packages/core/agent-loop/src/runtime-context.ts

/**
 * Durable projection state for the two loop-owned surface messages the system
 * prompt plugin forms: the system prompt (surface node 0 and any in-history
 * replacement) and the dynamic runtime-context snapshot.
 * @module @deepseek-ai/dsh-agent-loop/runtime-context
 */

import { createSystemMessage, createUserMessage } from '@deepseek-ai/dsh-llm'
import type { ContextSnapshotSection, Message } from '@deepseek-ai/dsh-llm'
import type { Session, SessionEvent, SessionSeq, SurfaceIntent, SystemMessage, UserMessage } from '@deepseek-ai/dsh-session'
import { isReplacementSurfaceEvent } from '@deepseek-ai/dsh-session'
import type { Context } from '@deepseek-ai/cordis'

const SOURCE = '@deepseek-ai/dsh-system-prompt'
const CLEARED = 'Current runtime context: none. Earlier runtime-context snapshots no longer apply.'

function isOwned(message: UserMessage): boolean {
  return message.source.kind === 'plugin' && message.source.plugin === SOURCE
}

function textOf(message: Message): string | undefined {
  const [block] = message.content
  return message.content.length === 1 && block?.type === 'text' ? block.text : undefined
}

/** One uncommitted system-prompt surface operation for request admission or reconciliation. */
export interface SystemPromptCommit {
  /** Rendered prompt or empty content: an empty head records no prompt; empty tails are dormant. */
  message: SystemMessage
  /** `append` for a new system node, otherwise a replacement of one surviving system node. */
  intent: SurfaceIntent<'system/message'>
}

/** The request-series facts one prompt decision is made under. */
export interface SystemPromptDecisionInput {
  /** Whether the prepared route for this attempt reads a later `system` message as the effective prompt. */
  inHistory: boolean
  /**
   * Whether this step's request starts a new model-message series: a pre-step
   * listener declared one, the surface was replaced since the last request, or
   * the assembled tool schemas differ from the logged header.
   */
  startsSeries: boolean
}

/** Committed events from the newest backward; the restore scans stop at the first match. */
function eventsNewestFirst(session: Session): readonly SessionEvent[] {
  // oxlint-disable-next-line typescript/no-deprecated -- Existing Session history read; migration deferred.
  return session.snapshotEvents().toReversed()
}

/**
 * Decides how a rendered system prompt reaches the surface without owning the
 * commit. The first prompt, even empty, reserves surface node 0.
 * A capable continuing series appends changed nonempty text after the
 * cached history. An incapable route, broken series, or cleared prompt instead
 * normalizes the first system node and empties later active nodes. Dormant empty
 * tails do not supply effective text or require repeated replacements.
 */
export class SystemPromptProjection {
  constructor(private readonly session: Session) {}

  /** The surviving `system/message` nodes in surface order. */
  private systemNodes(): { seq: SessionSeq; text: string | undefined }[] {
    const nodes: { seq: SessionSeq; text: string | undefined }[] = []
    for (const seq of this.session.surface.nodes) {
      // oxlint-disable-next-line typescript/no-deprecated -- Existing Session history read; migration deferred.
      const event = this.session.eventAt(seq)
      if (event?.type !== 'system/message') continue
      const content = event.data.message.content
      const text = content.length === 0 ? '' : textOf(event.data.message)
      nodes.push({ seq, text })
    }
    return nodes
  }

  /**
   * Reconcile effective text and retained nodes with the prepared route and series.
   * @param rendered - the fully rendered system prompt; `''` when none is active.
   * @param input - the route capability and series facts for this step.
   * @returns ordered per-node updates; an empty list means no update is needed.
   */
  project(rendered: string, input: SystemPromptDecisionInput): SystemPromptCommit[] {
    const nodes = this.systemNodes()
    const head = nodes[0]
    if (head === undefined) {
      return [{ message: createSystemMessage(rendered, SOURCE), intent: { surfaceOp: 'append' } }]
    }
    const latest = nodes.findLast(node => node.text !== '') ?? head
    if (!input.inHistory || input.startsSeries || rendered.length === 0) {
      const updates = nodes.slice(1).filter(node => node.text !== '')
        .map(node => this.replace(node.seq, ''))
      if (head.text !== rendered) updates.push(this.replace(head.seq, rendered))
      return updates
    }
    if (latest.text === rendered) return []
    return [{ message: createSystemMessage(rendered, SOURCE), intent: { surfaceOp: 'append' } }]
  }

  private replace(seq: SessionSeq, text: string): SystemPromptCommit {
    return {
      message: createSystemMessage(text, SOURCE),
      intent: { surfaceOp: { op: 'replace', startSeq: seq, endSeq: seq }, sourceEventSeqs: [seq] },
    }
  }
}

/** Tracks the last retained runtime-context snapshot without owning its commit. */
export class RuntimeContextProjection {
  /** `undefined` means no snapshot ever existed; `null` means none is retained. */
  private retained: { seq: SessionSeq; text: string | undefined } | null | undefined

  /**
   * Restore projection state once, then follow authoritative session events.
   * @param ctx - agent-scoped event context.
   * @param session - session receiving projected messages.
   */
  constructor(ctx: Context, session: Session) {
    const surface = new Set(session.surface.nodes)
    for (const event of eventsNewestFirst(session)) {
      if (event.type !== 'user/message' || !isOwned(event.data)) continue
      this.retained ??= null
      if (surface.has(event.seq)) {
        this.retained = { seq: event.seq, text: textOf(event.data) }
        break
      }
    }

    ctx.on('session/event', (subject, event) => {
      if (subject !== session) return
      if (event.type === 'user/message' && isOwned(event.data)) {
        this.retained = { seq: event.seq, text: textOf(event.data) }
      } else if (this.retained
        && isReplacementSurfaceEvent(event)
        && event.sourceEventSeqs?.includes(this.retained.seq) === true) {
        this.retained = null
      }
    })
  }

  /**
   * Create an uncommitted snapshot only when the retained value differs.
   * @param current - fully rendered dynamic context.
   * @param sections - named contributions that formed the current snapshot.
   * @returns a candidate user message, or `undefined` when no update is needed.
   */
  project(current: string, sections: readonly ContextSnapshotSection[]): UserMessage | undefined {
    if (this.retained === undefined && current.length === 0) return
    const snapshot = current.length === 0 ? CLEARED : current
    if (this.retained?.text === snapshot) return
    return createUserMessage({
      content: [{ type: 'text', text: snapshot }],
      // The cleared marker has no contributions left to attribute.
      source: sections.length === 0
        ? { kind: 'plugin', plugin: SOURCE }
        : { kind: 'plugin', plugin: SOURCE, form: 'snapshot', sections },
    })
  }
}