import type { ContextUsage } from "@earendil-works/pi-coding-agent";
import type { EpochConfig } from "./constants.ts";

export type PressureBand = "unknown" | "normal" | "soft" | "critical";

export interface PressureEvaluation {
  band: PressureBand;
  /** Tokens still available before the model context window is exhausted. */
  remaining: number | null;
  remainingRatio: number | null;
  tokens: number | null;
  contextWindow: number | null;
}

export function evaluatePressure(usage: ContextUsage | undefined, config: EpochConfig): PressureEvaluation {
  if (!usage || usage.contextWindow <= 0 || usage.tokens === null) {
    return { band: "unknown", remaining: null, remainingRatio: null, tokens: null, contextWindow: usage?.contextWindow ?? null };
  }
  const remaining = Math.max(0, usage.contextWindow - usage.tokens);
  const remainingRatio = remaining / usage.contextWindow;
  let band: PressureBand = "normal";
  if (remainingRatio <= config.criticalRemainingRatio) band = "critical";
  else if (remainingRatio <= config.softRemainingRatio) band = "soft";
  return { band, remaining, remainingRatio, tokens: usage.tokens, contextWindow: usage.contextWindow };
}

function percent(ratio: number): string {
  return `${Math.round(ratio * 100)}%`;
}

/** One-shot reminder texts. Never include running counters to avoid prompt churn. */
export function reminderText(band: "soft" | "critical", evaluation: PressureEvaluation): string {
  const remaining = evaluation.remainingRatio !== null ? percent(evaluation.remainingRatio) : "a small fraction";
  if (band === "soft") {
    return (
      `Context pressure is high: about ${remaining} of the model context window remains. ` +
      "If there is non-reconstructable state (explicit user decisions, constraints, blockers, acceptance criteria), preserve it with context_notes. " +
      "When the current working set is no longer useful, call new_context to start a fresh context epoch in the same session."
    );
  }
  return (
    `Context is near its safety boundary: about ${remaining} of the model context window remains. ` +
    "Preserve essential non-reconstructable state with context_notes now, then call new_context to continue in a fresh context epoch."
  );
}