From fe7936d4bbb1d2db0e97711c0d8626ecd65fee78 Mon Sep 17 00:00:00 2001
From: 0xFEEDC0DE64 <daniel@brunner.ninja>
Date: Tue, 23 Aug 2022 15:24:44 +0200
Subject: [PATCH] Add base64_decode

---
 src/strutils.cpp | 52 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/strutils.h   |  2 ++
 2 files changed, 54 insertions(+)

diff --git a/src/strutils.cpp b/src/strutils.cpp
index a088d4f..f42cc0c 100644
--- a/src/strutils.cpp
+++ b/src/strutils.cpp
@@ -107,6 +107,58 @@ std::string toBase64String(std::basic_string_view<unsigned char> buf)
     return out;
 }
 
+namespace {
+constexpr std::string_view base64_chars =
+        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+        "abcdefghijklmnopqrstuvwxyz"
+        "0123456789+/";
+bool is_base64(char c) {
+  return (isalnum(c) || (c == '+') || (c == '/'));
+}
+}
+
+std::string base64_decode(std::string const& encoded_string)
+{
+  int in_len = encoded_string.size();
+  int i = 0;
+  int j = 0;
+  int in_ = 0;
+  char char_array_4[4], char_array_3[3];
+  std::string ret;
+
+  while (in_len-- && ( encoded_string[in_] != '=') && is_base64(encoded_string[in_])) {
+    char_array_4[i++] = encoded_string[in_]; in_++;
+    if (i ==4) {
+      for (i = 0; i <4; i++)
+        char_array_4[i] = base64_chars.find(char_array_4[i]);
+
+      char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
+        char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
+        char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];
+
+        for (i = 0; (i < 3); i++)
+            ret.push_back(char_array_3[i]);
+        i = 0;
+      }
+    }
+
+    if (i) {
+      for (j = i; j <4; j++)
+        char_array_4[j] = 0;
+
+      for (j = 0; j <4; j++)
+        char_array_4[j] = base64_chars.find(char_array_4[j]);
+
+      char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
+      char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
+      char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];
+
+      for (j = 0; (j < i - 1); j++) ret.push_back(char_array_3[j]);
+    }
+
+    return ret;
+  }
+
 bool stringEqualsIgnoreCase(std::string_view a, std::string_view b)
 {
     if (a.size() != b.size())
diff --git a/src/strutils.h b/src/strutils.h
index 1fc4638..ed867aa 100644
--- a/src/strutils.h
+++ b/src/strutils.h
@@ -50,6 +50,8 @@ inline std::string toBase64String(std::string_view str)
     return toBase64String(std::basic_string_view<unsigned char>{reinterpret_cast<const unsigned char *>(str.data()), str.size()});
 }
 
+std::string base64_decode(std::string const& encoded_string);
+
 bool stringEqualsIgnoreCase(std::string_view a, std::string_view b);
 bool stringStartsWith(std::string_view fullString, std::string_view begin);
 bool stringEndsWith(std::string_view fullString, std::string_view ending);