New rule: rand zero

spec/ameba/rule/rand_zero_spec.cr

@@ -0,0 +1,36 @@
+require "../../spec_helper"
+
+module Ameba::Rule
+  describe RandZero do
+    subject = RandZero.new
+
+    it "passes if it is not rand(1) or rand(0)" do
+      s = Source.new %(
+        rand(1.0)
+        rand(0.11)
+        rand(2)
+      )
+      subject.catch(s).should be_valid
+    end
+
+    it "fails if it is rand(0)" do
+      s = Source.new "rand(0)"
+      subject.catch(s).should_not be_valid
+    end
+
+    it "fails if it is rand(1)" do
+      s = Source.new "rand(1)"
+      subject.catch(s).should_not be_valid
+    end
+
+    it "reports rule, location and a message" do
+      s = Source.new "rand(1)", "source.cr"
+      subject.catch(s).should_not be_valid
+      error = s.errors.first
+
+      error.rule.should_not be_nil
+      error.location.to_s.should eq "source.cr:1:1"
+      error.message.should eq "rand(1) always returns 0"
+    end
+  end
+end

src/ameba/rule/rand_zero.cr

@@ -0,0 +1,46 @@
+module Ameba::Rule
+  # A rule that disallows `rand(0)` and `rand(1)` calls.
+  # Such calls always return `0`.
+  #
+  # For example:
+  #
+  # ```
+  # rand(1)
+  # ```
+  #
+  # Should be written as:
+  #
+  # ```
+  # rand
+  # # or
+  # rand(2)
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # RandZero:
+  #   Enabled: true
+  # ```
+  #
+  struct RandZero < Base
+    properties do
+      description = "Disallows rand zero calls"
+    end
+
+    def test(source)
+      AST::Visitor.new self, source
+    end
+
+    def test(source, node : Crystal::Call)
+      return unless node.name == "rand" &&
+                    node.args.size == 1 &&
+                    (arg = node.args.first) &&
+                    (arg.is_a? Crystal::NumberLiteral) &&
+                    (value = arg.value) &&
+                    (value == "0" || value == "1")
+
+      source.error self, node.location, "#{node} always returns 0"
+    end
+  end
+end