Rename the variable 'quality' to 'brotli_quality'

Right now the quality is only used in the brotli compressor. And it
would be ambiguous if we add another compressor in the future; since the
compressors may have different interpretion on the quality value.

Test: unit tests pass; run bsdiff
Change-Id: I23af06a135b2b47df5209171f7db0773ac02a326
diff --git a/bsdiff_arguments.h b/bsdiff_arguments.h
index b330834..1907e73 100644
--- a/bsdiff_arguments.h
+++ b/bsdiff_arguments.h
@@ -14,18 +14,19 @@
 
 namespace bsdiff {
 
-// Class to store the patch writer options about format, type and quality.
+// Class to store the patch writer options about format, type and
+// brotli_quality.
 class BsdiffArguments {
  public:
   BsdiffArguments()
       : format_(BsdiffFormat::kLegacy),
         compressor_type_(CompressorType::kBZ2),
-        compression_quality_(-1) {}
+        brotli_quality_(-1) {}
 
-  BsdiffArguments(BsdiffFormat format, CompressorType type, int quality)
+  BsdiffArguments(BsdiffFormat format, CompressorType type, int brotli_quality)
       : format_(format),
         compressor_type_(type),
-        compression_quality_(quality) {}
+        brotli_quality_(brotli_quality) {}
 
   // Check if the compressor type is compatible with the bsdiff format.
   bool IsValid() const;
@@ -37,7 +38,7 @@
 
   CompressorType compressor_type() const { return compressor_type_; }
 
-  int compression_quality() const { return compression_quality_; }
+  int brotli_quality() const { return brotli_quality_; }
 
   // Parse the command line arguments of the main function and set all the
   // fields accordingly.
@@ -52,8 +53,12 @@
   // Parse the bsdiff format from string.
   static bool ParseBsdiffFormat(const std::string& str, BsdiffFormat* format);
 
-  // Parse the compression quality (for brotli) from string.
-  static bool ParseQuality(const std::string& str, int* quality);
+  // Parse the compression quality (for brotli) from string; also check if the
+  // value is within the valid range.
+  static bool ParseQuality(const std::string& str,
+                           int* quality,
+                           int min,
+                           int max);
 
  private:
   // Current format supported are the legacy "BSDIFF40" or "BSDF2".
@@ -62,9 +67,8 @@
   // The algorithm to compress the patch, i.e. BZ2 or Brotli.
   CompressorType compressor_type_;
 
-  // The quality of compression, only valid when using brotli as the
-  // compression algorithm.
-  int compression_quality_;
+  // The quality of brotli compressor.
+  int brotli_quality_;
 
   size_t min_length_{0};
 };