/**
 * Session-projection seam: the merge-extensible `SessionProjectionMap` type
 * table, the `ProjectionDefinition` state-driven computation unit contract,
 * and the `ctx.sessionProjections` registry that DRIVES every registered unit
 * forward eagerly over committed session events. Domain host plugins
 * contribute pure mathematics (init/apply/view); the framework owns the
 * subscription, the per-session watermark cache, and change notification;
 * carriers consume the snapshot read face and the change feed. Neither side
 * knows the other
 * (capability-seam three-way split). Design authority: the session-projection
 * RFC (.agents/notes/proposed/architecture/2026-07-27-session-projection-and-command-log.md).
 *
 * Whole-value event rule (load-bearing): a state-carrying log event MUST
 * carry the complete post-change state, never a bare delta — it keeps every
 * unit's transition trivially cheap and every served value self-describing.
 *
 * @module @deepseek-ai/dsh-session-projection
 */

import { Context, Service } from 'cordis'
import type { ZodType } from 'zod'
import type { Session, SessionEvent } from '@deepseek-ai/dsh-session'

declare module 'cordis' {
  interface Context {
    sessionProjections: SessionProjectionRegistry
  }
}

import type { SessionProjectionMap } from './types.ts'

export type { SessionProjectionMap } from './types.ts'

/**
 * One domain's state-driven computation unit: three pure synchronous
 * functions plus declarations — never an opaque getter. The framework drives
 * `apply` on every committed session event; the domain holds no
 * subscriptions and owns only the mathematics. All three functions MUST be
 * synchronous (an async unit would tear the carriers' consistency cut) and
 * `state` MUST be plain JSON (the persisted-cache precondition).
 */
export interface ProjectionDefinition<K extends keyof SessionProjectionMap, S> {
  /** The projection key this unit owns (its `SessionProjectionMap` entry). */
  key: K
  /** Validates the wire payload (`view` output) before it leaves the host. */
  schema: ZodType<SessionProjectionMap[K]>
  /**
   * State for the empty log.
   * @returns the initial state.
   */
  init(): S
  /**
   * Pure transition: previous state + one committed event → next state. A
   * unit uninterested in an event MUST return the same state reference — an
   * unchanged reference (`Object.is`) produces zero downstream work.
   * @param state - the state covering all prior events.
   * @param event - the next committed session event.
   * @returns the next state (same reference when the event is not the unit's).
   */
  apply(state: S, event: SessionEvent): S
  /**
   * State → wire payload (the read-side projection).
   * @param state - the current state.
   * @returns the whole current value for this unit's key.
   */
  view(state: S): SessionProjectionMap[K]
  /**
   * Persisted-cache invalidation anchor: bump whenever the state shape or the
   * fold semantics change, so persisted `(sessionId, key, ver, seq, val)`
   * rows from an older unit are discarded instead of being forward-applied
   * into garbage. Non-negative integer.
   */
  stateVersion: number
}

/**
 * Change-feed listener: one unit's value changed for one session. `value` is
 * the schema-validated `view` output; `seq` is the unit's watermark at
 * emission (the seq of the event that caused the change).
 */
export type ProjectionChangeListener = (
  session: Session,
  key: Extract<keyof SessionProjectionMap, string>,
  value: unknown,
  seq: number,
) => void

/**
 * One consistent read cut over every registered unit for one session.
 * `asOfSeq` is the shared watermark — the seq of the last event every value
 * reflects (`-1` for an empty log, mirroring `session/subscribed.lastSeq`).
 */
export interface ProjectionSnapshot {
  /** Seq of the last event the values reflect; -1 for an empty log. */
  asOfSeq: number
  /** Whole current value per registered key. */
  values: Partial<SessionProjectionMap>
}

/**
 * One unit's checkpoint: its internal state (plain JSON by the unit
 * contract), the seq of the last event folded into it, and the unit
 * `stateVersion` that produced it — the persisted projection-cache row
 * `(sessionId, key, ver, seq, val)` minus the two outer keys. A row is
 * never authoritative, only a fold shortcut: `restore` discards it on a
 * version mismatch or when it claims events past the stored log end.
 */
export interface ProjectionCheckpointRow {
  /** The registering unit's `stateVersion` at fold time. */
  ver: number
  /** Seq of the last event folded into `val`; -1 for the empty log. */
  seq: number
  /** The unit's internal state — plain JSON per the unit contract. */
  val: unknown
}

/** Checkpoint rows keyed by projection key (one session's persisted cache value). */
export type ProjectionCheckpoint = Record<string, ProjectionCheckpointRow>

/** Type-erased unit view the drive machinery works with (the register seam already proved the typed contract). */
interface ErasedDefinition {
  key: string
  schema: { parse(value: unknown): unknown }
  init(): unknown
  apply(state: unknown, event: SessionEvent): unknown
  view(state: unknown): unknown
  stateVersion: number
}

/** Per-session per-unit watermark cache row. */
interface UnitCell {
  state: unknown
  /** Seq of the last event passed through `apply` (regardless of change). */
  observedSeq: number
}

/** One live registration: the unit plus its per-session cells (dropped whole on disposal). */
interface Registration {
  readonly def: ErasedDefinition
  readonly cells: WeakMap<Session, UnitCell>
}

/**
 * `ctx.sessionProjections`: the projection unit table and its drive. The
 * service subscribes to `session/event` once; every committed event passes
 * every registered unit's `apply` (eager drive), and a changed state
 * reference notifies the change feed with the schema-validated view.
 * Cells build lazily — a unit registered after events flowed, or a session
 * older than the registry, folds `init` over the in-memory log on first
 * touch (event or read). Registration is an effect (disposer rides the
 * calling fiber): an unloaded domain plugin's key disappears from snapshots
 * and clients read it as capability absence. Duplicate keys throw. Domain
 * plugins register under `ctx.inject(['sessionProjections'], …)` so headless
 * assemblies without the registry stay unaffected.
 */
export class SessionProjectionRegistry extends Service {
  private readonly registrations = new Map<string, Registration>()
  private readonly listeners = new Set<ProjectionChangeListener>()

  /**
   * Create and install the registry as `ctx.sessionProjections`.
   * @param ctx - Cordis context that owns the service.
   */
  constructor(ctx: Context) {
    super(ctx, 'sessionProjections')
    ctx.on('session/event', (session: Session, event: SessionEvent) => {
      this.drive(session, event)
    })
  }

  /**
   * Register one domain's unit. The registration is an effect on the calling
   * context's fiber: disposing the fiber (or calling the returned disposer)
   * removes the key — and the unit's cached cells — from subsequent drives
   * and snapshots.
   * @param definition - key, boundary schema, pure unit functions, and stateVersion.
   * @returns the exact disposer that unregisters this unit.
   */
  register<K extends keyof SessionProjectionMap, S>(definition: ProjectionDefinition<K, S>): () => void {
    if (!Number.isSafeInteger(definition.stateVersion) || definition.stateVersion < 0) {
      throw new Error(`session projection ${JSON.stringify(definition.key)} stateVersion must be a non-negative integer, got ${String(definition.stateVersion)}`)
    }
    const dispose = this.ctx.effect(function* (this: SessionProjectionRegistry) {
      const key = definition.key as string
      if (this.registrations.has(key)) {
        throw new Error(`session projection key ${JSON.stringify(key)} is already registered`)
      }
      this.registrations.set(key, { def: definition, cells: new WeakMap() })
      yield () => {
        this.registrations.delete(key)
      }
    }.bind(this), 'sessionProjections.register()')
    return () => void dispose()
  }

  /**
   * Subscribe to the change feed. The registration is an effect on the
   * calling context's fiber.
   * @param listener - called once per unit whose state reference changed, per committed event.
   * @returns the exact disposer that unsubscribes.
   */
  onChanged(listener: ProjectionChangeListener): () => void {
    const dispose = this.ctx.effect(() => {
      this.listeners.add(listener)
      return () => {
        this.listeners.delete(listener)
      }
    }, 'sessionProjections.onChanged()')
    return () => void dispose()
  }

  /**
   * One consistent cut over every registered unit for one session, read from
   * the watermark cache (missing cells fold lazily over the in-memory log).
   * Fully synchronous — every value and `asOfSeq` reflect the same log
   * position. Each value passes its unit's schema before leaving.
   * @param session - the session whose projection values are read.
   * @returns the snapshot; `values` is empty when no unit is registered.
   */
  snapshot(session: Session): ProjectionSnapshot {
    const values: Record<string, unknown> = {}
    for (const registration of this.registrations.values()) {
      const cell = this.cellFor(registration, session)
      values[registration.def.key] = registration.def.schema.parse(registration.def.view(cell.state))
    }
    return { asOfSeq: session.seq - 1, values: values }
  }

  /**
   * State-level checkpoint of every registered unit for one session, read
   * from the watermark cache (missing cells fold lazily over the in-memory
   * log). This is the write side of the persisted projection cache: the
   * returned rows are the `(key → {ver, seq, val})` part of the durable
   * `(sessionId, key, ver, seq, val)`
   * rows. Every `val` is a DETACHED structured clone — never the live
   * cell reference: the watermark cache is this registry's authoritative
   * mutable state, and a caller reaching the live reference could corrupt
   * every subsequent snapshot and frame through it (plain JSON by the unit
   * contract, so the clone is total).
   * @param session - the session whose unit states are checkpointed.
   * @returns one row per registered key; empty when no unit is registered.
   */
  checkpoint(session: Session): ProjectionCheckpoint {
    const rows: ProjectionCheckpoint = {}
    for (const registration of this.registrations.values()) {
      const cell = this.cellFor(registration, session)
      rows[registration.def.key] = {
        ver: registration.def.stateVersion,
        seq: cell.observedSeq,
        val: structuredClone(cell.state),
      }
    }
    return rows
  }

  /**
   * The stored seq a {@link restore} tail read over `checkpoint` must start
   * at: one event BELOW the lowest usable watermark (a row is usable when
   * its `ver` matches the live unit's `stateVersion`; an absent or mismatched row
   * pulls the floor to `0` — that key must refold the full log). The
   * one-below anchor is load-bearing: the tail then proves how far the
   * stored log still extends, so {@link restore} can detect a log that
   * shrank below a row's watermark (crash-repair truncation) instead of
   * serving the stale row as current — an empty tail read from the anchor
   * yields an end below every watermark and the restore rejects for a full
   * re-read.
   * @param checkpoint - persisted rows for one session (possibly stale or empty).
   * @returns the seq to hand the persistence `readFrom`, or `undefined`
   *   when no unit is registered (no read needed — {@link restore} would
   *   serve empty values regardless).
   */
  restoreFloor(checkpoint: ProjectionCheckpoint): number | undefined {
    let floor: number | undefined
    for (const registration of this.registrations.values()) {
      const row = checkpoint[registration.def.key]
      const need = row !== undefined && row.ver === registration.def.stateVersion
        ? Math.max(row.seq + 1, 0)
        : 0
      floor = floor === undefined ? need : Math.min(floor, need)
    }
    return floor === undefined ? undefined : Math.max(floor - 1, 0)
  }

  /**
   * View a checkpoint's rows without any log read: for every registered
   * unit whose row's `ver` matches, serve the schema-validated
   * `view` of the stored state; mismatched or absent rows leave their key
   * absent (a cold or listing consumer treats it as not-yet-available and a
   * fuller read path refolds it). The zero-I/O rung of the read ladder —
   * values are as stale as their rows, never wrong.
   * @param checkpoint - persisted rows for one session (possibly stale or empty).
   * @returns whole values per key with a usable row; empty when none.
   */
  viewCheckpoint(checkpoint: ProjectionCheckpoint): Partial<SessionProjectionMap> {
    const values: Record<string, unknown> = {}
    for (const registration of this.registrations.values()) {
      const def = registration.def
      const row = checkpoint[def.key]
      if (row === undefined || row.ver !== def.stateVersion) continue
      values[def.key] = def.schema.parse(def.view(row.val))
    }
    return values
  }

  /**
   * Cold read: fold every registered unit over a stored log suffix, seeding
   * each from its checkpoint row when usable — the one read recipe (cached
   * state + forward tail replay + `view`) applied without a live `Session`.
   * Call with the events returned by a persistence
   * `readFrom(id, restoreFloor(checkpoint))` and that same floor as
   * `baseSeq`; the floor's one-below anchor makes the supplied end honest,
   * so a shrunk log is detected here. A row is usable iff its
   * `ver` matches the live unit's `stateVersion`, it does not predate `baseSeq`
   * (`seq >= baseSeq - 1`), and it does not claim events past the
   * supplied end (`seq <= endSeq`); an unusable row is discarded
   * and its key refolds from `init` — which is only sound over the full
   * log, so a discarded row with `baseSeq > 0` throws (the caller re-reads
   * from seq 0, e.g. after a crash-repair truncation shrank the log below
   * a row's watermark).
   * @param checkpoint - persisted rows for one session (possibly stale or empty).
   * @param events - the stored events with `seq >= baseSeq`, in seq order.
   * @param baseSeq - the seq `events` starts at (its first event's seq when non-empty).
   * @returns the snapshot cut at the supplied log end (`asOfSeq` is the last
   *   supplied event's seq, `baseSeq - 1` for an empty tail) plus the
   *   refreshed checkpoint rows at that cut, ready for a durable write-back.
   */
  restore(checkpoint: ProjectionCheckpoint, events: readonly SessionEvent[], baseSeq: number):
  { snapshot: ProjectionSnapshot; checkpoint: ProjectionCheckpoint } {
    const endSeq = events.at(-1)?.seq ?? baseSeq - 1
    const values: Record<string, unknown> = {}
    const refreshed: ProjectionCheckpoint = {}
    for (const registration of this.registrations.values()) {
      const def = registration.def
      const row = checkpoint[def.key]
      const usable = row !== undefined
        && row.ver === def.stateVersion
        && row.seq >= baseSeq - 1
        && row.seq <= endSeq
      if (!usable && baseSeq > 0) {
        throw new Error(
          `session projection ${JSON.stringify(def.key)} cannot restore from seq ${baseSeq}: `
          + 'its checkpoint row is missing, version-mismatched, or beyond the supplied log end; re-read from seq 0',
        )
      }
      let state = usable ? row.val : def.init()
      const from = usable ? row.seq : baseSeq - 1
      for (const event of events) {
        if (event.seq > from) state = def.apply(state, event)
      }
      values[def.key] = def.schema.parse(def.view(state))
      refreshed[def.key] = { ver: def.stateVersion, seq: endSeq, val: state }
    }
    return {
      snapshot: { asOfSeq: endSeq, values: values },
      checkpoint: refreshed,
    }
  }

  /** Fold one unit from init over `events`, producing a cell watermarked at the last folded event. */
  private buildCell(def: ErasedDefinition, events: readonly SessionEvent[]): UnitCell {
    let state = def.init()
    for (const event of events) state = def.apply(state, event)
    return { state, observedSeq: (events.at(-1)?.seq ?? -1) }
  }

  /** Read (or lazily build, folding the full in-memory log) one unit's cell. */
  private cellFor(registration: Registration, session: Session): UnitCell {
    let cell = registration.cells.get(session)
    if (cell === undefined) {
      cell = this.buildCell(registration.def, session.events)
      registration.cells.set(session, cell)
    }
    return cell
  }

  /** Eager drive: pass one committed event through every registered unit; notify on changed references. */
  private drive(session: Session, event: SessionEvent): void {
    for (const registration of this.registrations.values()) {
      let cell = registration.cells.get(session)
      if (cell === undefined) {
        // Late build mid-stream: fold history before this event (seq = log
        // index, so the prefix slice is exact), then take the normal gate.
        cell = this.buildCell(registration.def, session.events.slice(0, event.seq))
        registration.cells.set(session, cell)
      }
      const next = registration.def.apply(cell.state, event)
      const changed = !Object.is(next, cell.state)
      cell.state = next
      cell.observedSeq = event.seq
      if (changed && this.listeners.size > 0) {
        const value = registration.def.schema.parse(registration.def.view(next))
        for (const listener of this.listeners) {
          listener(session, registration.def.key as Extract<keyof SessionProjectionMap, string>, value, event.seq)
        }
      }
    }
  }
}

export default SessionProjectionRegistry