Implement pipeline expressions parser

6 files changed, 49 insertions, 5 deletions

diff --git a/lib/gitlab/ci/pipeline/expression/lexeme.rb b/lib/gitlab/ci/pipeline/expression/lexeme.rb
index bbf3fec7407..91ba333d8dd 100644
--- a/lib/gitlab/ci/pipeline/expression/lexeme.rb
+++ b/lib/gitlab/ci/pipeline/expression/lexeme.rb
@@ -7,6 +7,10 @@ module Gitlab
             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)
diff --git a/lib/gitlab/ci/pipeline/expression/lexer.rb b/lib/gitlab/ci/pipeline/expression/lexer.rb
index 8432b36b066..ee202ebce69 100644
--- a/lib/gitlab/ci/pipeline/expression/lexer.rb
+++ b/lib/gitlab/ci/pipeline/expression/lexer.rb
@@ -5,7 +5,8 @@ module Gitlab
         class Lexer
           LEXEMES = [
             Expression::Variable,
-            Expression::String
+            Expression::String,
+            Expression::Equals
           ]
 
           MAX_CYCLES = 5
diff --git a/lib/gitlab/ci/pipeline/expression/statement.rb b/lib/gitlab/ci/pipeline/expression/statement.rb
index 91d91195f5a..417b3f806b1 100644
--- a/lib/gitlab/ci/pipeline/expression/statement.rb
+++ b/lib/gitlab/ci/pipeline/expression/statement.rb
@@ -3,6 +3,8 @@ module Gitlab
     module Pipeline
       module Expression
         class Statement
+          ParserError = Class.new(StandardError)
+
           GRAMMAR = [
             %w[variable equals string],
             %w[variable equals variable],
@@ -12,14 +14,41 @@ module Gitlab
             %w[variable]
           ]
 
-          def initialize(pipeline, statement)
+          def initialize(statement, pipeline)
             @pipeline = pipeline
-            @statement = statement
+            @lexer = Expression::Lexer.new(statement)
           end
 
           def variables
           end
 
+          def tokens
+            @lexer.tokenize
+          end
+
+          def lexemes
+            @lexemes ||= tokens.map(&:to_lexeme)
+          end
+
+          ##
+          # Our syntax is very simple, so we don't need yet to implement a
+          # recurisive parser, we can use the most simple approach to create
+          # a reverse descent parse tree "by hand".
+          #
+          def parse_tree
+            raise ParserError if lexemes.empty?
+
+            unless GRAMMAR.find { |syntax| syntax == lexemes }
+              raise ParserError, 'Unknown pipeline expression!'
+            end
+
+            if lexemes.many?
+              Expression::Equals.new(tokens.first.build, tokens.last.build)
+            else
+              tokens.first.build
+            end
+          end
+
           def evaluate
           end
         end
diff --git a/lib/gitlab/ci/pipeline/expression/string.rb b/lib/gitlab/ci/pipeline/expression/string.rb
index e6cc50c56d8..6a59b81467c 100644
--- a/lib/gitlab/ci/pipeline/expression/string.rb
+++ b/lib/gitlab/ci/pipeline/expression/string.rb
@@ -14,6 +14,7 @@ module Gitlab
           end
 
           def self.build(string)
+            new(string.match(PATTERN)[:string])
           end
         end
       end
diff --git a/lib/gitlab/ci/pipeline/expression/token.rb b/lib/gitlab/ci/pipeline/expression/token.rb
index 03a47a17d40..3b957c83016 100644
--- a/lib/gitlab/ci/pipeline/expression/token.rb
+++ b/lib/gitlab/ci/pipeline/expression/token.rb
@@ -10,7 +10,12 @@ module Gitlab
             @type = type
           end
 
+          def build
+            @type.build(@value)
+          end
+
           def to_lexeme
+            type.name.demodulize.downcase
           end
         end
       end
diff --git a/lib/gitlab/ci/pipeline/expression/variable.rb b/lib/gitlab/ci/pipeline/expression/variable.rb
index ec6c03d336a..d6a7a655220 100644
--- a/lib/gitlab/ci/pipeline/expression/variable.rb
+++ b/lib/gitlab/ci/pipeline/expression/variable.rb
@@ -5,12 +5,16 @@ module Gitlab
         class Variable < Expression::Lexeme
           PATTERN = /\$(?<name>\w+)/.freeze
 
-          def initialize(value)
-            @value = value
+          def initialize(name)
+            @name = name
           end
 
           def evaluate(**variables)
           end
+
+          def self.build(string)
+            new(string.match(PATTERN)[:name])
+          end
         end
       end
     end