import type { StepMetrics, TurnMetrics, Usage } from "@dispatch/wire";
import type { CacheRateView, StepMetricsView, TurnMetricsView } from "./types";

function formatTokens(n: number): string {
	return n.toLocaleString("en-US");
}

function formatDuration(ms: number | undefined): string | null {
	if (ms === undefined || ms <= 0) return null;
	if (ms < 1000) return `${Math.round(ms)}ms`;
	return `${(ms / 1000).toFixed(1)}s`;
}

function formatTps(tps: number | null): string | null {
	if (tps === null) return null;
	if (tps < 10) return `${tps.toFixed(1)} tok/s`;
	return `${Math.round(tps)} tok/s`;
}

/**
 * Format the current context size for display. A defined count renders as
 * `"<n> tokens in context"` (thousands-separated); `undefined` ("unknown" — no
 * per-step usage reported yet) renders the placeholder `"context size unknown"`.
 * Never renders `0` for the unknown case.
 */
export function formatContextSize(n: number | undefined): string {
	if (n === undefined) return "context size unknown";
	return `${formatTokens(n)} tokens in context`;
}

/**
 * Compact token count for a slim status bar: `812`, `12.3k`, `1.2M`. Full
 * thousands-separated numbers live elsewhere; this trades precision for width.
 */
export function formatCompactTokens(n: number): string {
	if (n < 1000) return `${n}`;
	if (n < 1_000_000) {
		const k = n / 1000;
		return `${k >= 100 ? Math.round(k) : k.toFixed(1)}k`;
	}
	const m = n / 1_000_000;
	return `${m >= 100 ? Math.round(m) : m.toFixed(1)}M`;
}

/**
 * Context-window occupancy: the current size against a max window limit.
 *
 * `current` is the latest turn's context size (0 when unknown); `max` is the
 * model's window limit (or `null` when unknown). `percent` is
 * `current / max * 100` clamped to [0, 100], UNROUNDED (the UI picks the
 * precision) — so a few-thousand-token context against a 1,000,000 window still
 * reads non-zero. `percent` is `null` when `max` is unknown (no bar/denominator).
 */
export interface ContextUsage {
	readonly current: number;
	readonly max: number | null;
	readonly percent: number | null;
}

export function computeContextUsage(
	contextSize: number | undefined,
	contextLimit: number | null | undefined,
): ContextUsage {
	const current = contextSize ?? 0;
	const max = typeof contextLimit === "number" && contextLimit > 0 ? contextLimit : null;
	const percent = max === null ? null : Math.max(0, Math.min(100, (current / max) * 100));
	return { current, max, percent };
}

/** Compute tokens-per-second. Returns null when elapsed time is absent or zero. */
export function computeTps(outputTokens: number, elapsedMs: number | undefined): number | null {
	if (elapsedMs === undefined || elapsedMs <= 0) return null;
	return outputTokens / (elapsedMs / 1000);
}

function totalTokens(u: Usage): number {
	return u.inputTokens + u.outputTokens;
}

function formatBreakdown(u: Usage): string {
	let s = `${formatTokens(u.inputTokens)} in / ${formatTokens(u.outputTokens)} out`;
	if (u.cacheReadTokens !== undefined && u.cacheReadTokens > 0) {
		s += ` / ${formatTokens(u.cacheReadTokens)} cache`;
	}
	return s;
}

/** Build a formatted view of a single step's metrics. */
export function viewStepMetrics(step: StepMetrics, index: number): StepMetricsView {
	const total = totalTokens(step.usage);
	const tps = computeTps(step.usage.outputTokens, step.decodeMs ?? step.genTotalMs);
	return {
		label: `step ${index + 1}`,
		tokensLabel: `${formatTokens(total)} tok`,
		tps: formatTps(tps),
		ttft: formatDuration(step.ttftMs),
		decode: formatDuration(step.decodeMs),
		genTotal: formatDuration(step.genTotalMs),
	};
}

/**
 * Cache hit rate as a 0..100 integer percentage: `cacheReadTokens / inputTokens`,
 * clamped to [0,1]. Absent cache field counts as 0; a 0% rate is legitimate (not
 * missing data). Returns 0 when there are no input tokens.
 */
export function computeCachePct(u: Usage): number {
	const read = u.cacheReadTokens ?? 0;
	if (u.inputTokens <= 0) return 0;
	const rate = read / u.inputTokens;
	const clamped = rate < 0 ? 0 : rate > 1 ? 1 : rate;
	return Math.round(clamped * 100);
}

/** Colour severity for a cache hit percentage (badge colour). */
function cacheLevel(pct: number): "success" | "warning" | "error" {
	if (pct >= 66) return "success";
	if (pct >= 33) return "warning";
	return "error";
}

/** Build a view of a cache hit rate (percentage + colour level + hit flag). */
export function viewCacheRate(u: Usage): CacheRateView {
	const pct = computeCachePct(u);
	return { pct, level: cacheLevel(pct), isHit: (u.cacheReadTokens ?? 0) > 0 };
}

/**
 * Expected cache (retention): of the cache that existed going INTO this turn, how
 * much was read back — `clamp01(cacheRead_N / (cacheRead_{N-1} + cacheWrite_{N-1}))`.
 * The denominator is the PRIOR turn's cached prefix (what it read + what it wrote).
 * Ideally ~100% on every turn after the first; <100% = the cache busted/expired.
 *
 * Returns `null` when it cannot be derived: no prior turn (`prev === null`) or the
 * prior turn cached nothing (denominator <= 0) — distinct from a real 0%.
 */
export function computeExpectedCachePct(current: Usage, prev: Usage | null): number | null {
	if (prev === null) return null;
	const denom = (prev.cacheReadTokens ?? 0) + (prev.cacheWriteTokens ?? 0);
	if (denom <= 0) return null;
	const read = current.cacheReadTokens ?? 0;
	const rate = read / denom;
	const clamped = rate < 0 ? 0 : rate > 1 ? 1 : rate;
	return Math.round(clamped * 100);
}

/**
 * Build a view of the cross-turn retention (percentage + colour level + hit flag),
 * or `null` when it can't be derived (see `computeExpectedCachePct`).
 */
export function viewExpectedCache(current: Usage, prev: Usage | null): CacheRateView | null {
	const pct = computeExpectedCachePct(current, prev);
	if (pct === null) return null;
	return { pct, level: cacheLevel(pct), isHit: (current.cacheReadTokens ?? 0) > 0 };
}

/** Build a formatted view of a turn's aggregate metrics. */
export function viewTurnMetrics(turn: TurnMetrics, turnNumber?: number): TurnMetricsView {
	const total = totalTokens(turn.usage);
	let totalGenMs: number | undefined;
	for (const step of turn.steps) {
		const stepMs = step.decodeMs ?? step.genTotalMs;
		if (stepMs !== undefined) {
			totalGenMs = (totalGenMs ?? 0) + stepMs;
		}
	}
	const tps = computeTps(turn.usage.outputTokens, totalGenMs);
	return {
		label: turnNumber !== undefined ? `turn ${turnNumber}` : "turn",
		tokensLabel: `${formatTokens(total)} tok`,
		breakdown: formatBreakdown(turn.usage),
		tps: formatTps(tps),
		duration: formatDuration(turn.durationMs),
	};
}