/**
 * earth-now.co
 * Author:  Simon-Pierre Boucher
 * Contact: contact@spboucher.ai
 * File:    apps/web/lib/smoothing.ts
 * Purpose: Pure 60 s linear blend between an old and a new CounterModel — no visible jump when SSE delivers a refit (time is a parameter)
 */

import { counterValue, type CounterModel } from "@earth-now/counter";

/** Guardrail duration: a model swap is smoothed over 60 s (CLAUDE.md production rule). */
export const SMOOTHING_DURATION_MS = 60_000;

/** Blend weight for the new model at time t: 0 before swapStart, 1 after swapStart+duration. */
export function smoothingAlpha(
  tMs: number,
  swapStartMs: number,
  durationMs: number = SMOOTHING_DURATION_MS,
): number {
  if (durationMs <= 0 || tMs >= swapStartMs + durationMs) return 1;
  if (tMs <= swapStartMs) return 0;
  return (tMs - swapStartMs) / durationMs;
}

/** Linear blend of two already-evaluated values across the smoothing window. */
export function blendValues(
  oldValue: number,
  newValue: number,
  tMs: number,
  swapStartMs: number,
  durationMs: number = SMOOTHING_DURATION_MS,
): number {
  const alpha = smoothingAlpha(tMs, swapStartMs, durationMs);
  if (alpha === 0) return oldValue;
  if (alpha === 1) return newValue;
  return oldValue * (1 - alpha) + newValue * alpha;
}

/**
 * Evaluator that linearly cross-fades counterValue(oldModel) → counterValue(newModel)
 * over [swapStartMs, swapStartMs + durationMs]. Exactly the old value at swapStart,
 * exactly the new value from swapStart+duration onwards. Pure — never reads the clock.
 */
export function makeSmoothedEvaluator(
  oldModel: CounterModel,
  newModel: CounterModel,
  swapStartMs: number,
  durationMs: number = SMOOTHING_DURATION_MS,
): (tMs: number) => number {
  return (tMs: number): number => {
    const alpha = smoothingAlpha(tMs, swapStartMs, durationMs);
    if (alpha === 1) return counterValue(newModel, tMs);
    const oldValue = counterValue(oldModel, tMs);
    if (alpha === 0) return oldValue;
    return oldValue * (1 - alpha) + counterValue(newModel, tMs) * alpha;
  };
}