Use CoverageData everywhere for consistency

This allows us to get rid off duplicated and inconsistent
calculations. As a side effect of this, #565 is fixed now.

So, fixes #565.

The bigger purpose is multi step though where the access
to coverage data is normalized and hence methods that work
with expecting a minimum amount of coverage and formatters
can work with the known good same data base as opposed
to memoizing all different method names.

At first I wanted to only do it for FileList and leave the
rest for later. But the strength computation needed the
total number of lines of code and so to pass it in I'd have
had to calculate it but I wanted to calculate it only in
CoverageData and there the avalanche started....

Commit also includes moving methods around in SourceFile to be
private - it happened in the moment, forgive me for no extra
commit (it was hard to determine what I could delete/what I need
to adjust etc.).

lib/simplecov/coverage_data.rb

@@ -13,21 +13,44 @@ module SimpleCov
   class CoverageData
     attr_reader :total, :covered, :missed, :strength, :percent
 
+    def self.from(coverage_data)
+      covered, missed, total_strength =
+        coverage_data.reduce([0, 0, 0.0]) do |(covered, missed, total_strength), file_coverage_data|
+          [
+            covered + file_coverage_data.covered,
+            missed + file_coverage_data.missed,
+            # gotta remultiply with loc because files have different strenght and loc
+            # giving them a different "weight" in total
+            total_strength + (file_coverage_data.strength * file_coverage_data.total)
+          ]
+        end
+
+      new(covered: covered, missed: missed, total_strength: total_strength)
+    end
+
     # Requires only covered, missed and strength to be initialized.
     #
     # Other values are computed by this class.
-    def initialize(covered:, missed:, strength: nil)
+    def initialize(covered:, missed:, total_strength: 0.0)
       @covered  = covered
       @missed   = missed
-      @strength = strength
       @total    = covered + missed
       @percent  = compute_percent(covered, total)
+      @strength = compute_strength(total_strength, @total)
     end
 
+  private
+
     def compute_percent(covered, total)
       return 100.0 if total.zero?
 
       Float(covered * 100.0 / total)
     end
+
+    def compute_strength(total_strength, total)
+      return 0.0 if total.zero?
+
+      Float(total_strength.to_f / total)
+    end
   end
 end

lib/simplecov/file_list.rb

@@ -13,7 +13,7 @@ class FileList
                    # also delegating methods implemented in Enumerable as they have
                    # custom Array implementations which are presumably better/more
                    # resource efficient
-                   :size, :map,
+                   :size, :map, :count,
                    # surprisingly not in Enumerable
                    :empty?, :length,
                    # still act like we're kinda an array
@@ -24,24 +24,17 @@ def initialize(files)
     end
 
     def coverage
-      {
-        **line_coverage,
-        **branch_coverage
-      }
+      @coverage ||= compute_coverage
     end
 
     # Returns the count of lines that have coverage
     def covered_lines
-      return 0.0 if empty?
-
-      map { |f| f.covered_lines.count }.inject(:+)
+      coverage[:line]&.covered
     end
 
     # Returns the count of lines that have been missed
     def missed_lines
-      return 0.0 if empty?
-
-      map { |f| f.missed_lines.count }.inject(:+)
+      coverage[:line]&.missed
     end
 
     # Returns the count of lines that are not relevant for coverage
@@ -71,75 +64,51 @@ def least_covered_file
 
     # Returns the overall amount of relevant lines of code across all files in this list
     def lines_of_code
-      covered_lines + missed_lines
+      coverage[:line]&.total
     end
 
     # Computes the coverage based upon lines covered and lines missed
     # @return [Float]
     def covered_percent
-      percent(covered_lines, lines_of_code)
+      coverage[:line]&.percent
     end
 
     # Computes the strength (hits / line) based upon lines covered and lines missed
     # @return [Float]
     def covered_strength
-      return 0.0 if empty? || lines_of_code.zero?
-
-      Float(map { |f| f.covered_strength * f.lines_of_code }.inject(:+) / lines_of_code)
+      coverage[:line]&.strength
     end
 
     # Return total count of branches in all files
     def total_branches
-      return 0 if empty?
-
-      map { |file| file.total_branches.count }.inject(:+)
+      coverage[:branch]&.total
     end
 
     # Return total count of covered branches
     def covered_branches
-      return 0 if empty?
-
-      map { |file| file.covered_branches.count }.inject(:+)
+      coverage[:branch]&.covered
     end
 
     # Return total count of covered branches
     def missed_branches
-      return 0 if empty?
-
-      map { |file| file.missed_branches.count }.inject(:+)
+      coverage[:branch]&.missed
     end
 
     def branch_covered_percent
-      percent(covered_branches, total_branches)
+      coverage[:branch]&.percent
     end
 
   private
 
-    def percent(covered, total)
-      return 100.0 if empty? || total.zero?
-
-      Float(covered * 100.0 / total)
-    end
-
-    def line_coverage
-      {
-        line: CoverageData.new(
-          strength: covered_strength,
-          covered: covered_lines,
-          missed: missed_lines
-        )
-      }
-    end
-
-    def branch_coverage
-      return nil unless SimpleCov.branch_coverage?
+    def compute_coverage
+      total_coverage_data = @files.each_with_object(line: [], branch: []) do |file, together|
+        together[:line] << file.coverage[:line]
+        together[:branch] << file.coverage[:branch] if SimpleCov.branch_coverage?
+      end
 
-      {
-        branch: CoverageData.new(
-          covered: covered_branches,
-          missed: missed_branches
-        )
-      }
+      coverage_data = {line: CoverageData.from(total_coverage_data[:line])}
+      coverage_data[:branch] = CoverageData.from(total_coverage_data[:branch]) if SimpleCov.branch_coverage?
+      coverage_data
     end
   end
 end