Let ameba explain the issue at the specified location (#86)

spec/ameba/formatter/explain_formatter_spec.cr

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+require "../../spec_helper"
+
+module Ameba
+  def explanation(source)
+    output = IO::Memory.new
+    ErrorRule.new.catch(source)
+    location = {file: "source.cr", line: 1, column: 1}
+    Formatter::ExplainFormatter.new(output, location).finished([source])
+    output.to_s
+  end
+
+  describe Formatter::ExplainFormatter do
+    describe "#location" do
+      it "returns crystal location" do
+        location = Formatter::ExplainFormatter
+          .new(STDOUT, {file: "compiler.cr", line: 3, column: 8}).location
+
+        location.is_a?(Crystal::Location).should be_true
+        location.filename.should eq "compiler.cr"
+        location.line_number.should eq 3
+        location.column_number.should eq 8
+      end
+    end
+
+    describe "#output" do
+      it "returns io" do
+        output = Formatter::ExplainFormatter
+          .new(STDOUT, {file: "compiler.cr", line: 3, column: 8}).output
+        output.should eq STDOUT
+      end
+    end
+
+    describe "#finished" do
+      it "writes issue info" do
+        source = Source.new "a = 42", "source.cr"
+        output = explanation(source)
+        output.should contain "ISSUE INFO"
+        output.should contain "This rule always adds an error"
+        output.should contain "source.cr:1:1"
+      end
+
+      it "writes affected code" do
+        source = Source.new "a = 42", "source.cr"
+        output = explanation(source)
+        output.should contain "AFFECTED CODE"
+        output.should contain "a = 42"
+      end
+
+      it "writes rule info" do
+        source = Source.new "a = 42", "source.cr"
+        output = explanation(source)
+        output.should contain "RULE INFO"
+        output.should contain "Ameba/ErrorRule"
+        output.should contain "Always adds an error at 1:1"
+      end
+
+      it "writes detailed description" do
+        source = Source.new "a = 42", "source.cr"
+        output = explanation(source)
+        output.should contain "DETAILED DESCRIPTION"
+        output.should contain "TO BE DONE..."
+      end
+
+      it "writes nothing if location not found" do
+        source = Source.new "a = 42", "another_source.cr"
+        explanation(source).should be_empty
+      end
+    end
+  end
+end

spec/ameba/formatter/util_spec.cr

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+require "../../spec_helper"
+
+module Ameba::Formatter
+  class Subject
+    include Util
+  end
+
+  subject = Subject.new
+
+  describe Util do
+    describe "#affected_code" do
+      it "returns nil if there is no such a line number" do
+        source = Source.new %(
+          a = 1
+        )
+        location = Crystal::Location.new("filename", 2, 1)
+        subject.affected_code(source, location).should be_nil
+      end
+
+      it "returns correct line if it is found" do
+        source = Source.new %(
+          a = 1
+        )
+        location = Crystal::Location.new("filename", 1, 1)
+        subject.affected_code(source, location).should eq "> a = 1\n  \e[33m^\e[0m"
+      end
+    end
+  end
+end