "http" && u.Scheme != "https") {
		return "", fmt.Errorf("objectlink: endpoint %q is not http(s)://host[:port]", r.Endpoint)
	}
	region := r.Region
	if region == "" {
		region = defaultRegion
	}
	now := r.Now
	if now.IsZero() {
		now = time.Now()
	}
	path := "/" + encode(r.Bucket, false) + "/" + encode(r.Key, false)
	query := sign(r.Method, u.Host, path, r.AccessKey, r.SecretKey, region, now.UTC(), int64(r.Expires/time.Second))
	return u.Scheme + "://" + u.Host + path + "?" + query, nil
}

// sign returns the query string of a presigned link, signature last. The path is already encoded.
func sign(method, host, path, accessKey, secretKey, region string, now time.Time, expires int64) string {
	date := now.Format("20060102")
	stamp := now.Format("20060102T150405Z")
	scope := date + "/" + region + "/s3/aws4_request"
	params := map[string]string{
		"X-Amz-Algorithm":     algorithm,
		"X-Amz-Credential":    accessKey + "/" + scope,
		"X-Amz-Date":          stamp,
		"X-Amz-Expires":       fmt.Sprint(expires),
		"X-Amz-SignedHeaders": "host",
	}
	names := make([]string, 0, len(params))
	for k := range params {
		names = append(names, k)
	}
	sort.Strings(names)
	var query []string
	for _, k := range names {
		query = append(query, encode(k, true)+"="+encode(params[k], true))
	}
	canonicalQuery := strings.Join(query, "&")
	canonical := strings.Join([]string{method, path, canonicalQuery, "host:" + host + "\n", "host", "UNSIGNED-PAYLOAD"}, "\n")
	digest := sha256.Sum256([]byte(canonical))
	toSign := strings.Join([]string{algorithm, stamp, scope, hex.EncodeToString(digest[:])}, "\n")
	key := mac([]byte("AWS4"+secretKey), date)
	key = mac(key, region)
	key = mac(key, "s3")
	key = mac(key, "aws4_request")
	return canonicalQuery + "&X-Amz-Signature=" + hex.EncodeToString(mac(key, toSign))
}

func mac(key []byte, data string) []byte {
	h := hmac.New(sha256.New, key)
	h.Write([]byte(data))
	return h.Sum(nil)
}

// encode is AWS's URI encoding: everything but A-Z a-z 0-9 - _ . ~ as %XX, and the slash kept in a path.
func encode(s string, slash bool) string {
	var b strings.Builder
	for i := 0; i < len(s); i++ {
		c := s[i]
		switch {
		case c >= 'A' && c <= 'Z', c >= 'a' && c <= 'z', c >= '0' && c <= '9', c == '-', c == '_', c == '.', c == '~':
			b.WriteByte(c)
		case c == '/' && !slash:
			b.WriteByte(c)
		default:
			fmt.Fprintf(&b, "%%%02X", c)
		}
	}
	return b.String()
}

// Result is what moved: its size and the SHA-256 of its bytes, in hex.
type Result struct {
	Size   int64
	SHA256 string
}

// StoreError is a refusal by the store: its status and the code and message of its XML error body.
type StoreError struct {
	Status  int
	Code    string
	Message string
}

func (e *StoreError) Error() string {
	if e.Code == "" {
		return fmt.Sprintf("objectlink: the store answered %d", e.Status)
	}
	return fmt.Sprintf("objectlink: the store answered %d %s: %s", e.Status, e.Code, e.Message)
}

func storeError(resp *http.Response) error {
	body, _ := io.ReadAll(io.LimitReader(resp.Body, 64<<10))
	var parsed struct {
		Code    string `xml:"Code"`
		Message string `xml:"Message"`
	}
	_ = xml.Unmarshal(body, &parsed)
	return &StoreError{Status: resp.StatusCode, Code: parsed.Code, Message: parsed.Message}
}

// tooBig is the error of a document past its limit.
func tooBig(limit int64) error {
	return fmt.Errorf("objectlink: the document is over %d bytes (ADR 0468 §3)", limit)
}

// exact counts and hashes what passes, and fails a stream that is longer than declared, or ends before it.
type exact struct {
	r    io.Reader
	size int64
	n    int64
	hash interface{ Write([]byte) (int, error) }
}

func (l *exact) Read(p []byte) (int, error) {
	n, err := l.r.Read(p)
	l.n += int64(n)
	if l.n > l.size {
		return 0, fmt.Errorf("objectlink: the body is longer than the declared %d bytes", l.size)
	}
	l.hash.Write(p[:n])
	if err == io.EOF && l.n < l.size {
		return n, fmt.Errorf("objectlink: the body ended at %d bytes, short of the declared %d", l.n, l.size)
	}
	return n, err
}

// Put streams body to a link made for PUT, never holding it in memory. The caller gives the body's exact size, which
// is always sent as the Content-Length (a store answers 411 to a PUT without one). A negative size is refused, and so
// is one over limit (MaxDocument when limit is not above zero, ADR 0468 §3), both before any request. A body that
// yields more or fewer bytes than size fails the request, so a PUT whose body does not end as declared stores nothing.
func Put(ctx context.Context, client *http.Client, link string, body io.Reader, size, limit int64) (Result, error) {
	if limit <= 0 {
		limit = MaxDocument
	}
	if client == nil {
		client = http.DefaultClient
	}
	if size < 0 {
		return Result{}, errors.New("objectlink: put needs the size of the body; a negative or unknown size is refused")
	}
	if size > limit {
		return Result{}, tooBig(limit)
	}
	hash := sha256.New()
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()
	counted := &exact{r: body, size: size, hash: hash}
	req, err := http.NewRequestWithContext(ctx, http.MethodPut, link, io.NopCloser(counted))
	if err != nil {
		return Result{}, err
	}
	req.ContentLength = size
	if size == 0 {
		req.Body = http.NoBody
	}
	resp, err := client.Do(req)
	if err != nil {
		cancel()
		return Result{}, fmt.Errorf("objectlink: put: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		return Result{}, storeError(resp)
	}
	if counted.n != size {
		return Result{}, fmt.Errorf("objectlink: put: sent %d bytes of the declared %d", counted.n, size)
	}
	return Result{Size: counted.n, SHA256: hex.EncodeToString(hash.Sum(nil))}, nil
}

// Get streams the object of a link made for GET into w, refusing more than limit bytes (MaxDocument when limit is
// not above zero). When wantSHA256 is given and the bytes hash to another value the call fails: the caller removes
// what it wrote.
func Get(ctx context.Context, client *http.Client, link string, w io.Writer, limit int64, wantSHA256 string) (Result, error) {
	if limit <= 0 {
		limit = MaxDocument
	}
	if client == nil {
		client = http.DefaultClient
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, link, nil)
	if err != nil {
		return Result{}, err
	}
	resp, err := client.Do(req)
	if err != nil {
		return Result{}, fmt.Errorf("objectlink: get: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		return Result{}, storeError(resp)
	}
	if resp.ContentLength > limit {
		return Result{}, tooBig(limit)
	}
	hash := sha256.New()
	n, err := io.Copy(io.MultiWriter(w, hash), io.LimitReader(resp.Body, limit+1))
	if err != nil {
		return Result{}, fmt.Errorf("objectlink: get: %w", err)
	}
	if n > limit {
		return Result{}, tooBig(limit)
	}
	got := hex.EncodeToString(hash.Sum(nil))
	if wantSHA256 != "" && !strings.EqualFold(wantSHA256, got) {
		return Result{}, fmt.Errorf("objectlink: the document hashes to %s, not the %s expected", got, strings.ToLower(wantSHA256))
	}
	return Result{Size: n, SHA256: got}, nil
}