/**
 * A Pattern is a function that matches against a string starting at a given index.
 *
 * If it matches successfully, it returns some captured value, and the index following the match.
 *
 * @template T
 * @typedef {{
 *   (source: string, index: number): ([T, number] | null)
 * }} Peg.Pattern
 */
var Peg = window.Peg ?? {};

/**
 * Makes a pattern from a function, adding helper methods.
 *
 * @template T
 * @param {(source: string, index: number) => ([T, number] | null)} pattern
 * @returns {Peg.Pattern<T>}
 */
Peg.WrapPattern = function (pattern) {
  return /** @type {Peg.Pattern<T>} */ (pattern);
};

/**
 * Proxies to a pattern retrieved from an accessor function.
 *
 * Allows using a pattern recursively in its own definition, by returning the value of the const assigned to.
 *
 * @template T
 * @param {() => Peg.Pattern<T>} getPattern
 * @returns {Peg.Pattern<T>}
 */
Peg.Use = function (getPattern) {
  return Peg.WrapPattern(function (source, index) {
    return getPattern()(source, index);
  });
};

/**
 * Creates a pattern that wraps another pattern, transforming the returned value on a match
 * @template T, U
 * @param {Peg.Pattern<T>} pattern
 * @param {(value: T)=> U} map
 * @return {Peg.Pattern<U>}
 */
Peg.Map = function (pattern, map) {
  return Peg.WrapPattern(function (source, index) {
    const match = pattern(source, index);
    return match ? [map(match[0]), match[1]] : null;
  });
};

/**
 * Creates a pattern matching a regex & returning any captures. The regex needs to be sticky (using the //y modifier)
 * @param {RegExp} regex
 * @return {Peg.Pattern<RegExpExecArray>}
 */
Peg.Regex = function (regex) {
  return Peg.WrapPattern(function (source, index) {
    regex.lastIndex = index;
    const matches = regex.exec(source);
    return matches ? [matches, regex.lastIndex] : null;
  });
};

/**
 * Creates a pattern that tries the given patterns, in order, until it finds one that matches at the current index.
 * @template T
 * @param {...Peg.Pattern<T>} patterns
 * @return {Peg.Pattern<T>}
 */
Peg.Choose = function (...patterns) {
  return Peg.WrapPattern(function (source, index) {
    for (const pattern of patterns) {
      const match = pattern(source, index);
      if (match) {
        return match;
      }
    }
    return null;
  });
};

/**
 * Creates a pattern that concatenates the given patterns, returning a tuple of their captured values.
 *
 * For example, if A matches "a" and captures 1, while B matches "b" and captures null,
 * then `Sequence(A,B)` will match "ab" and capture [1, null]
 * @template {unknown[]} T
 * @param {{[K in keyof T]: Peg.Pattern<T[K]>}} patterns
 * @return {Peg.Pattern<T>}
 */
Peg.Sequence = function (...patterns) {
  return Peg.WrapPattern(function (source, index) {
    const values = /** @type {T} */ (/** @type {unknown} */ ([]));
    for (const pattern of patterns) {
      const match = pattern(source, index);
      if (match == null) {
        return null;
      }
      values.push(match[0]);
      index = match[1];
    }
    return [values, index];
  });
};

/**
 * Creates a pattern that matches consecutive runs of the given pattern, returning an array of all captures.
 *
 * The match only succeeds if the run is at least {min} instances long.
 *
 * If the given pattern does not consume input, the matching will be terminated to prevent an eternal loop.
 *
 * Note that if the minimum run is zero, this pattern will always succeed, but might not consume any input.
 * @template {unknown} T
 * @param {number} min
 * @param {Peg.Pattern<T>} pattern
 * @return {Peg.Pattern<T[]>}
 */
Peg.AtLeast = function (min, pattern) {
  return Peg.WrapPattern(function (source, index) {
    const values = /** @type {T[]} */ ([]);
    do {
      const match = pattern(source, index);
      if (match == null) break;
      values.push(match[0]);
      if (index == match[1]) break;
      index = match[1];
    } while (true);
    if (values.length >= min) {
      return [values, index];
    } else {
      return null;
    }
  });
};

/**
 * Pattern that matches the end of input
 * @type {Peg.Pattern<true>}
 */
Peg.End = Peg.WrapPattern(function End(source, index) {
  if (source.length == index) {
    return [true, index];
  } else {
    return null;
  }
});