/lib/clacky/agent/message_compressor.rb

# frozen_string_literal: true

module Clacky
  # Message compressor using Insert-then-Compress strategy
  #
  # New Strategy: Instead of creating a separate API call for compression,
  # we insert a compression instruction into the current conversation flow.
  # This allows us to reuse the existing cache (system prompt + tools) and
  # only pay for processing the new compression instruction.
  #
  # Flow:
  # 1. Agent detects compression threshold is reached
  # 2. Compressor builds a compression instruction message
  # 3. Agent inserts this message and calls LLM (with cache reuse!)
  # 4. LLM returns compressed summary
  # 5. Compressor rebuilds message list: system + summary + recent messages
  # 6. Agent continues with new message list (cache will rebuild from here)
  #
  # Benefits:
  # - Compression call reuses existing cache (huge token savings)
  # - Only one cache rebuild after compression (vs two with old approach)
  #
  class MessageCompressor
    COMPRESSION_PROMPT = <<~PROMPT.freeze
      ═══════════════════════════════════════════════════════════════
      CRITICAL: TASK CHANGE - MEMORY COMPRESSION MODE
      ═══════════════════════════════════════════════════════════════
      The conversation above has ENDED. You are now in MEMORY COMPRESSION MODE.

      CRITICAL INSTRUCTIONS - READ CAREFULLY:

      1. This is NOT a continuation of the conversation
      2. DO NOT respond to any requests in the conversation above
      3. DO NOT call ANY tools or functions
      4. DO NOT use tool_calls in your response
      5. Your response MUST be PURE TEXT ONLY

      YOUR ONLY TASK: Create a comprehensive summary of the conversation above.

      REQUIRED RESPONSE FORMAT:
      First output a <topics> line listing 3-6 key topic phrases (comma-separated, concise).
      Then output a <continues_previous> line: "true" if this conversation is a direct
      continuation of the SAME task/topic as the PREVIOUS chunk shown below, "false" if it
      has moved on to a different task or topic. If there is no previous chunk, output "false".
      Then output the full summary wrapped in <summary> tags.

      Example format:
      <topics>Rails setup, database config, deploy pipeline, Tailwind CSS</topics>
      <continues_previous>false</continues_previous>
      <summary>
      ...full summary text...
      </summary>

      Focus on:
      - User's explicit requests and intents
      - Key technical concepts and code changes
      - Files examined and modified
      - Errors encountered and fixes applied
      - Current work status and pending tasks

      Begin your response NOW. Remember: PURE TEXT only, starting with <topics> then
      <continues_previous> then <summary>.
    PROMPT

    def initialize(client, model: nil)
      @client = client
      @model = model
    end

    # Generate compression instruction message to be inserted into conversation
    # This enables cache reuse by using the same API call with tools
    # 
    # SIMPLIFIED APPROACH:
    # - Don't duplicate conversation history in the compression message
    # - LLM can already see all messages, just ask it to compress
    # - Keep the instruction small for better cache efficiency
    #
    # @param messages [Array<Hash>] Original conversation messages
    # @param recent_messages [Array<Hash>] Recent messages to keep uncompressed (optional)
    # @param previous_topics [String, nil] Topics of the most recent chunk on disk,
    #   shown to the LLM so it can decide whether the current conversation is a
    #   continuation (drives the <continues_previous> output for chunk merging).
    # @return [Hash] Compression instruction message to insert, or nil if nothing to compress
    def build_compression_message(messages, recent_messages: [], previous_topics: nil)
      # Get messages to compress (exclude system message and recent messages)
      messages_to_compress = messages.reject { |m| m[:role] == "system" || recent_messages.include?(m) }

      # If nothing to compress, return nil
      return nil if messages_to_compress.empty?

      content = COMPRESSION_PROMPT
      if previous_topics && !previous_topics.strip.empty?
        content = "#{COMPRESSION_PROMPT}\n\nPREVIOUS CHUNK TOPICS (for <continues_previous> judgement): #{previous_topics}"
      end

      {
        role: "user",
        content: content,
        system_injected: true
      }
    end

    # Parse LLM response and rebuild message list with compression
    # @param compressed_content [String] The compressed summary from LLM
    # @param original_messages [Array<Hash>] Original messages before compression
    # @param recent_messages [Array<Hash>] Recent messages to preserve
    # @param chunk_path [String, nil] Path to the archived chunk MD file (if saved)
    # @param pulled_back_messages [Array<Hash>] Messages temporarily popped from the
    #   tail of @history before the compression LLM call (to free up token budget so
    #   the compression call itself doesn't overflow context). These are NOT discarded —
    #   they are reattached to the tail of the rebuilt history so recent task progress
    #   is preserved. Default: [] (normal compression path doesn't need this).
    # @return [Array<Hash>] Rebuilt message list: system + compressed + recent + pulled_back
    def rebuild_with_compression(compressed_content, original_messages:, recent_messages:, chunk_path: nil, topics: nil, previous_chunks: [], pulled_back_messages: [])
      # Find and preserve system message
      system_msg = original_messages.find { |m| m[:role] == "system" }

      # Parse the compressed result, embedding previous chunk references so the
      # new summary carries a complete index of all older archives. This avoids
      # keeping all prior compressed_summary messages in active history while
      # still giving the AI a path to find old conversations via file_reader.
      parsed_messages = parse_compressed_result(compressed_content,
                                                chunk_path: chunk_path,
                                                topics: topics,
                                                previous_chunks: previous_chunks)

      # If parsing fails or returns empty, raise error
      if parsed_messages.nil? || parsed_messages.empty?
        raise "LLM compression failed: unable to parse compressed messages"
      end

      # Return system message + compressed messages + recent messages + pulled_back messages.
      # Strip any system messages from recent_messages as a safety net —
      # get_recent_messages_with_tool_pairs already excludes them, but this
      # guard ensures we never end up with duplicate system prompts even if
      # the caller passes an unfiltered list.
      #
      # pulled_back_messages: messages that were temporarily popped from the tail
      # of @history before the compression LLM call (to free up token budget so
      # the compression call itself doesn't overflow context). They are reattached
      # here to preserve recent task progress.
      safe_recent = recent_messages.reject { |m| m[:role] == "system" }
      safe_pulled_back = pulled_back_messages.reject { |m| m[:role] == "system" }
      [system_msg, *parsed_messages, *safe_recent, *safe_pulled_back].compact
    end


    # Parse topics tag from compressed content.
    # Returns the topics string if found, nil otherwise.
    # e.g. "<topics>Rails setup, database config</topics>" → "Rails setup, database config"
    def parse_topics(content)
      return nil if content.nil? || content.to_s.empty?
      m = content.to_s.match(/<topics>(.*?)<\/topics>/m)
      m ? m[1].strip : nil
    end

    # Parse the <continues_previous> tag. Returns true only when the LLM
    # explicitly says "true"; missing tag or any other value → false.
    # This conservative default ensures we never merge unless the model is sure.
    def parse_continues_previous(content)
      return false if content.nil? || content.to_s.empty?
      m = content.to_s.match(/<continues_previous>(.*?)<\/continues_previous>/m)
      m ? m[1].strip.downcase == "true" : false
    end

    def parse_compressed_result(result, chunk_path: nil, topics: nil, previous_chunks: [])
      # Return the compressed result as a single user message (role: "user").
      #
      # Why role:"user" instead of "assistant":
      #   When all original user messages get archived into the chunk during compression
      #   (e.g. a long single-turn `/slash` task), the rebuilt history can end up as
      #   `system → assistant(summary) → assistant(tool_calls) → tool → …` with NO user
      #   message anywhere. Strict providers (notably DeepSeek V4 thinking mode) reject
      #   this as a malformed turn structure with a misleading
      #   "reasoning_content must be passed back" 400 error.
      #
      # Marking it as a user message gives the conversation a valid turn boundary.
      # `system_injected: true` ensures the UI's replay_history still hides it from
      # the chat panel (the real-user filter excludes system_injected messages), while
      # INTERNAL_FIELDS in MessageHistory strips the marker before the API payload is
      # built — so DeepSeek/OpenAI/Anthropic only see a plain `{role:"user", content:…}`.
      #
      # The `compressed_summary: true` flag is preserved so that replay_history still
      # routes this message through the chunk-expansion path (which keys off that flag,
      # not the role).
      #
      # @param topics [String, nil] Short topic description extracted from <topics> tag
      # @param previous_chunks [Array<Hash>] Info about older chunk files
      #   Each hash: { basename:, path:, topics: }
      content = result.to_s.strip

      if content.empty?
        []
      else
        # Strip out the <topics> and <continues_previous> blocks — they're
        # metadata for chunk handling, not for AI context.
        content_without_topics = content.gsub(/<topics>.*?<\/topics>\n*/m, "")
                                        .gsub(/<continues_previous>.*?<\/continues_previous>\n*/m, "")
                                        .strip

        # Build previous chunks index section — links to older chunk files so the AI
        # can find earlier conversations without keeping all prior compressed_summary
        # messages in the active history. Shows newest chunks first (reverse order),
        # capped at 10 to keep the message size bounded.
        previous_chunks_section = ""
        if previous_chunks.any?
          max_visible = 10
          visible = previous_chunks.last(max_visible).reverse
          older_count = previous_chunks.size - visible.size

          previous_chunks_section = "\n\n---\n📁 **Previous chunks (newest first):**\n"
          visible.each do |pc|
            topic_str = pc[:topics] ? " — #{pc[:topics]}" : ""
            previous_chunks_section += "- `#{pc[:basename]}`#{topic_str}\n"
          end

          if older_count > 0
            oldest = previous_chunks.first
            previous_chunks_section += "- ... and #{older_count} older chunks back to `#{oldest[:basename]}`\n"
          end

          previous_chunks_section += "_Use `file_reader` to recall details from these chunks._"
        end

        # Inject chunk anchor so AI knows where to find original conversation for THIS chunk
        anchor = ""
        if chunk_path
          anchor = "\n\n---\n📁 **Current chunk archived at:** `#{chunk_path}`\n" \
                   "_Use `file_reader` tool to recall details from this chunk._"
        end

        # Prefix lets the model recognise this is injected context, not a user utterance.
        # Order: summary → previous chunks → current anchor (chronological)
        framed_content = "[Compressed conversation summary — previous turns archived]\n\n" \
                         "#{content_without_topics}" \
                         "#{previous_chunks_section}" \
                         "#{anchor}"

        [{
          role: "user",
          content: framed_content,
          compressed_summary: true,
          chunk_path: chunk_path,
          topics: topics,
          system_injected: true
        }]
      end
    end
  end
end