import type { MapScale } from './map.types';

/**
 * Maps a data value to a pixel size for proportional symbology.
 *
 * @description
 * Nine of the sixteen surveyed applications hand-rolled this same calculation
 * before plotting circles. It lives here so that a marker's radius and the size
 * key in `MapLegend` are guaranteed to come from identical math — a legend that
 * disagrees with the symbols it describes is worse than no legend.
 *
 * `'sqrt'` is the default because it makes the rendered *area* proportional to
 * the value. `'linear'` scales the radius directly and therefore exaggerates
 * large values; use it only when the mark is a bar or a line, not a disc.
 *
 * Degenerate inputs collapse to `range[0]` rather than producing `NaN`:
 * a zero-width domain, a non-finite value, and (for `'log'`) any non-positive
 * bound or value.
 */
export function resolveScale(scale: MapScale): number {
  const { value, domain, range, method = 'sqrt' } = scale;
  const [d0, d1] = domain;
  const [r0, r1] = range;

  if (!Number.isFinite(value) || !Number.isFinite(d0) || !Number.isFinite(d1)) return r0;
  if (d0 === d1) return r0;

  // Clamp into the domain so out-of-range data cannot escape the size range.
  const lo = Math.min(d0, d1);
  const hi = Math.max(d0, d1);
  const clamped = Math.min(Math.max(value, lo), hi);

  let t: number;

  if (method === 'log') {
    // Logs are undefined at and below zero; a domain that reaches zero has no
    // meaningful log mapping, so fall back to the smallest size.
    if (lo <= 0 || clamped <= 0) return r0;
    t = (Math.log(clamped) - Math.log(lo)) / (Math.log(hi) - Math.log(lo));
  } else if (method === 'sqrt') {
    // Shift by the domain floor so negative domains still produce a real root.
    const span = hi - lo;
    t = Math.sqrt((clamped - lo) / span);
  } else {
    t = (clamped - lo) / (hi - lo);
  }

  if (!Number.isFinite(t)) return r0;

  return r0 + t * (r1 - r0);
}

/**
 * Default marker radius, in pixels, when `shape: 'circle'` is used with neither
 * `radiusPx` nor `scale`. Sized to read as a deliberate dot next to the 32px
 * pin without competing with it.
 */
export const DEFAULT_MARKER_RADIUS_PX = 8;