"use strict";

// Prefix predictor for the mnemonicode wordlist. Words are not distinct at
// any fixed prefix length (one pair, "capital"/"capitan", needs 6 of 7
// chars), so matching always refines on the full typed prefix rather than
// assuming a fixed cutoff.

function lowerBound(sorted, target) {
  let lo = 0;
  let hi = sorted.length;
  while (lo < hi) {
    const mid = (lo + hi) >>> 1;
    if (sorted[mid] < target) lo = mid + 1;
    else hi = mid;
  }
  return lo;
}

class Predictor {
  constructor(words) {
    this.words = words.slice().sort();
  }

  // All words starting with `prefix`, in alphabetical order.
  matches(prefix) {
    const lo = lowerBound(this.words, prefix);
    const hi = lowerBound(this.words, prefix + "￿");
    return this.words.slice(lo, hi);
  }

  // Prediction state for a prefix the user has typed so far.
  predict(prefix) {
    const matches = this.matches(prefix);
    return {
      prefix,
      matches,
      isWord: matches.includes(prefix),
      completion: matches.length === 1 ? matches[0] : null,
    };
  }
}

module.exports = { Predictor };