// Author: Simon-Pierre Boucher
// Contact: contact@spboucher.ai
// Project: chat.spboucher.ai

// Context compiler: DB history and model context are separate concepts.
// Compiles the active thread into a request that fits the model's window.

import type { ChatMessageInput } from "@/lib/openrouter";
import type { MessageRow } from "@/lib/conversations";

/** Cheap token estimate: ~4 characters per token. Deliberately conservative. */
export function estimateTokens(text: string): number {
  return Math.ceil(text.length / 3.6);
}

export interface CompiledContext {
  messages: ChatMessageInput[];
  estimatedPromptTokens: number;
  trimmedCount: number;
}

const RESERVED_OUTPUT_TOKENS = 4096;

/**
 * Compile the thread for a request: keep system instructions, keep the most
 * recent messages, trim oldest low-value context when over budget.
 */
export function compileContext(
  thread: MessageRow[],
  contextLength: number | undefined,
  systemPrompt?: string
): CompiledContext {
  const budget = Math.max((contextLength ?? 128_000) - RESERVED_OUTPUT_TOKENS, 8_000);

  const system: ChatMessageInput[] = systemPrompt
    ? [{ role: "system", content: systemPrompt }]
    : [];
  const systemTokens = systemPrompt ? estimateTokens(systemPrompt) : 0;

  // Only completed / meaningful messages reach the model.
  const usable = thread.filter(
    (m) => (m.role === "user" || m.role === "assistant") && m.content.length > 0
  );

  // Walk from newest to oldest, accumulating until the budget is spent.
  const kept: MessageRow[] = [];
  let used = systemTokens;
  for (let i = usable.length - 1; i >= 0; i--) {
    const t = estimateTokens(usable[i].content) + 6; // per-message overhead
    if (used + t > budget && kept.length > 0) break;
    kept.push(usable[i]);
    used += t;
  }
  kept.reverse();

  return {
    messages: [
      ...system,
      ...kept.map((m): ChatMessageInput => ({ role: m.role as "user" | "assistant", content: m.content })),
    ],
    estimatedPromptTokens: used,
    trimmedCount: usable.length - kept.length,
  };
}