Refactor file layer

test/scan.h

@@ -136,51 +136,92 @@ class string_scan_buffer : public scan_buffer {
       : scan_buffer(s.begin(), s.end(), true) {}
 };
 
-class file_scan_buffer : public scan_buffer {
- private:
-  FILE* file_;
-  char next_;
-  bool filled_ = false;
+// A FILE wrapper. F is FILE defined as a template parameter to make
+// system-specific API detection work.
+template <typename F> class file_base {
+ protected:
+  F* file_;
+
+  ~file_base() = default;
+
+ public:
+  file_base(F* file) : file_(file) {}
+  operator F*() const { return file_; }
+
+  // Reads a code unit from the stream.
+  auto get() -> int {
+    int result = getc(file_);
+    if (result == EOF && ferror(file_) != 0)
+      FMT_THROW(system_error(errno, FMT_STRING("getc failed")));
+    return result;
+  }
+
+  // Puts the code unit back into the stream buffer.
+  void unget(char c) {
+    if (ungetc(c, file_) == EOF)
+      FMT_THROW(system_error(errno, FMT_STRING("ungetc failed")));
+  }
+};
+
+// A FILE wrapper for Apple's libc.
+template <typename F> class apple_file : public file_base<F> {
+ public:
+  using file_base<F>::file_base;
 
   // Returns the file's read buffer as a string_view.
-  template <typename F, FMT_ENABLE_IF(sizeof(F::_p) != 0)>
-  auto get_buffer(F* file) -> string_view {  // Apple libc
-    char* ptr = reinterpret_cast<char*>(file->_p);
-    return {ptr, to_unsigned(file->_r)};
+  auto buffer() const -> string_view {
+    return {reinterpret_cast<char*>(this->file_->_p),
+            to_unsigned(this->file_->_r)};
+  }
+};
+
+// A fallback FILE wrapper.
+template <typename F> class fallback_file : public file_base<F> {
+ private:
+  char next_;  // The next unconsumed character in the buffer.
+  bool has_next_ = false;
+
+ public:
+  using file_base<F>::file_base;
+
+  auto buffer() const -> string_view { return {&next_, has_next_ ? 1u : 0u}; }
+
+  auto get() -> int {
+    has_next_ = false;
+    return file_base<F>::get();
   }
-  auto get_buffer(...) -> string_view {
-    return {&next_, (filled_ ? 1u : 0u)};
+
+  void unget(char c) {
+    file_base<F>::unget(c);
+    next_ = c;
+    has_next_ = true;
   }
+};
+
+template <typename F, FMT_ENABLE_IF(sizeof(F::_p) != 0)>
+auto get_file(F* file, int) -> apple_file<F> {
+  return file;
+}
+auto get_file(FILE* file, ...) -> fallback_file<FILE> { return file; }
+
+class file_scan_buffer : public scan_buffer {
+ private:
+  decltype(get_file(static_cast<FILE*>(nullptr), 0)) file_;
 
   void do_fill() {
-    string_view buf = get_buffer(file_);
+    string_view buf = file_.buffer();
     if (buf.size() == 0) {
-      int result = getc(file_);
-      if (result != EOF) {
-        // Put the character back since we are only filling the buffer.
-        if (ungetc(result, file_) == EOF)
-          FMT_THROW(system_error(errno, FMT_STRING("I/O error")));
-        next_ = static_cast<char>(result);
-        filled_ = true;
-      } else {
-        if (ferror(file_) != 0)
-          FMT_THROW(system_error(errno, FMT_STRING("I/O error")));
-        filled_ = false;
-      } 
-      buf = get_buffer(file_);
+      int c = file_.get();
+      // Put the character back since we are only filling the buffer.
+      if (c != EOF) file_.unget(static_cast<char>(c));
+      buf = file_.buffer();
     }
     this->set(buf.begin(), buf.end());
   }
 
   void consume() override {
     // Consume the current buffer content.
-    string_view buf = get_buffer(file_);
-    for (size_t i = 0, n = buf.size(); i != n; ++i) {
-      int result = getc(file_);
-      if (result == EOF && ferror(file_) != 0)
-        FMT_THROW(system_error(errno, FMT_STRING("I/O error")));
-    }
-    filled_ = false;
+    for (size_t i = 0, n = file_.buffer().size(); i != n; ++i) file_.get();
     do_fill();
   }