/**
 * Translate DeepSeek wire chunks into the harness `StreamChunk` protocol.
 *
 * A small state machine over the SSE payload stream:
 * - `delta.content` / `delta.reasoning_content` / `delta.tool_calls[i]` each
 *   own one harness block (index allocated on first sight). The first
 *   thinking-mode chunk carries `reasoning_content: ""` — that must NOT open
 *   a reasoning block.
 * - `finish_reason` and `usage` are DEFERRED: emitted only at the `[DONE]`
 *   sentinel, so the wire's two usage shapes (attached to the finish chunk,
 *   or a trailing usage-only chunk) both work and nothing ever follows
 *   `finish`. Last usage wins.
 *
 * @module dsh-llm-deepseek/translate
 */

import { CallId, LlmError } from '@deepseek-ai/dsh-llm'
import type { ContentBlock, FinishReason, StreamChunk, TokenUsage } from '@deepseek-ai/dsh-llm'
import { DONE } from './sse.ts'
import type { WireChunk, WireUsage } from './types.ts'

/** One open block under assembly. */
interface OpenBlock {
  index: number
  kind: 'text' | 'reasoning' | 'tool-call'
  text: string
  /** tool-call only */
  callId?: string
  name?: string
}

/** Map the wire finish_reason vocabulary to the harness FinishReason. */
export function mapFinishReason(reason: string): FinishReason {
  switch (reason) {
    case 'stop': return { kind: 'stop' }
    case 'tool_calls': return { kind: 'tool-calls' }
    case 'length': return { kind: 'max-tokens' }
    default:
      // content_filter, insufficient_system_resource, future additions.
      return { kind: 'error', message: `model stopped: ${reason}`, code: reason.toUpperCase() }
  }
}

/**
 * Map wire usage fields. DeepSeek's `prompt_tokens` INCLUDES cache hits
 * (`prompt_tokens = prompt_cache_hit_tokens + prompt_cache_miss_tokens`,
 * api/create-chat-completion); the harness TokenUsage convention is
 * DISJOINT counts, so cache reads are subtracted out of `inputTokens`.
 */
export function mapUsage(usage: WireUsage): TokenUsage {
  const cacheRead = usage.prompt_tokens_details?.cached_tokens ?? usage.prompt_cache_hit_tokens
  const reasoning = usage.completion_tokens_details?.reasoning_tokens
  return {
    inputTokens: usage.prompt_tokens - (cacheRead ?? 0),
    outputTokens: usage.completion_tokens,
    ...cacheRead !== undefined ? { cacheReadTokens: cacheRead } : {},
    ...reasoning !== undefined ? { reasoningTokens: reasoning } : {},
  }
}

/** Assemble the final ContentBlock for one open block. */
function closeBlock(block: OpenBlock): ContentBlock {
  switch (block.kind) {
    case 'text': return { type: 'text', text: block.text }
    case 'reasoning': return { type: 'reasoning', text: block.text }
    case 'tool-call': return {
      type: 'tool-call',
      id: CallId(block.callId ?? ''),
      name: block.name ?? '',
      arguments: block.text,
    }
  }
}

/**
 * Consume SSE data payloads (ending with `[DONE]`) and yield StreamChunks.
 * Malformed JSON payloads abort the stream with `MALFORMED_RESPONSE`.
 */
export async function* translate(payloads: AsyncIterable<string>): AsyncGenerator<StreamChunk> {
  let nextIndex = 0
  let textBlock: OpenBlock | undefined
  let reasoningBlock: OpenBlock | undefined
  const toolBlocks = new Map<number, OpenBlock>()
  const order: OpenBlock[] = []
  let pendingFinish: FinishReason | undefined
  let pendingUsage: TokenUsage | undefined

  function open(kind: OpenBlock['kind']): OpenBlock {
    const block: OpenBlock = { index: nextIndex++, kind, text: '' }
    order.push(block)
    return block
  }

  for await (const payload of payloads) {
    if (payload === DONE) {
      for (const block of order) {
        yield { type: 'block-end', index: block.index, block: closeBlock(block) }
      }
      if (pendingUsage) yield { type: 'usage', usage: pendingUsage }
      yield { type: 'finish', reason: pendingFinish ?? { kind: 'stop' } }
      return
    }

    let chunk: WireChunk
    try {
      chunk = JSON.parse(payload) as WireChunk
    } catch {
      throw new LlmError(`malformed SSE payload: ${payload.slice(0, 120)}`, 'MALFORMED_RESPONSE')
    }

    for (const choice of chunk.choices ?? []) {
      const delta = choice.delta

      // Reasoning first: thinking mode interleaves it before text. The
      // empty-string first chunk must not open a block.
      const reasoning = delta?.reasoning_content
      if (typeof reasoning === 'string' && reasoning.length > 0) {
        if (!reasoningBlock) {
          reasoningBlock = open('reasoning')
          yield { type: 'block-start', index: reasoningBlock.index, blockType: 'reasoning' }
        }
        reasoningBlock.text += reasoning
        yield { type: 'reasoning-delta', index: reasoningBlock.index, text: reasoning }
      }

      const content = delta?.content
      if (typeof content === 'string' && content.length > 0) {
        if (!textBlock) {
          textBlock = open('text')
          yield { type: 'block-start', index: textBlock.index, blockType: 'text' }
        }
        textBlock.text += content
        yield { type: 'text-delta', index: textBlock.index, text: content }
      }

      for (const call of delta?.tool_calls ?? []) {
        let block = toolBlocks.get(call.index)
        if (!block) {
          block = open('tool-call')
          toolBlocks.set(call.index, block)
          yield { type: 'block-start', index: block.index, blockType: 'tool-call' }
        }
        if (call.id !== undefined) block.callId = call.id
        if (call.function?.name !== undefined) block.name = call.function.name
        const fragment = call.function?.arguments ?? ''
        block.text += fragment
        yield {
          type: 'tool-call-delta',
          index: block.index,
          id: CallId(block.callId ?? ''),
          ...block.name !== undefined ? { name: block.name } : {},
          argumentsDelta: fragment,
        }
      }

      if (typeof choice.finish_reason === 'string') {
        pendingFinish = mapFinishReason(choice.finish_reason)
      }
    }

    // Usage may arrive attached to the finish chunk or as a trailing
    // usage-only chunk — keep the latest.
    if (chunk.usage) pendingUsage = mapUsage(chunk.usage)
  }

  // parseSse guarantees the [DONE] sentinel (or throws); reaching here means
  // the payload source violated that contract.
  throw new LlmError('SSE payload stream ended without [DONE]', 'STREAM_CLOSED')
}