string_bytes: Make base64 encode/decode reusable

Node already has support for base64 encoding and decoding that is not
visible outside the string_bytes.cc file. Our work on providing a
support for the Chrome inspector protocol
(#6792) requires base64 encoding
support. Rather then introducing a second copy of the base64 encoder,
we suggest moving this code into a separate header.

PR-URL: #6910
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: ofrobots - Ali Ijaz Sheikh <ofrobots@google.com>

src/base64.h

@@ -0,0 +1,194 @@
+#ifndef SRC_BASE64_H_
+#define SRC_BASE64_H_
+
+#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#include "util.h"
+
+#include <stddef.h>
+
+namespace node {
+//// Base 64 ////
+#define base64_encoded_size(size) ((size + 2 - ((size + 2) % 3)) / 3 * 4)
+
+
+// Doesn't check for padding at the end.  Can be 1-2 bytes over.
+static inline size_t base64_decoded_size_fast(size_t size) {
+  size_t remainder = size % 4;
+
+  size = (size / 4) * 3;
+  if (remainder) {
+    if (size == 0 && remainder == 1) {
+      // special case: 1-byte input cannot be decoded
+      size = 0;
+    } else {
+      // non-padded input, add 1 or 2 extra bytes
+      size += 1 + (remainder == 3);
+    }
+  }
+
+  return size;
+}
+
+template <typename TypeName>
+size_t base64_decoded_size(const TypeName* src, size_t size) {
+  if (size == 0)
+    return 0;
+
+  if (src[size - 1] == '=')
+    size--;
+  if (size > 0 && src[size - 1] == '=')
+    size--;
+
+  return base64_decoded_size_fast(size);
+}
+
+
+extern const int8_t unbase64_table[256];
+
+
+#define unbase64(x)                                                           \
+  static_cast<uint8_t>(unbase64_table[static_cast<uint8_t>(x)])
+
+
+template <typename TypeName>
+size_t base64_decode_slow(char* dst, size_t dstlen,
+                          const TypeName* src, size_t srclen) {
+  uint8_t hi;
+  uint8_t lo;
+  size_t i = 0;
+  size_t k = 0;
+  for (;;) {
+#define V(expr)                                                               \
+    while (i < srclen) {                                                      \
+      const uint8_t c = src[i];                                               \
+      lo = unbase64(c);                                                       \
+      i += 1;                                                                 \
+      if (lo < 64)                                                            \
+        break;  /* Legal character. */                                        \
+      if (c == '=')                                                           \
+        return k;                                                             \
+    }                                                                         \
+    expr;                                                                     \
+    if (i >= srclen)                                                          \
+      return k;                                                               \
+    if (k >= dstlen)                                                          \
+      return k;                                                               \
+    hi = lo;
+    V(/* Nothing. */);
+    V(dst[k++] = ((hi & 0x3F) << 2) | ((lo & 0x30) >> 4));
+    V(dst[k++] = ((hi & 0x0F) << 4) | ((lo & 0x3C) >> 2));
+    V(dst[k++] = ((hi & 0x03) << 6) | ((lo & 0x3F) >> 0));
+#undef V
+  }
+  UNREACHABLE();
+}
+
+
+template <typename TypeName>
+size_t base64_decode_fast(char* const dst, const size_t dstlen,
+                          const TypeName* const src, const size_t srclen,
+                          const size_t decoded_size) {
+  const size_t available = dstlen < decoded_size ? dstlen : decoded_size;
+  const size_t max_i = srclen / 4 * 4;
+  const size_t max_k = available / 3 * 3;
+  size_t i = 0;
+  size_t k = 0;
+  while (i < max_i && k < max_k) {
+    const uint32_t v =
+        unbase64(src[i + 0]) << 24 |
+        unbase64(src[i + 1]) << 16 |
+        unbase64(src[i + 2]) << 8 |
+        unbase64(src[i + 3]);
+    // If MSB is set, input contains whitespace or is not valid base64.
+    if (v & 0x80808080) {
+      break;
+    }
+    dst[k + 0] = ((v >> 22) & 0xFC) | ((v >> 20) & 0x03);
+    dst[k + 1] = ((v >> 12) & 0xF0) | ((v >> 10) & 0x0F);
+    dst[k + 2] = ((v >>  2) & 0xC0) | ((v >>  0) & 0x3F);
+    i += 4;
+    k += 3;
+  }
+  if (i < srclen && k < dstlen) {
+    return k + base64_decode_slow(dst + k, dstlen - k, src + i, srclen - i);
+  }
+  return k;
+}
+
+
+template <typename TypeName>
+size_t base64_decode(char* const dst, const size_t dstlen,
+                     const TypeName* const src, const size_t srclen) {
+  const size_t decoded_size = base64_decoded_size(src, srclen);
+  return base64_decode_fast(dst, dstlen, src, srclen, decoded_size);
+}
+
+static size_t base64_encode(const char* src,
+                            size_t slen,
+                            char* dst,
+                            size_t dlen) {
+  // We know how much we'll write, just make sure that there's space.
+  CHECK(dlen >= base64_encoded_size(slen) &&
+        "not enough space provided for base64 encode");
+
+  dlen = base64_encoded_size(slen);
+
+  unsigned a;
+  unsigned b;
+  unsigned c;
+  unsigned i;
+  unsigned k;
+  unsigned n;
+
+  static const char table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                              "abcdefghijklmnopqrstuvwxyz"
+                              "0123456789+/";
+
+  i = 0;
+  k = 0;
+  n = slen / 3 * 3;
+
+  while (i < n) {
+    a = src[i + 0] & 0xff;
+    b = src[i + 1] & 0xff;
+    c = src[i + 2] & 0xff;
+
+    dst[k + 0] = table[a >> 2];
+    dst[k + 1] = table[((a & 3) << 4) | (b >> 4)];
+    dst[k + 2] = table[((b & 0x0f) << 2) | (c >> 6)];
+    dst[k + 3] = table[c & 0x3f];
+
+    i += 3;
+    k += 4;
+  }
+
+  if (n != slen) {
+    switch (slen - n) {
+      case 1:
+        a = src[i + 0] & 0xff;
+        dst[k + 0] = table[a >> 2];
+        dst[k + 1] = table[(a & 3) << 4];
+        dst[k + 2] = '=';
+        dst[k + 3] = '=';
+        break;
+
+      case 2:
+        a = src[i + 0] & 0xff;
+        b = src[i + 1] & 0xff;
+        dst[k + 0] = table[a >> 2];
+        dst[k + 1] = table[((a & 3) << 4) | (b >> 4)];
+        dst[k + 2] = table[(b & 0x0f) << 2];
+        dst[k + 3] = '=';
+        break;
+    }
+  }
+
+  return dlen;
+}
+}  // namespace node
+
+
+#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#endif  // SRC_BASE64_H_