/**
 * Runtime of one open domain: authoritative in-memory state, the single
 * per-domain write chain, and change-event emission. Reads are synchronous
 * from memory; every write queues on the chain, mutates memory, awaits
 * backend durability, then emits `domain/changed` — so events carry values
 * that equal the in-memory state at emission and arrive in write order.
 * @module @deepseek-ai/dsh-domain/src/domain
 */

import type { Context } from 'cordis'
import type { KvUnit } from '@deepseek-ai/dsh-storage'
import { DomainError } from './error.ts'
import type { DomainSpec, DomainGlobalSpec, TableKeyOf, TableValueOf } from './spec.ts'
import type { DomainChanged } from './events.ts'

/** Handle on a domain's global singleton. */
export interface DomainGlobal<G> {
  /**
   * Current value, synchronously from the authoritative in-memory state.
   * Before the first `set` this is the spec's `initial`.
   * @returns the current global value.
   */
  get(): G

  /**
   * Replace the value durably. Queued on the domain's write chain; the first
   * `set` is what materializes the global on the medium.
   * @param value - New value; must satisfy the spec's schema (not re-checked
   * here — validation happens at the durable read boundary).
   * @returns resolution after durability and event emission.
   */
  set(value: G): Promise<void>
}

/**
 * Handle on one declared table. Records are plain immutable data: returned
 * values are the stored objects themselves (no defensive copies) and must not
 * be mutated in place — replace via `put`/`update`.
 */
export interface KvTable<K extends string, V> {
  /**
   * Read one record, synchronously from memory.
   * @param key - Record key.
   * @returns the record, or `undefined` when absent.
   */
  get(key: K): V | undefined

  /**
   * Snapshot iterator over `[key, record]` pairs. A snapshot, not a live
   * view: iteration stays stable while queued writes land.
   * @returns the pair iterator.
   */
  entries(): IterableIterator<[K, V]>

  /**
   * Snapshot iterator over keys.
   * @returns the key iterator.
   */
  keys(): IterableIterator<K>

  /** Current record count. */
  readonly size: number

  /**
   * Insert or overwrite one record durably.
   * @param key - Record key.
   * @param value - The full new record (no partial merge).
   * @returns resolution after durability and event emission.
   */
  put(key: K, value: V): Promise<void>

  /**
   * Delete one record durably.
   * @param key - Record key.
   * @returns `true` when the record existed, `false` when it was already
   * absent (no write and no event in that case).
   */
  delete(key: K): Promise<boolean>

  /**
   * Atomic read-modify-write on the domain's write chain: `fn` sees the
   * value current at its queue slot, so concurrent updates never interleave.
   * @param key - Record key; a missing key rejects with `missing-key`.
   * @param fn - Synchronous pure transform from current to next record.
   * @returns the stored next record.
   */
  update(key: K, fn: (current: V) => V): Promise<V>
}

/** Global handle of a spec: typed when declared, `never` (inaccessible) when not. */
export type DomainGlobalHandleOf<S extends DomainSpec> =
  S extends { readonly global: DomainGlobalSpec<infer G> } ? DomainGlobal<G> : never

/** One open domain, typed by its spec. */
export interface Domain<S extends DomainSpec> {
  /** Domain name from the spec. */
  readonly name: string
  /** Global singleton handle; a spec without `global` has no usable handle (`never`). */
  readonly global: DomainGlobalHandleOf<S>
  /**
   * Resolve one declared table handle. Handles are stable — repeated calls
   * return the same instance.
   * @param name - Declared table name.
   * @returns the typed table handle.
   */
  table<N extends keyof S['tables'] & string>(name: N): KvTable<TableKeyOf<S, N>, TableValueOf<S, N>>
}

/** Internal seam handing table handles their domain-owned write machinery. */
interface TableHost {
  readonly domainName: string
  readonly unit: KvUnit
  /** Queue one job on the domain's single write chain. */
  enqueue<T>(job: () => Promise<T>): Promise<T>
  /** Throw `closed` once the domain has fully closed (reads stay valid while draining). */
  assertReadable(): void
  /** Emit `domain/changed` for one durably landed write. */
  emitChanged(change: DomainChanged): void
}

const noop = () => {}

/**
 * The single domain implementation behind the {@link Domain} interface. The
 * facility constructs it from a validated `loadAll` snapshot and erases it to
 * `Domain<S>`; nothing outside this package constructs one.
 */
export class DomainImpl {
  /** Domain name from the spec. */
  readonly name: string

  private readonly tables = new Map<string, KvTableImpl<string, unknown>>()
  private globalValue: unknown
  private readonly globalHandle?: DomainGlobal<unknown>

  /** Tail of the write chain; every link settles (rejections are observed by the caller's slice). */
  private chain: Promise<void> = Promise.resolve()
  /** Set when dispose begins: new writes reject while already-queued writes drain. */
  private disposing = false
  /** Set when dispose finishes (chain drained, unit closed): reads reject from here on. */
  private closed = false
  private disposal?: Promise<void>

  /**
   * @param ctx - Context that carries `domain/changed` emissions.
   * @param spec - The domain declaration.
   * @param unit - The opened backend unit; this instance owns its lifecycle.
   * @param records - Validated per-table records from the unit's `loadAll`.
   * @param globalValue - Validated stored global, or the spec's `initial`
   * when the medium held none; `undefined` when the spec declares no global.
   */
  constructor(
    private readonly ctx: Context,
    spec: DomainSpec,
    private readonly unit: KvUnit,
    records: Map<string, Map<string, unknown>>,
    globalValue: unknown,
  ) {
    this.name = spec.name
    const host: TableHost = {
      domainName: spec.name,
      unit,
      enqueue: (job) => this.enqueue(job),
      assertReadable: () => this.assertReadable(),
      emitChanged: (change) => this.ctx.emit('domain/changed', change),
    }
    for (const table of Object.keys(spec.tables)) {
      this.tables.set(table, new KvTableImpl(host, table, records.get(table) ?? new Map()))
    }
    if (spec.global !== undefined) {
      this.globalValue = globalValue
      this.globalHandle = {
        get: () => {
          this.assertReadable()
          return this.globalValue
        },
        set: (value) => this.enqueue(async () => {
          this.globalValue = value
          await this.unit.setGlobal(value)
          host.emitChanged({ domain: this.name, table: '', key: '', operation: 'put', value })
        }),
      }
    }
  }

  /** Global singleton handle; accessing it on a spec that declares no global is a caller bug and throws. */
  get global(): DomainGlobal<unknown> {
    if (this.globalHandle === undefined) {
      throw new Error(`domain '${this.name}' declares no global`)
    }
    return this.globalHandle
  }

  /**
   * Resolve one declared table handle; an undeclared name is a caller bug
   * and throws.
   * @param name - Declared table name.
   * @returns the stable table handle.
   */
  table(name: string): KvTable<string, unknown> {
    const table = this.tables.get(name)
    if (table === undefined) {
      throw new Error(`domain '${this.name}' declares no table '${name}'`)
    }
    return table
  }

  /**
   * Close this domain: reject new writes immediately, drain already-queued
   * writes (their events still emit), then close the unit. Idempotent —
   * repeated calls share one teardown.
   * @returns resolution after the unit is released.
   */
  dispose(): Promise<void> {
    this.disposal ??= this.runDispose()
    return this.disposal
  }

  private async runDispose(): Promise<void> {
    this.disposing = true
    // Chain links never reject (each is settled via then(noop, noop)), so
    // this await is a pure drain barrier.
    await this.chain
    await this.unit.close()
    this.closed = true
  }

  private enqueue<T>(job: () => Promise<T>): Promise<T> {
    if (this.disposing) {
      return Promise.reject(new DomainError('closed', `domain '${this.name}' is closed`))
    }
    const result = this.chain.then(job)
    this.chain = result.then(noop, noop)
    return result
  }

  private assertReadable(): void {
    if (this.closed) {
      throw new DomainError('closed', `domain '${this.name}' is closed`)
    }
  }
}

/** Table handle bound to one in-memory record map and its domain's write chain. */
class KvTableImpl<K extends string, V> implements KvTable<K, V> {
  constructor(
    private readonly host: TableHost,
    private readonly tableName: string,
    private readonly records: Map<string, unknown>,
  ) {}

  get(key: K): V | undefined {
    this.host.assertReadable()
    return this.records.get(key) as V | undefined
  }

  entries(): IterableIterator<[K, V]> {
    this.host.assertReadable()
    return ([...this.records.entries()] as [K, V][])[Symbol.iterator]()
  }

  keys(): IterableIterator<K> {
    this.host.assertReadable()
    return ([...this.records.keys()] as K[])[Symbol.iterator]()
  }

  get size(): number {
    this.host.assertReadable()
    return this.records.size
  }

  put(key: K, value: V): Promise<void> {
    return this.host.enqueue(async () => {
      this.records.set(key, value)
      await this.host.unit.putRecord(this.tableName, key, value)
      this.emitPut(key, value)
    })
  }

  delete(key: K): Promise<boolean> {
    return this.host.enqueue(async () => {
      // Existence is decided at this job's chain slot, not at call time: an
      // earlier queued put of the same key makes this delete observe it.
      if (!this.records.has(key)) return false
      this.records.delete(key)
      await this.host.unit.deleteRecord(this.tableName, key)
      this.host.emitChanged({
        domain: this.host.domainName,
        table: this.tableName,
        key,
        operation: 'deleted',
      })
      return true
    })
  }

  update(key: K, fn: (current: V) => V): Promise<V> {
    return this.host.enqueue(async () => {
      if (!this.records.has(key)) {
        throw new DomainError(
          'missing-key',
          `domain '${this.host.domainName}' table '${this.tableName}' has no record '${key}' to update`,
        )
      }
      const next = fn(this.records.get(key) as V)
      this.records.set(key, next)
      await this.host.unit.putRecord(this.tableName, key, next)
      this.emitPut(key, next)
      return next
    })
  }

  private emitPut(key: K, value: V): void {
    this.host.emitChanged({
      domain: this.host.domainName,
      table: this.tableName,
      key,
      operation: 'put',
      value,
    })
  }
}