instagram: new extraction method

new extraction method first tries to fetch content directly from instagram graphql API, fallback to html embed page. in case every method fail, rely on 3rd party

util/misc.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"crypto/rand"
 	"fmt"
 	"govd/models"
 	"net/http"
@@ -95,6 +96,44 @@ func GetLastError(err error) error {
 	return lastErr
 }
 
+func RandomBase64(length int) string {
+	const letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
+	const mask = 63 // 6 bits, since len(letters) == 64
+
+	result := make([]byte, length)
+	random := make([]byte, length)
+	_, err := rand.Read(random)
+	if err != nil {
+		return strings.Repeat("A", length)
+	}
+	for i, b := range random {
+		result[i] = letters[int(b)&mask]
+	}
+	return string(result)
+}
+
+func RandomAlphaString(length int) string {
+	const letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+	const lettersLen = byte(len(letters))
+	const maxByte = 255 - (255 % lettersLen) // 255 - (255 % 52) = 255 - 47 = 208
+
+	result := make([]byte, length)
+	i := 0
+	for i < length {
+		b := make([]byte, 1)
+		_, err := rand.Read(b)
+		if err != nil {
+			return strings.Repeat("a", length)
+		}
+		if b[0] > maxByte {
+			continue // avoid bias
+		}
+		result[i] = letters[b[0]%lettersLen]
+		i++
+	}
+	return string(result)
+}
+
 func ParseCookieFile(fileName string) ([]*http.Cookie, error) {
 	cachedCookies, ok := cookiesCache[fileName]
 	if ok {