1 files changed, 92 insertions, 0 deletions
diff --git a/sax/src/guessing_decoder.cc b/sax/src/guessing_decoder.cc
new file mode 100644
index 0000000..e72dab3
--- /dev/null
+++ b/sax/src/guessing_decoder.cc
@@ -0,0 +1,92 @@
+#include "guessing_decoder.hh"
+
+#include "decoder.hh"
+#include "sax_decoder.hh"
+#include "utf8.hh"
+#include "utf_error.hh"
+
+#include <cassert>
+
+using namespace std::string_view_literals;
+
+namespace modxml {
+namespace sax {
+
+namespace {
+
+bool eq(std::span<uint8_t const> a, std::size_t& a_offset, std::string_view b) {
+  if (a.size() - a_offset < b.size())
+    return false;
+  for (size_t i = 0; i < b.size(); ++i)
+    if (a[a_offset + i] != b[i])
+      return false;
+  return true;
+}
+
+class GuessingDecoder : public Decoder {
+ public:
+  State decode(std::span<uint8_t const> in, std::size_t& in_offset,
+               std::span<uint8_t> out, std::size_t& out_offset) override {
+    assert(in_offset <= in.size());
+
+    if (!decided_) {
+      if (eq(in, in_offset, "\xef\xbb\xbf"sv)) {
+        decided_ = create_utf8_decoder();
+      } else if (eq(in, in_offset, "\xfe\xff\x00\x00"sv)) {
+        in_offset += 4;
+        decided_ = create_utf32be_decoder();
+      } else if (eq(in, in_offset, "\xfe\xff"sv)) {
+        // Could be UTF-32 BOM, need more data to decide
+        // (note, an xml document encoded in UTF-16 that is less than 4 bytes
+        //  is rather impossible).
+        if (in.size() - in_offset < 4)
+          return State::NEED_MORE;
+        in_offset += 2;
+        decided_ = create_utf16be_decoder();
+      } else if (eq(in, in_offset, "\xff\xfe"sv)) {
+        in_offset += 2;
+        decided_ = create_utf16le_decoder();
+      } else if (eq(in, in_offset, "\x00\x00\xff\xfe"sv)) {
+        in_offset += 4;
+        decided_ = create_utf32le_decoder();
+      } else {
+        auto avail = in.size() - in_offset;
+        if (avail == 0)
+          return State::NEED_MORE;
+        if (avail >= 4 && in[in_offset] == 0 && in[in_offset + 1] == 0
+            && in[in_offset + 2] == 0 && in[in_offset + 3] != 0) {
+          decided_ = create_utf32le_decoder();
+        } else if (avail >= 4 && in[in_offset] != 0 && in[in_offset + 1] == 0
+                   && in[in_offset + 2] == 0 && in[in_offset + 3] == 0) {
+          decided_ = create_utf32be_decoder();
+        } else if (avail >= 2 && in[in_offset] == 0 && in[in_offset + 1] != 0) {
+          decided_ = create_utf16le_decoder();
+        } else if (avail >= 2 && in[in_offset] != 0 && in[in_offset + 1] == 0) {
+          decided_ = create_utf16be_decoder();
+        } else {
+          auto tmp = in_offset;
+          auto ret = utf::read8(in, tmp);
+          if (ret == utf::NEED_MORE)
+            return State::NEED_MORE;
+          if (ret == utf::INVALID)
+            return State::INVALID;
+          // UTF-8 should be good enough to read the XML declaration.
+          decided_ = create_utf8_decoder();
+        }
+      }
+    }
+    return decided_->decode(in, in_offset, out, out_offset);
+  }
+
+ private:
+  std::unique_ptr<Decoder> decided_;
+};
+
+}  // namespace
+
+std::unique_ptr<Decoder> create_guessing_decoder() {
+  return std::make_unique<GuessingDecoder>();
+}
+
+}  // namespace sax
+}  // namespace modxml