DEV: adding a concept of prompt builder

This is half baked and not working, but starts demonstrating the idea

lib/modules/ai_bot/bot.rb

@@ -219,6 +219,22 @@ module DiscourseAi
         0
       end
 
+      def bot_prompt_with_topic_context2(post)
+        builder = tokenizer.build_prompt(max_tokens: prompt_limit)
+        builder << { user: bot_user.username, content: rendered_system_prompt, type: :system }
+
+        conversation_context(post).each do |raw, username, function|
+          builder.unshift(
+            user: username,
+            content: raw,
+            type: :username == bot_user.username ? :assistant : :user,
+          )
+          break if builder.full?
+        end
+
+        builder.generate
+      end
+
       def bot_prompt_with_topic_context(post, prompt: "topic")
         messages = []
         conversation = conversation_context(post)

lib/shared/prompting/prompt_builder.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module DiscourseAi
+  module Prompting
+    class PromptBuilder
+      def initialize(tokenizer:, max_tokens:)
+        @tokenizer = tokenizer
+        @max_tokens = max_tokens
+        @contents = []
+      end
+
+      def full?
+      end
+
+      def <<(content:, type:, user: nil)
+        validate_type(type)
+
+        @contents << { content: content, type: type, user: user }
+      end
+
+      def unshift(content:, type:, user: nil)
+        validate_type(type)
+
+        @contents.unshift(content: content, type: type, user: user)
+      end
+
+      def generate
+        raise NotImplemented
+      end
+
+      def validate_type(type)
+        if !%i[system assistant user].include?(type)
+          raise ArgumentError, "type must be one of :system, :assistant, :user"
+        end
+      end
+    end
+  end
+end