import type { Theme } from "@earendil-works/pi-coding-agent";
import type { AssistantUsageMetric } from "./types.js";
import { formatPercent, hitRateColor } from "./format-utils.js";

/** Below this leverage (cached tokens per fresh token) cache economics are poor. */
const LEVERAGE_FLOOR = 5;

function formatIdle(ms: number): string {
  const min = Math.round(ms / 60_000);
  if (min >= 60) return `${(min / 60).toFixed(1).replace(/\.0$/, "")}h`;
  return `${min}m`;
}

function formatMoney(usd: number): string {
  if (usd >= 1) return `$${usd.toFixed(2)}`;
  if (usd >= 0.0005) return `$${usd.toFixed(3)}`;
  if (usd > 0) return "<$0.001";
  return "$0";
}

/**
 * Human-readable reasons a turn's cache prefix was broken. All signals are
 * computed in session-data; this only renders them. Tags intentionally
 * co-occur: a restart, for example, creates both a long idle gap and a
 * rebuilt prefix, and hiding the stronger signal behind the weaker one is
 * exactly how a restart miss gets mislabeled as pure cache expiry.
 * Ordering runs from recorded causes to evidence to conclusion.
 */
export function reasonTags(m: AssistantUsageMetric): string[] {
  const tags: string[] = [];
  if (m.modelChanged) tags.push("model switch");
  if (m.thinkingChanged) tags.push("thinking change");
  if (m.afterCompact) tags.push("after compact");
  if (m.promptShrunk) tags.push("prefix shrank");
  if (m.contextInvalidated) tags.push("context rebuilt");
  // Precomputed in session-data against the TTL applicable to this model's
  // API protocol; the tag is rendered only when that verdict says expired.
  if (m.idleExpired === true) tags.push(`idle ${formatIdle(m.idleMs!)}`);
  return tags;
}

/**
 * Miss analysis: every turn below the 80% hit-rate threshold, worst first,
 * annotated with the likely cause and the estimated extra cost of the miss.
 * Long lists are browsed with j/k scroll (handled by the dialog).
 */
export function renderMissesView(theme: Theme, messages: AssistantUsageMetric[], _width: number): string[] {
  const lines: string[] = [];

  lines.push(theme.fg("accent", theme.bold("▎ Miss analysis")));

  // A turn is listed only when the cacheable prefix was broken (miss ratio
  // above the cached-length-scaled threshold). Appended information — first
  // turns, searches, file reads, large tool outputs — never breaks the
  // prefix and is deliberately not listed.
  const misses = messages
    .filter((m) => m.missFlagged)
    .sort((a, b) => a.cacheHitPercent - b.cacheHitPercent);

  if (misses.length === 0) {
    lines.push(theme.fg("success", "No cache-prefix breaks detected — cache is healthy."));
    return lines;
  }

  // Session-wide economics: total est. waste + cache leverage (cached tokens
  // served per fresh token paid). Same spirit as pi's cache-waste tracking.
  const totalWaste = misses.reduce((sum, m) => sum + (m.missedCost ?? 0), 0);
  const totalRead = messages.reduce((sum, m) => sum + m.cacheRead, 0);
  const totalFresh = messages.reduce((sum, m) => sum + m.input + m.cacheWrite, 0);
  const leverage = totalFresh > 0 ? totalRead / totalFresh : undefined;

  const summaryParts = [
    `${misses.length} of ${messages.length} turns flagged (low hit rate or abnormal miss)`,
  ];
  if (totalWaste > 0) summaryParts.push(`est. waste ${formatMoney(totalWaste)}`);
  if (leverage !== undefined) {
    const text = `leverage ${leverage.toFixed(1)}x`;
    summaryParts.push(leverage < LEVERAGE_FLOOR ? theme.fg("warning", text) : theme.fg("success", text));
  }
  lines.push(theme.fg("dim", summaryParts.join(" · ")));
  lines.push(theme.fg("dim", "* = on current active branch"));

  for (const message of misses) {
    const label = `#${String(message.sequence).padStart(2, " ")}${message.isOnActiveBranch ? "*" : " "}`;
    const tags = reasonTags(message);
    const tagText = tags.length > 0 ? tags.join(" · ") : "—";
    const mc = message.missedCost ?? 0;
    const cost = mc > 0 ? `+${formatMoney(mc)}` : "";
    lines.push(
      `${theme.fg("muted", label)} ${hitRateColor(theme, message.cacheHitPercent, formatPercent(message.cacheHitPercent).padStart(6, " "))}  ` +
        theme.fg("accent", cost.padEnd(10)) +
        theme.fg("dim", `${message.provider}/${message.model}`) +
        theme.fg("dim", `  ${tagText}`),
    );
  }
  return lines;
}