#include "battery_est.h"
#include <Arduino.h>

// Anchor-based, not a ring buffer (cf. usage_rate.cpp): we remember (time, pct)
// at the moment discharge began and project from the average rate since then.
//   rate = (anchor_pct - pct_now) / minutes_elapsed
//   minutes_left = pct_now / rate
// A long baseline beats a short ring for a signal that only ticks every few
// minutes — the estimate just keeps tightening as the session runs. We withhold
// a number until the cell has dropped a meaningful amount over a few minutes, so
// the first figure isn't built on fuel-gauge settling noise right after boot.
#define EST_MIN_DROP_PCT     3         // need >= 3% drained since the anchor
#define EST_MIN_ELAPSED_MS   300000UL  // ...over at least 5 minutes
#define EST_MAX_MINUTES      (100 * 60)

static bool     have_anchor   = false;
static uint32_t anchor_ms     = 0;
static int      anchor_pct    = -1;
static int      last_pct      = -1;
static bool     last_charging = false;

void battery_est_update(int percent, bool charging) {
    last_charging = charging;
    last_pct      = percent;

    if (charging || percent < 0) {
        // On USB / charging / no reading the estimate is meaningless. Drop the
        // anchor so the next discharge starts from a fresh baseline.
        have_anchor = false;
        return;
    }
    // Discharging. Start an anchor, or re-anchor if the gauge ticked UP (it
    // relaxes upward when load drops) so we never compute a negative rate.
    if (!have_anchor || percent > anchor_pct) {
        have_anchor = true;
        anchor_ms   = millis();
        anchor_pct  = percent;
    }
}

int battery_est_minutes(void) {
    if (last_charging || last_pct < 0) return -2;
    if (!have_anchor)                  return -1;

    int      drop = anchor_pct - last_pct;
    uint32_t dt   = millis() - anchor_ms;
    if (drop < EST_MIN_DROP_PCT || dt < EST_MIN_ELAPSED_MS) return -1;

    // Minutes each 1% has been taking, extrapolated across the remaining pct.
    float minutes_per_pct = (float)dt / 60000.0f / (float)drop;
    float mins = (float)last_pct * minutes_per_pct;
    if (mins < 0)              return -1;
    if (mins > EST_MAX_MINUTES) mins = EST_MAX_MINUTES;
    return (int)(mins + 0.5f);
}