base64: Add new module

Encodes and decodes base64. No linebreaks, always padding.

2 files changed, 167 insertions, 0 deletions

diff --git a/src/base64.cc b/src/base64.cc
new file mode 100644
index 0000000..ce7b41d
--- /dev/null
+++ b/src/base64.cc
@@ -0,0 +1,142 @@
+#include "base64.hh"
+
+#include <cstddef>
+#include <cstdint>
+#include <optional>
+#include <span>
+#include <string>
+#include <string_view>
+#include <vector>
+
+namespace base64 {
+
+namespace {
+
+std::string_view kAlphabet(
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    "abcdefghijklmnopqrstuvwxyz"
+    "0123456789+/");
+
+[[nodiscard]]
+inline char encode(uint8_t c) {
+  return kAlphabet[c];
+}
+
+[[nodiscard]]
+inline uint8_t decode(char c) {
+  if (c >= 'A' && c <= 'Z') {
+    return c - 'A';
+  }
+  if (c >= 'a' && c <= 'z') {
+    return 26 + (c - 'a');
+  }
+  if (c >= '0' && c <= '9') {
+    return 52 + (c - '0');
+  }
+  if (c == '+')
+    return 62;
+  if (c == '/')
+    return 63;
+  return 255;
+}
+
+}  // namespace
+
+std::string encode(std::span<uint8_t const> data) {
+  std::string ret;
+  encode(data, ret);
+  return ret;
+}
+
+void encode(std::span<uint8_t const> in, std::string& out) {
+  out.reserve((in.size() * 4 + 3) / 3);
+  size_t const leftover = in.size() % 3;
+  size_t const end = in.size() - leftover;
+  size_t i = 0;
+  for (; i < end; i += 3) {
+    out.push_back(encode(in[i] >> 2));
+    out.push_back(encode(((in[i] & 0x03) << 4) | (in[i + 1] >> 4)));
+    out.push_back(encode(((in[i + 1] & 0x0f) << 2) | (in[i + 2] >> 6)));
+    out.push_back(encode(in[i + 2] & 0x3f));
+  }
+  switch (leftover) {  // NOLINT(bugprone-switch-missing-default-case)
+    case 0:
+      return;
+    case 1:
+      out.push_back(encode(in[i] >> 2));
+      out.push_back(encode((in[i] & 0x03) << 4));
+      out.push_back('=');
+      break;
+    case 2:
+      out.push_back(encode(in[i] >> 2));
+      out.push_back(encode(((in[i] & 0x03) << 4) | (in[i + 1] >> 4)));
+      out.push_back(encode((in[i + 1] & 0x0f) << 2));
+      break;
+  }
+  out.push_back('=');
+}
+
+std::optional<std::vector<uint8_t>> decode(std::string_view value) {
+  std::vector<uint8_t> ret;
+  if (decode(value, ret))
+    return ret;
+  return std::nullopt;
+}
+
+bool decode(std::string_view in, std::vector<uint8_t>& out) {
+  if (in.size() % 4)
+    return false;
+
+  if (in.empty())
+    return true;
+
+  size_t pad;
+  if (in.back() == '=') {
+    if (in[in.size() - 2] == '=') {
+      pad = 2;
+    } else {
+      pad = 1;
+    }
+  } else {
+    pad = 0;
+  }
+
+  size_t const end = in.size() - (pad ? 4 : 0);
+  size_t i = 0;
+  for (; i < end; i += 4) {
+    auto v1 = decode(in[i]);
+    auto v2 = decode(in[i + 1]);
+    auto v3 = decode(in[i + 2]);
+    auto v4 = decode(in[i + 3]);
+    if (v1 == 255 || v2 == 255 || v3 == 255 || v4 == 255)
+      return false;
+    out.push_back((v1 << 2) | (v2 >> 4));
+    out.push_back(((v2 & 0xf) << 4) | (v3 >> 2));
+    out.push_back(((v3 & 0x3) << 6) | v4);
+  }
+  switch (pad) {  // NOLINT(bugprone-switch-missing-default-case)
+    case 0:
+      break;
+    case 1: {
+      auto v1 = decode(in[i]);
+      auto v2 = decode(in[i + 1]);
+      auto v3 = decode(in[i + 2]);
+      if (v1 == 255 || v2 == 255 || v3 == 255)
+        return false;
+      out.push_back((v1 << 2) | (v2 >> 4));
+      out.push_back(((v2 & 0xf) << 4) | (v3 >> 2));
+      break;
+    }
+    case 2: {
+      auto v1 = decode(in[i]);
+      auto v2 = decode(in[i + 1]);
+      if (v1 == 255 || v2 == 255)
+        return false;
+      out.push_back((v1 << 2) | (v2 >> 4));
+      break;
+    }
+  }
+  return true;
+}
+
+}  // namespace base64
diff --git a/src/base64.hh b/src/base64.hh
new file mode 100644
index 0000000..491755c
--- /dev/null
+++ b/src/base64.hh
@@ -0,0 +1,25 @@
+#ifndef BASE64_HH
+#define BASE64_HH
+
+#include <cstdint>
+#include <optional>
+#include <span>
+#include <string>
+#include <vector>
+
+namespace base64 {
+
+[[nodiscard]]
+std::string encode(std::span<uint8_t const> data);
+
+void encode(std::span<uint8_t const> in, std::string& out);
+
+[[nodiscard]]
+std::optional<std::vector<uint8_t>> decode(std::string_view value);
+
+[[nodiscard]]
+bool decode(std::string_view in, std::vector<uint8_t>& out);
+
+}  // namespace base64
+
+#endif  // BASE64_HH