package openai

import (
	"bufio"
	"encoding/json"
	"fmt"
	"io"
	"strings"
	"sync"

	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
)

// stream consumes the data-only SSE stream of chat.completion.chunk events.
//
// Delivery contract: TextDelta events as content fragments arrive; ToolCall
// events only once fully assembled (fragments are buffered internally and
// flushed at stream end — simplest correct handling of interleaved parallel
// calls); exactly one final Response event; then io.EOF.
type stream struct {
	m    *model
	body io.ReadCloser
	sc   *bufio.Scanner

	closeOnce sync.Once
	closeErr  error

	queue []llm.StreamEvent
	done  bool // finalize ran; drain queue then io.EOF

	text    strings.Builder
	calls   []*toolCallAcc // first-appearance order
	byIndex map[int]*toolCallAcc
	finish  string
	usage   llm.Usage
}

// toolCallAcc accumulates one tool call's fragments. The id and name arrive
// on the first fragment for an index; arguments arrive as string pieces to
// concatenate.
type toolCallAcc struct {
	id   string
	name string
	args strings.Builder
}

// Next implements llm.Stream.
func (s *stream) Next() (llm.StreamEvent, error) {
	for {
		if len(s.queue) > 0 {
			ev := s.queue[0]
			s.queue = s.queue[1:]
			return ev, nil
		}
		if s.done {
			return llm.StreamEvent{}, io.EOF
		}
		if !s.sc.Scan() {
			if err := s.sc.Err(); err != nil {
				return llm.StreamEvent{}, fmt.Errorf("openai: read stream: %w", err)
			}
			// Why: some compat servers close the body without a [DONE]
			// sentinel; a clean EOF still finalizes with what arrived.
			s.finalize()
			continue
		}
		line := strings.TrimSpace(s.sc.Text())
		if !strings.HasPrefix(line, "data:") {
			continue // SSE comments, event:/id: fields, blank separators
		}
		payload := strings.TrimSpace(strings.TrimPrefix(line, "data:"))
		if payload == "" {
			continue
		}
		if payload == "[DONE]" {
			s.finalize()
			continue
		}
		if err := s.handleChunk([]byte(payload)); err != nil {
			return llm.StreamEvent{}, err
		}
	}
}

// handleChunk folds one chat.completion.chunk into the stream state,
// queueing any events it produces.
func (s *stream) handleChunk(data []byte) error {
	var chunk streamChunk
	if err := json.Unmarshal(data, &chunk); err != nil {
		return fmt.Errorf("openai: decode stream chunk: %w", err)
	}
	if chunk.Error != nil {
		// Mid-stream error event on an otherwise-200 stream. Status stays 0:
		// there is no failing HTTP status to report.
		apiErr := &llm.APIError{
			Provider: s.m.p.name,
			Model:    s.m.id,
			Code:     chunk.Error.Code,
			Message:  chunk.Error.Message,
		}
		if apiErr.Code == "" {
			apiErr.Code = chunk.Error.Type
		}
		return apiErr
	}
	if chunk.Usage != nil {
		s.usage = chunk.Usage.toUsage()
	}
	// Why the guard: the include_usage chunk arrives with an EMPTY choices
	// array; indexing choices[0] unconditionally would panic on it.
	if len(chunk.Choices) == 0 {
		return nil
	}
	choice := chunk.Choices[0]
	if choice.FinishReason != "" {
		s.finish = choice.FinishReason
	}
	if choice.Delta.Content != "" {
		s.text.WriteString(choice.Delta.Content)
		s.queue = append(s.queue, llm.StreamEvent{TextDelta: choice.Delta.Content})
	}
	for _, tc := range choice.Delta.ToolCalls {
		acc := s.byIndex[tc.Index]
		if acc == nil {
			if s.byIndex == nil {
				s.byIndex = make(map[int]*toolCallAcc)
			}
			acc = &toolCallAcc{}
			s.byIndex[tc.Index] = acc
			s.calls = append(s.calls, acc)
		}
		if tc.ID != "" {
			acc.id = tc.ID
		}
		if tc.Function.Name != "" {
			acc.name = tc.Function.Name
		}
		acc.args.WriteString(tc.Function.Arguments)
	}
	return nil
}

// finalize assembles the buffered tool calls and the final Response, queues
// them (ToolCall events first, Response last), and marks the stream done.
func (s *stream) finalize() {
	if s.done {
		return
	}
	s.done = true
	resp := &llm.Response{Model: s.m.p.name + "/" + s.m.id, Usage: s.usage}
	if s.text.Len() > 0 {
		resp.Parts = []llm.Part{llm.TextPart{Text: s.text.String()}}
	}
	for i, acc := range s.calls {
		id := acc.id
		if id == "" {
			// Why: ToolResult.ID must echo ToolCall.ID; synthesize for
			// compat servers that stream calls without ids.
			id = fmt.Sprintf("call_%d", i)
		}
		resp.ToolCalls = append(resp.ToolCalls, llm.ToolCall{
			ID:        id,
			Name:      acc.name,
			Arguments: json.RawMessage(acc.args.String()),
		})
	}
	resp.FinishReason = mapFinish(s.finish, len(resp.ToolCalls) > 0)
	for i := range resp.ToolCalls {
		tc := resp.ToolCalls[i] // copy so the event doesn't alias the slice
		s.queue = append(s.queue, llm.StreamEvent{ToolCall: &tc})
	}
	s.queue = append(s.queue, llm.StreamEvent{Response: resp})
}

// Close implements llm.Stream. Closing the body unblocks any in-flight read
// and aborts the HTTP stream; safe to call at any time, including twice.
func (s *stream) Close() error {
	s.closeOnce.Do(func() { s.closeErr = s.body.Close() })
	return s.closeErr
}