11 files changed, 325 insertions, 0 deletions
diff --git a/lib/gitlab/ci/pipeline/expression/lexeme/base.rb b/lib/gitlab/ci/pipeline/expression/lexeme/base.rb
new file mode 100644
index 00000000000..047ab66e9b3
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexeme/base.rb
@@ -0,0 +1,25 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class Base
+            def evaluate(**variables)
+              raise NotImplementedError
+            end
+
+            def self.build(token)
+              raise NotImplementedError
+            end
+
+            def self.scan(scanner)
+              if scanner.scan(self::PATTERN)
+                Expression::Token.new(scanner.matched, self)
+              end
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/lexeme/equals.rb b/lib/gitlab/ci/pipeline/expression/lexeme/equals.rb
new file mode 100644
index 00000000000..3a2f0c6924e
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexeme/equals.rb
@@ -0,0 +1,26 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class Equals < Lexeme::Operator
+            PATTERN = /==/.freeze
+
+            def initialize(left, right)
+              @left = left
+              @right = right
+            end
+
+            def evaluate(variables = {})
+              @left.evaluate(variables) == @right.evaluate(variables)
+            end
+
+            def self.build(_value, behind, ahead)
+              new(behind, ahead)
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/lexeme/null.rb b/lib/gitlab/ci/pipeline/expression/lexeme/null.rb
new file mode 100644
index 00000000000..a2778716924
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexeme/null.rb
@@ -0,0 +1,25 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class Null < Lexeme::Value
+            PATTERN = /null/.freeze
+
+            def initialize(value = nil)
+              @value = nil
+            end
+
+            def evaluate(variables = {})
+              nil
+            end
+
+            def self.build(_value)
+              self.new
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/lexeme/operator.rb b/lib/gitlab/ci/pipeline/expression/lexeme/operator.rb
new file mode 100644
index 00000000000..f640d0b5855
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexeme/operator.rb
@@ -0,0 +1,15 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class Operator < Lexeme::Base
+            def self.type
+              :operator
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/lexeme/string.rb b/lib/gitlab/ci/pipeline/expression/lexeme/string.rb
new file mode 100644
index 00000000000..48bde213d44
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexeme/string.rb
@@ -0,0 +1,25 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class String < Lexeme::Value
+            PATTERN = /("(?<string>.+?)")|('(?<string>.+?)')/.freeze
+
+            def initialize(value)
+              @value = value
+            end
+
+            def evaluate(variables = {})
+              @value.to_s
+            end
+
+            def self.build(string)
+              new(string.match(PATTERN)[:string])
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/lexeme/value.rb b/lib/gitlab/ci/pipeline/expression/lexeme/value.rb
new file mode 100644
index 00000000000..f2611d65faf
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexeme/value.rb
@@ -0,0 +1,15 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class Value < Lexeme::Base
+            def self.type
+              :value
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/lexeme/variable.rb b/lib/gitlab/ci/pipeline/expression/lexeme/variable.rb
new file mode 100644
index 00000000000..b781c15fd67
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexeme/variable.rb
@@ -0,0 +1,25 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class Variable < Lexeme::Value
+            PATTERN = /\$(?<name>\w+)/.freeze
+
+            def initialize(name)
+              @name = name
+            end
+
+            def evaluate(variables = {})
+              HashWithIndifferentAccess.new(variables).fetch(@name, nil)
+            end
+
+            def self.build(string)
+              new(string.match(PATTERN)[:name])
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/lexer.rb b/lib/gitlab/ci/pipeline/expression/lexer.rb
new file mode 100644
index 00000000000..e1c68b7c3c2
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/lexer.rb
@@ -0,0 +1,59 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Lexer
+          include ::Gitlab::Utils::StrongMemoize
+
+          LEXEMES = [
+            Expression::Lexeme::Variable,
+            Expression::Lexeme::String,
+            Expression::Lexeme::Null,
+            Expression::Lexeme::Equals
+          ].freeze
+
+          SyntaxError = Class.new(Statement::StatementError)
+
+          MAX_TOKENS = 100
+
+          def initialize(statement, max_tokens: MAX_TOKENS)
+            @scanner = StringScanner.new(statement)
+            @max_tokens = max_tokens
+          end
+
+          def tokens
+            strong_memoize(:tokens) { tokenize }
+          end
+
+          def lexemes
+            tokens.map(&:to_lexeme)
+          end
+
+          private
+
+          def tokenize
+            tokens = []
+
+            @max_tokens.times do
+              @scanner.skip(/\s+/) # ignore whitespace
+
+              return tokens if @scanner.eos?
+
+              lexeme = LEXEMES.find do |type|
+                type.scan(@scanner).tap do |token|
+                  tokens.push(token) if token.present?
+                end
+              end
+
+              unless lexeme.present?
+                raise Lexer::SyntaxError, 'Unknown lexeme found!'
+              end
+            end
+
+            raise Lexer::SyntaxError, 'Too many tokens!'
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/parser.rb b/lib/gitlab/ci/pipeline/expression/parser.rb
new file mode 100644
index 00000000000..90f94d0b763
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/parser.rb
@@ -0,0 +1,40 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Parser
+          def initialize(tokens)
+            @tokens = tokens.to_enum
+            @nodes = []
+          end
+
+          ##
+          # This produces a reverse descent parse tree.
+          #
+          # It currently does not support precedence of operators.
+          #
+          def tree
+            while token = @tokens.next
+              case token.type
+              when :operator
+                token.build(@nodes.pop, tree).tap do |node|
+                  @nodes.push(node)
+                end
+              when :value
+                token.build.tap do |leaf|
+                  @nodes.push(leaf)
+                end
+              end
+            end
+          rescue StopIteration
+            @nodes.last || Lexeme::Null.new
+          end
+
+          def self.seed(statement)
+            new(Expression::Lexer.new(statement).tokens)
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/statement.rb b/lib/gitlab/ci/pipeline/expression/statement.rb
new file mode 100644
index 00000000000..4f0e101b730
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/statement.rb
@@ -0,0 +1,42 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Statement
+          StatementError = Class.new(StandardError)
+
+          GRAMMAR = [
+            %w[variable equals string],
+            %w[variable equals variable],
+            %w[variable equals null],
+            %w[string equals variable],
+            %w[null equals variable],
+            %w[variable]
+          ].freeze
+
+          def initialize(statement, pipeline)
+            @lexer = Expression::Lexer.new(statement)
+
+            @variables = pipeline.variables.map do |variable|
+              [variable.key, variable.value]
+            end
+          end
+
+          def parse_tree
+            raise StatementError if @lexer.lexemes.empty?
+
+            unless GRAMMAR.find { |syntax| syntax == @lexer.lexemes }
+              raise StatementError, 'Unknown pipeline expression!'
+            end
+
+            Expression::Parser.new(@lexer.tokens).tree
+          end
+
+          def evaluate
+            parse_tree.evaluate(@variables.to_h)
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/pipeline/expression/token.rb b/lib/gitlab/ci/pipeline/expression/token.rb
new file mode 100644
index 00000000000..58211800b88
--- /dev/null
+++ b/lib/gitlab/ci/pipeline/expression/token.rb
@@ -0,0 +1,28 @@
+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Token
+          attr_reader :value, :lexeme
+
+          def initialize(value, lexeme)
+            @value = value
+            @lexeme = lexeme
+          end
+
+          def build(*args)
+            @lexeme.build(@value, *args)
+          end
+
+          def type
+            @lexeme.type
+          end
+
+          def to_lexeme
+            @lexeme.name.demodulize.downcase
+          end
+        end
+      end
+    end
+  end
+end