policy/conditional: add first version of Engine

This is just a first very basic version of the conditional engine.

gateway/src/apicast/policy/conditional/engine.lua

@@ -0,0 +1,28 @@
+local _M = {}
+
+local value_of = {
+  request_method = function() return ngx.req.get_method() end,
+  request_host = function() return ngx.var.host end,
+  request_path = function() return ngx.var.uri end
+}
+
+function _M.evaluate(expression)
+  local match_attr = ngx.re.match(expression, [[^([\w]+)$]], 'oj')
+
+  if match_attr then
+    return value_of[match_attr[1]]()
+  end
+
+  local match_attr_and_value = ngx.re.match(expression, [[^([\w]+) == "([\w/]+)"$]], 'oj')
+
+  if not match_attr_and_value then
+    return nil, 'Error while parsing the condition'
+  end
+
+  local entity = match_attr_and_value[1]
+  local value = match_attr_and_value[2]
+
+  return value_of[entity]() == value
+end
+
+return _M