var growthCache = null;

export function loadGrowthLmsData() {
  if (growthCache) return Promise.resolve(growthCache);
  return fetch('/data/calculators/growth-lms.json', { credentials: 'same-origin' })
    .then(function(response) {
      if (!response.ok) throw new Error('Failed to load growth LMS data');
      return response.json();
    })
    .then(function(data) {
      growthCache = data.lms || {};
      return growthCache;
    });
}

export function interpolateLMS(lmsTable, val) {
  var keys = Object.keys(lmsTable).map(Number).sort(function(a,b){return a-b;});
  if (val <= keys[0]) return lmsTable[keys[0]];
  if (val >= keys[keys.length-1]) return lmsTable[keys[keys.length-1]];
  for (var i = 0; i < keys.length - 1; i++) {
    if (val >= keys[i] && val <= keys[i+1]) {
      var t = (val - keys[i]) / (keys[i+1] - keys[i]);
      var a = lmsTable[keys[i]], b = lmsTable[keys[i+1]];
      return { L: a.L + t*(b.L-a.L), M: a.M + t*(b.M-a.M), S: a.S + t*(b.S-a.S) };
    }
  }
  return lmsTable[keys[0]];
}

export function normalCDF(z) {
  var a1=0.254829592, a2=-0.284496736, a3=1.421413741, a4=-1.453152027, a5=1.061405429, p=0.3275911;
  var sign = z < 0 ? -1 : 1;
  z = Math.abs(z) / Math.sqrt(2);
  var t = 1 / (1 + p * z);
  var y = 1 - (((((a5*t+a4)*t)+a3)*t+a2)*t+a1)*t*Math.exp(-z*z);
  return 0.5 * (1 + sign * y);
}

export function calcZFromLMS(value, L, M, S) {
  if (Math.abs(L) < 0.001) return Math.log(value / M) / S;
  return (Math.pow(value / M, L) - 1) / (L * S);
}

export function calculateLmsPercentile(lmsTable, x, value) {
  var lms = interpolateLMS(lmsTable, x);
  var z = calcZFromLMS(value, lms.L, lms.M, lms.S);
  return { lms: lms, z: z, percentile: normalCDF(z) * 100 };
}