Refactor file layer

test/scan-test.cc

@@ -28,6 +28,8 @@ TEST(scan_test, read_int) {
   EXPECT_EQ(n, 42);
   fmt::scan("-42", "{}", n);
   EXPECT_EQ(n, -42);
+  EXPECT_THROW_MSG(fmt::scan(std::to_string(INT_MAX + 1u), "{}", n),
+                   fmt::format_error, "number is too big");
 }
 
 TEST(scan_test, read_longlong) {

test/scan.h

@@ -136,51 +136,90 @@ 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_;
+
+ 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)};
   }
-  auto get_buffer(...) -> string_view {
-    return {&next_, (filled_ ? 1u : 0u)};
+};
+
+// 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();
   }
 
+  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();
   }
 
@@ -317,18 +356,30 @@ struct scan_handler : error_handler {
   scan_arg arg_;
 
   template <typename T = unsigned> auto read_uint() -> T {
-    T value = 0;
     auto it = scan_ctx_.begin(), end = scan_ctx_.end();
-    char c = it != end ? *it : '\0';
+    char c = it != end ? *it : '\0', prev_digit;
     if (c < '0' || c > '9') on_error("invalid input");
+
+    int num_digits = 0;
+    T value = 0, prev = 0;
     do {
+      prev = value;
       value = value * 10 + static_cast<unsigned>(c - '0');
+      prev_digit = c;
       c = *++it;
+      ++num_digits;
       if (c < '0' || c > '9') break;
-      // TODO: check overflow
     } while (it != end);
     scan_ctx_.advance_to(it);
-    return value;
+    
+    // Check overflow.
+    if (num_digits <= std::numeric_limits<int>::digits10) return value;
+    const unsigned max = to_unsigned((std::numeric_limits<int>::max)());
+    if (num_digits == std::numeric_limits<int>::digits10 + 1 &&
+        prev * 10ull + unsigned(prev_digit - '0') <= max) {
+      return value;
+    }
+    throw format_error("number is too big");
   }
 
   template <typename T = int> auto read_int() -> T {