json-schema-to-grammar.mjs

// WARNING: This file was ported from json_schema_to_grammar.py, please fix bugs / add features there first.
const SPACE_RULE = '| " " | "\\n" [ \\t]{0,20}';

function _buildRepetition(itemRule, minItems, maxItems, opts={}) {
  if (minItems === 0 && maxItems === 1) {
    return `${itemRule}?`;
  }


  const separatorRule = opts.separatorRule ?? '';
  const itemRuleIsLiteral = opts.itemRuleIsLiteral ?? false

  if (separatorRule === '') {
    if (minItems === 1 && maxItems === undefined) {
      return `${itemRule}+`;
    } else if (minItems === 0 && maxItems === undefined) {
      return `${itemRule}*`;
    } else {
      return `${itemRule}{${minItems},${maxItems !== undefined ? maxItems : ''}}`;
    }
  }

  const result = itemRule + ' ' + _buildRepetition(`(${separatorRule} ${itemRule})`, minItems > 0 ? minItems - 1 : 0, maxItems !== undefined ? maxItems - 1 : undefined);
  return minItems === 0 ? `(${result})?` : result;
}

function _generateMinMaxInt(minValue, maxValue, out, decimalsLeft = 16, topLevel = true) {
  const hasMin = minValue !== null;
  const hasMax = maxValue !== null;

  function digitRange(fromChar, toChar) {
      out.push("[");
      if (fromChar === toChar) {
          out.push(fromChar);
      } else {
          out.push(fromChar);
          out.push("-");
          out.push(toChar);
      }
      out.push("]");
  }

  function moreDigits(minDigits, maxDigits) {
      out.push("[0-9]");
      if (minDigits === maxDigits && minDigits === 1) {
          return;
      }
      out.push("{");
      out.push(minDigits.toString());
      if (maxDigits !== minDigits) {
          out.push(",");
          if (maxDigits !== Number.MAX_SAFE_INTEGER) {
              out.push(maxDigits.toString());
          }
      }
      out.push("}");
  }

  function uniformRange(fromStr, toStr) {
      let i = 0;
      while (i < fromStr.length && fromStr[i] === toStr[i]) {
          i++;
      }
      if (i > 0) {
          out.push("\"");
          out.push(fromStr.slice(0, i));
          out.push("\"");
      }
      if (i < fromStr.length) {
          if (i > 0) {
              out.push(" ");
          }
          const subLen = fromStr.length - i - 1;
          if (subLen > 0) {
              const fromSub = fromStr.slice(i + 1);
              const toSub = toStr.slice(i + 1);
              const subZeros = "0".repeat(subLen);
              const subNines = "9".repeat(subLen);

              let toReached = false;
              out.push("(");
              if (fromSub === subZeros) {
                  digitRange(fromStr[i], String.fromCharCode(toStr.charCodeAt(i) - 1));
                  out.push(" ");
                  moreDigits(subLen, subLen);
              } else {
                  out.push("[");
                  out.push(fromStr[i]);
                  out.push("] ");
                  out.push("(");
                  uniformRange(fromSub, subNines);
                  out.push(")");
                  if (fromStr.charCodeAt(i) < toStr.charCodeAt(i) - 1) {
                      out.push(" | ");
                      if (toSub === subNines) {
                          digitRange(String.fromCharCode(fromStr.charCodeAt(i) + 1), toStr[i]);
                          toReached = true;
                      } else {
                          digitRange(String.fromCharCode(fromStr.charCodeAt(i) + 1), String.fromCharCode(toStr.charCodeAt(i) - 1));
                      }
                      out.push(" ");
                      moreDigits(subLen, subLen);
                  }
              }
              if (!toReached) {
                  out.push(" | ");
                  digitRange(toStr[i], toStr[i]);
                  out.push(" ");
                  uniformRange(subZeros, toSub);
              }
              out.push(")");
          } else {
              out.push("[");
              out.push(fromStr[i]);
              out.push("-");
              out.push(toStr[i]);
              out.push("]");
          }
      }
  }

  if (hasMin && hasMax) {
      if (minValue < 0 && maxValue < 0) {
          out.push("\"-\" (");
          _generateMinMaxInt(-maxValue, -minValue, out, decimalsLeft, true);
          out.push(")");
          return;
      }

      if (minValue < 0) {
          out.push("\"-\" (");
          _generateMinMaxInt(0, -minValue, out, decimalsLeft, true);
          out.push(") | ");
          minValue = 0;
      }

      let minS = minValue.toString();
      const maxS = maxValue.toString();
      const minDigits = minS.length;
      const maxDigits = maxS.length;

      for (let digits = minDigits; digits < maxDigits; digits++) {
          uniformRange(minS, "9".repeat(digits));
          minS = "1" + "0".repeat(digits);
          out.push(" | ");
      }
      uniformRange(minS, maxS);
      return;
  }

  const lessDecimals = Math.max(decimalsLeft - 1, 1);

  if (hasMin) {
      if (minValue < 0) {
          out.push("\"-\" (");
          _generateMinMaxInt(null, -minValue, out, decimalsLeft, false);
          out.push(") | [0] | [1-9] ");
          moreDigits(0, decimalsLeft - 1);
      } else if (minValue === 0) {
          if (topLevel) {
              out.push("[0] | [1-9] ");
              moreDigits(0, lessDecimals);
          } else {
              moreDigits(1, decimalsLeft);
          }
      } else if (minValue <= 9) {
          const c = minValue.toString();
          const range_start = topLevel ? '1' : '0';
          if (c > range_start) {
              digitRange(range_start, String.fromCharCode(c.charCodeAt(0) - 1));
              out.push(" ");
              moreDigits(1, lessDecimals);
              out.push(" | ");
          }
          digitRange(c, "9");
          out.push(" ");
          moreDigits(0, lessDecimals);
      } else {
          const minS = minValue.toString();
          const length = minS.length;
          const c = minS[0];

          if (c > "1") {
              digitRange(topLevel ? "1" : "0", String.fromCharCode(c.charCodeAt(0) - 1));
              out.push(" ");
              moreDigits(length, lessDecimals);
              out.push(" | ");
          }
          digitRange(c, c);
          out.push(" (");
          _generateMinMaxInt(parseInt(minS.slice(1)), null, out, lessDecimals, false);
          out.push(")");
          if (c < "9") {
              out.push(" | ");
              digitRange(String.fromCharCode(c.charCodeAt(0) + 1), "9");
              out.push(" ");
              moreDigits(length - 1, lessDecimals);
          }
      }
      return;
  }

  if (hasMax) {
      if (maxValue >= 0) {
          if (topLevel) {
              out.push("\"-\" [1-9] ");
              moreDigits(0, lessDecimals);
              out.push(" | ");
          }
          _generateMinMaxInt(0, maxValue, out, decimalsLeft, true);
      } else {
          out.push("\"-\" (");
          _generateMinMaxInt(-maxValue, null, out, decimalsLeft, false);
          out.push(")");
      }
      return;
  }

  throw new Error("At least one of minValue or maxValue must be set");
}

class BuiltinRule {
  constructor(content, deps) {
    this.content = content;
    this.deps = deps || [];
  }
}

const PRIMITIVE_RULES = {
  boolean        : new BuiltinRule('("true" | "false") space', []),
  'decimal-part' : new BuiltinRule('[0-9]{1,16}', []),
  'integral-part': new BuiltinRule('[0] | [1-9] [0-9]{0,15}', []),
  number         : new BuiltinRule('("-"? integral-part) ("." decimal-part)? ([eE] [-+]? integral-part)? space', ['integral-part', 'decimal-part']),
  integer        : new BuiltinRule('("-"? integral-part) space', ['integral-part']),
  value          : new BuiltinRule('object | array | string | number | boolean | null', ['object', 'array', 'string', 'number', 'boolean', 'null']),
  object         : new BuiltinRule('"{" space ( string ":" space value ("," space string ":" space value)* )? "}" space', ['string', 'value']),
  array          : new BuiltinRule('"[" space ( value ("," space value)* )? "]" space', ['value']),
  uuid           : new BuiltinRule('"\\"" [0-9a-fA-F]{8} "-" [0-9a-fA-F]{4} "-" [0-9a-fA-F]{4} "-" [0-9a-fA-F]{4} "-" [0-9a-fA-F]{12} "\\"" space', []),
  char           : new BuiltinRule(`[^"\\\\\\x7F\\x00-\\x1F] | [\\\\] (["\\\\bfnrt] | "u" [0-9a-fA-F]{4})`, []),
  string         : new BuiltinRule(`"\\"" char* "\\"" space`, ['char']),
  null           : new BuiltinRule('"null" space', []),
};

// TODO: support "uri", "email" string formats
const STRING_FORMAT_RULES = {
  'date'            : new BuiltinRule('[0-9]{4} "-" ( "0" [1-9] | "1" [0-2] ) "-" ( \"0\" [1-9] | [1-2] [0-9] | "3" [0-1] )', []),
  'time'            : new BuiltinRule('([01] [0-9] | "2" [0-3]) ":" [0-5] [0-9] ":" [0-5] [0-9] ( "." [0-9]{3} )? ( "Z" | ( "+" | "-" ) ( [01] [0-9] | "2" [0-3] ) ":" [0-5] [0-9] )', []),
  'date-time'       : new BuiltinRule('date "T" time', ['date', 'time']),
  'date-string'     : new BuiltinRule('"\\"" date "\\"" space', ['date']),
  'time-string'     : new BuiltinRule('"\\"" time "\\"" space', ['time']),
  'date-time-string': new BuiltinRule('"\\"" date-time "\\"" space', ['date-time']),
}

const RESERVED_NAMES = {'root': true, ...PRIMITIVE_RULES, ...STRING_FORMAT_RULES};

const INVALID_RULE_CHARS_RE = /[^\dA-Za-z-]+/g;
const GRAMMAR_LITERAL_ESCAPE_RE = /[\n\r"]/g;
const GRAMMAR_RANGE_LITERAL_ESCAPE_RE = /[\n\r"\]\-\\]/g;
const GRAMMAR_LITERAL_ESCAPES = { '\r': '\\r', '\n': '\\n', '"': '\\"', '-': '\\-', ']': '\\]' };

const NON_LITERAL_SET = new Set('|.()[]{}*+?');
const ESCAPED_IN_REGEXPS_BUT_NOT_IN_LITERALS = new Set('^$.[]()|{}*+?');

export class SchemaConverter {
  constructor(options) {
    this._propOrder = options.prop_order || {};
    this._allowFetch = options.allow_fetch || false;
    this._dotall = options.dotall || false;
    this._rules = {'space': SPACE_RULE};
    this._refs = {};
    this._refsBeingResolved = new Set();
  }

  _formatLiteral(literal) {
    const escaped = literal.replace(
      GRAMMAR_LITERAL_ESCAPE_RE,
      m => GRAMMAR_LITERAL_ESCAPES[m]
    );
    return `"${escaped}"`;
  }

  _formatRangeChar(literal) {
    return JSON.stringify(literal).slice(1, -1).replace(
      GRAMMAR_RANGE_LITERAL_ESCAPE_RE,
      m => GRAMMAR_LITERAL_ESCAPES[m]
    );
  }

  _addRule(name, rule) {
    let escName = name.replace(INVALID_RULE_CHARS_RE, '-');
    let key = escName;

    if (escName in this._rules) {
      if (this._rules[escName] === rule) {
        return key;
      }

      let i = 0;
      while ((`${escName}${i}` in this._rules) && (this._rules[`${escName}${i}`] !== rule)) {
        i += 1;
      }
      key = `${escName}${i}`;
    }

    this._rules[key] = rule;
    return key;
  }

  async resolveRefs(schema, url) {
    const visit = async (n) => {
      if (Array.isArray(n)) {
        return Promise.all(n.map(visit));
      } else if (typeof n === 'object' && n !== null) {
        let ref = n.$ref;
        let target;
        if (ref !== undefined && !this._refs[ref]) {
          if (ref.startsWith('https://')) {
            if (!this._allowFetch) {
              throw new Error('Fetching remote schemas is not allowed (use --allow-fetch for force)');
            }
            const fetch = (await import('node-fetch')).default;

            const fragSplit = ref.split('#');
            const baseUrl = fragSplit[0];

            target = this._refs[baseUrl];
            if (!target) {
              target = await this.resolveRefs(await fetch(ref).then(res => res.json()), baseUrl);
              this._refs[baseUrl] = target;
            }

            if (fragSplit.length === 1 || fragSplit[fragSplit.length - 1] === '') {
              return target;
            }
          } else if (ref.startsWith('#/')) {
            target = schema;
            ref = `${url}${ref}`;
            n.$ref = ref;
          } else {
            throw new Error(`Unsupported ref ${ref}`);
          }

          const selectors = ref.split('#')[1].split('/').slice(1);
          for (const sel of selectors) {
            if (!target || !(sel in target)) {
              throw new Error(`Error resolving ref ${ref}: ${sel} not in ${JSON.stringify(target)}`);
            }
            target = target[sel];
          }

          this._refs[ref] = target;
        } else {
          await Promise.all(Object.values(n).map(visit));
        }
      }

      return n;
    };

    return visit(schema);
  }

  _generateUnionRule(name, altSchemas) {
    return altSchemas
      .map((altSchema, i) => this.visit(altSchema, `${name ?? ''}${name ? '-' : 'alternative-'}${i}`))
      .join(' | ');
  }

  _visitPattern(pattern, name) {
    if (!pattern.startsWith('^') || !pattern.endsWith('$')) {
      throw new Error('Pattern must start with "^" and end with "$"');
    }
    pattern = pattern.slice(1, -1);
    const subRuleIds = {};

    let i = 0;
    const length = pattern.length;

    const getDot = () => {
      let rule;
      if (this._dotall) {
        rule = '[\\U00000000-\\U0010FFFF]';
      } else {
        // Accept any character... except \n and \r line break chars (\x0A and \xOD)
        rule = '[^\\x0A\\x0D]';
      }
      return this._addRule('dot', rule);
    };


    const toRule = ([s, isLiteral]) => isLiteral ? "\"" + s + "\"" : s;

    const transform = () => {
      const start = i;
      // For each component of this sequence, store its string representation and whether it's a literal.
      // We only need a flat structure here to apply repetition operators to the last item, and
      // to merge literals at the and (we're parsing grouped ( sequences ) recursively and don't treat '|' specially
      // (GBNF's syntax is luckily very close to regular expressions!)
      const seq = [];

      const joinSeq = () => {
        const ret = [];
        for (const [isLiteral, g] of groupBy(seq, x => x[1])) {
          if (isLiteral) {
            ret.push([[...g].map(x => x[0]).join(''), true]);
          } else {
            ret.push(...g);
          }
        }
        if (ret.length === 1) {
          return ret[0];
        }
        return [ret.map(x => toRule(x)).join(' '), false];
      };

      while (i < length) {
        const c = pattern[i];
        if (c === '.') {
          seq.push([getDot(), false]);
          i += 1;
        } else if (c === '(') {
          i += 1;
          if (i < length) {
            if (pattern[i] === '?') {
              throw new Error(`Unsupported pattern syntax "${pattern[i]}" at index ${i} of /${pattern}/`);
            }
          }
          seq.push([`(${toRule(transform())})`, false]);
        } else if (c === ')') {
          i += 1;
          if (start <= 0 || pattern[start - 1] !== '(') {
            throw new Error(`Unbalanced parentheses; start = ${start}, i = ${i}, pattern = ${pattern}`);
          }
          return joinSeq();
        } else if (c === '[') {
          let squareBrackets = c;
          i += 1;
          while (i < length && pattern[i] !== ']') {
            if (pattern[i] === '\\') {
              squareBrackets += pattern.slice(i, i + 2);
              i += 2;
            } else {
              squareBrackets += pattern[i];
              i += 1;
            }
          }
          if (i >= length) {
            throw new Error(`Unbalanced square brackets; start = ${start}, i = ${i}, pattern = ${pattern}`);
          }
          squareBrackets += ']';
          i += 1;
          seq.push([squareBrackets, false]);
        } else if (c === '|') {
          seq.push(['|', false]);
          i += 1;
        } else if (c === '*' || c === '+' || c === '?') {
          seq[seq.length - 1] = [toRule(seq[seq.length - 1]) + c, false];
          i += 1;
        } else if (c === '{') {
          let curlyBrackets = c;
          i += 1;
          while (i < length && pattern[i] !== '}') {
            curlyBrackets += pattern[i];
            i += 1;
          }
          if (i >= length) {
            throw new Error(`Unbalanced curly brackets; start = ${start}, i = ${i}, pattern = ${pattern}`);
          }
          curlyBrackets += '}';
          i += 1;
          const nums = curlyBrackets.slice(1, -1).split(',').map(s => s.trim());
          let minTimes, maxTimes;
          if (nums.length === 1) {
            minTimes = parseInt(nums[0], 10);
            maxTimes = minTimes;
          } else {
            if (nums.length !== 2) {
              throw new Error(`Invalid quantifier ${curlyBrackets}`);
            }
            minTimes = nums[0] ? parseInt(nums[0], 10) : 0;
            maxTimes = nums[1] ? parseInt(nums[1], 10) : Infinity;
          }

          let [sub, subIsLiteral] = seq[seq.length - 1];

          if (!subIsLiteral) {
            let id = subRuleIds[sub];
            if (id === undefined) {
              id = this._addRule(`${name}-${Object.keys(subRuleIds).length + 1}`, sub);
              subRuleIds[sub] = id;
            }
            sub = id;
          }

          seq[seq.length - 1] = [
            _buildRepetition(subIsLiteral ? `"${sub}"` : sub, minTimes, maxTimes, {itemRuleIsLiteral: subIsLiteral}),
            false
          ];
        } else {
          let literal = '';
          while (i < length) {
            if (pattern[i] === '\\' && i < length - 1) {
              const next = pattern[i + 1];
              if (ESCAPED_IN_REGEXPS_BUT_NOT_IN_LITERALS.has(next)) {
                i += 1;
                literal += pattern[i];
                i += 1;
              } else {
                literal += pattern.slice(i, i + 2);
                i += 2;
              }
            } else if (pattern[i] === '"') {
              literal += '\\"';
              i += 1;
            } else if (!NON_LITERAL_SET.has(pattern[i]) &&
                (i === length - 1 || literal === '' || pattern[i + 1] === '.' || !NON_LITERAL_SET.has(pattern[i+1]))) {
              literal += pattern[i];
              i += 1;
            } else {
              break;
            }
          }
          if (literal !== '') {
            seq.push([literal, true]);
          }
        }
      }

      return joinSeq();
    };

    return this._addRule(name, "\"\\\"\" " + toRule(transform()) + " \"\\\"\" space")
  }

  _notStrings(strings) {
    class TrieNode {
      constructor() {
        this.children = {};
        this.isEndOfString = false;
      }

      insert(str) {
        let node = this;
        for (const c of str) {
          node = node.children[c] = node.children[c] || new TrieNode();
        }
        node.isEndOfString = true;
      }
    }

    const trie = new TrieNode();
    for (const s of strings) {
      trie.insert(s);
    }

    const charRuleName = this._addPrimitive('char', PRIMITIVE_RULES['char']);
    const out = ['["] ( '];

    const visit = (node) => {
      const rejects = [];
      let first = true;
      for (const c of Object.keys(node.children).sort()) {
        const child = node.children[c];
        rejects.push(c);
        if (first) {
          first = false;
        } else {
          out.push(' | ');
        }
        out.push(`[${c}]`);
        if (Object.keys(child.children).length > 0) {
          out.push(' (');
          visit(child);
          out.push(')');
        } else if (child.isEndOfString) {
          out.push(` ${charRuleName}+`);
        }
      }
      if (Object.keys(node.children).length > 0) {
        if (!first) {
          out.push(' | ');
        }
        out.push(`[^"${rejects.join('')}] ${charRuleName}*`);
      }
    };

    visit(trie);

    out.push(` )${trie.isEndOfString ? '' : '?'} ["] space`);
    return out.join('');
  }

  _resolveRef(ref) {
    let refName = ref.split('/').pop();
    if (!(refName in this._rules) && !this._refsBeingResolved.has(ref)) {
      this._refsBeingResolved.add(ref);
      const resolved = this._refs[ref];
      refName = this.visit(resolved, refName);
      this._refsBeingResolved.delete(ref);
    }
    return refName;
  }

  _generateConstantRule(value) {
    return this._formatLiteral(JSON.stringify(value));
  }

  visit(schema, name) {
    const schemaType = schema.type;
    const schemaFormat = schema.format;
    const ruleName = name in RESERVED_NAMES ? name + '-' : name == '' ? 'root' : name;

    const ref = schema.$ref;
    if (ref !== undefined) {
      return this._addRule(ruleName, this._resolveRef(ref));
    } else if (schema.oneOf || schema.anyOf) {
      return this._addRule(ruleName, this._generateUnionRule(name, schema.oneOf || schema.anyOf));
    } else if (Array.isArray(schemaType)) {
      return this._addRule(ruleName, this._generateUnionRule(name, schemaType.map(t => ({...schema, type: t}))));
    } else if ('const' in schema) {
      return this._addRule(ruleName, this._generateConstantRule(schema.const) + ' space');
    } else if ('enum' in schema) {
      const rule = '(' + schema.enum.map(v => this._generateConstantRule(v)).join(' | ') + ') space';
      return this._addRule(ruleName, rule);
    } else if ((schemaType === undefined || schemaType === 'object') &&
               ('properties' in schema ||
                ('additionalProperties' in schema && schema.additionalProperties !== true))) {
      const required = new Set(schema.required || []);
      const properties = Object.entries(schema.properties ?? {});
      return this._addRule(ruleName, this._buildObjectRule(properties, required, name, schema.additionalProperties));
    } else if ((schemaType === undefined || schemaType === 'object') && 'allOf' in schema) {
      const required = new Set();
      const properties = [];
      const addComponent = (compSchema, isRequired) => {
        const ref = compSchema.$ref;
        if (ref !== undefined) {
          compSchema = this._refs[ref];
        }

        if ('properties' in compSchema) {
          for (const [propName, propSchema] of Object.entries(compSchema.properties)) {
            properties.push([propName, propSchema]);
            if (isRequired) {
              required.add(propName);
            }
          }
        }
      };

      for (const t of schema.allOf) {
        if ('anyOf' in t) {
          for (const tt of t.anyOf) {
            addComponent(tt, false);
          }
        } else {
          addComponent(t, true);
        }
      }

      return this._addRule(ruleName, this._buildObjectRule(properties, required, name, null));
    } else if ((schemaType === undefined || schemaType === 'array') && ('items' in schema || 'prefixItems' in schema)) {
      const items = schema.items ?? schema.prefixItems;
      if (Array.isArray(items)) {
        return this._addRule(
          ruleName,
          '"[" space ' +
            items.map((item, i) => this.visit(item, `${name ?? ''}${name ? '-' : ''}tuple-${i}`)).join(' "," space ') +
            ' "]" space'
        );
      } else {
        const itemRuleName = this.visit(items, `${name ?? ''}${name ? '-' : ''}item`);
        const minItems = schema.minItems || 0;
        const maxItems = schema.maxItems;
        return this._addRule(ruleName, '"[" space ' + _buildRepetition(itemRuleName, minItems, maxItems, {separatorRule: '"," space'}) + ' "]" space');
      }
    } else if ((schemaType === undefined || schemaType === 'string') && 'pattern' in schema) {
      return this._visitPattern(schema.pattern, ruleName);
    } else if ((schemaType === undefined || schemaType === 'string') && /^uuid[1-5]?$/.test(schema.format || '')) {
      return this._addPrimitive(
        ruleName === 'root' ? 'root' : schemaFormat,
        PRIMITIVE_RULES['uuid']
      );
    } else if ((schemaType === undefined || schemaType === 'string') && `${schema.format}-string` in STRING_FORMAT_RULES) {
      const primName = `${schema.format}-string`
      return this._addRule(ruleName, this._addPrimitive(primName, STRING_FORMAT_RULES[primName]));
    } else if (schemaType === 'string' && ('minLength' in schema || 'maxLength' in schema)) {
      const charRuleName = this._addPrimitive('char', PRIMITIVE_RULES['char']);
      const minLen = schema.minLength || 0;
      const maxLen = schema.maxLength;
      return this._addRule(ruleName, '"\\\"" ' + _buildRepetition(charRuleName, minLen, maxLen) + ' "\\\"" space');
    } else if (schemaType === 'integer' && ('minimum' in schema || 'exclusiveMinimum' in schema || 'maximum' in schema || 'exclusiveMaximum' in schema)) {
      let minValue = null;
      let maxValue = null;
      if ('minimum' in schema) {
        minValue = schema.minimum;
      } else if ('exclusiveMinimum' in schema) {
        minValue = schema.exclusiveMinimum + 1;
      }
      if ('maximum' in schema) {
        maxValue = schema.maximum;
      } else if ('exclusiveMaximum' in schema) {
        maxValue = schema.exclusiveMaximum - 1;
      }

      const out = ["("];
      _generateMinMaxInt(minValue, maxValue, out);
      out.push(") space");
      return this._addRule(ruleName, out.join(''));
    } else if ((schemaType === 'object') || (Object.keys(schema).length === 0)) {
      return this._addRule(ruleName, this._addPrimitive('object', PRIMITIVE_RULES['object']));
    } else {
      if (!(schemaType in PRIMITIVE_RULES)) {
        throw new Error(`Unrecognized schema: ${JSON.stringify(schema)}`);
      }
      // TODO: support minimum, maximum, exclusiveMinimum, exclusiveMaximum at least for zero
      return this._addPrimitive(ruleName === 'root' ? 'root' : schemaType, PRIMITIVE_RULES[schemaType]);
    }
  }

  _addPrimitive(name, rule) {
    let n = this._addRule(name, rule.content);
    for (const dep of rule.deps) {
      const depRule = PRIMITIVE_RULES[dep] || STRING_FORMAT_RULES[dep];
      if (!depRule) {
        throw new Error(`Rule ${dep} not known`);
      }
      if (!(dep in this._rules)) {
        this._addPrimitive(dep, depRule);
      }
    }
    return n;
  }

  _buildObjectRule(properties, required, name, additionalProperties) {
    const propOrder = this._propOrder;
    // sort by position in prop_order (if specified) then by original order
    const sortedProps = properties.map(([k]) => k).sort((a, b) => {
      const orderA = propOrder[a] || Infinity;
      const orderB = propOrder[b] || Infinity;
      return orderA - orderB || properties.findIndex(([k]) => k === a) - properties.findIndex(([k]) => k === b);
    });

    const propKvRuleNames = {};
    for (const [propName, propSchema] of properties) {
      const propRuleName = this.visit(propSchema, `${name ?? ''}${name ? '-' : ''}${propName}`);
      propKvRuleNames[propName] = this._addRule(
        `${name ?? ''}${name ? '-' : ''}${propName}-kv`,
        `${this._formatLiteral(JSON.stringify(propName))} space ":" space ${propRuleName}`
      );
    }
    const requiredProps = sortedProps.filter(k => required.has(k));
    const optionalProps = sortedProps.filter(k => !required.has(k));

    if (additionalProperties) {
      const subName = `${name ?? ''}${name ? '-' : ''}additional`;
      const valueRule =
        additionalProperties != null && typeof additionalProperties === 'object' ? this.visit(additionalProperties, `${subName}-value`)
        : this._addPrimitive('value', PRIMITIVE_RULES['value']);

      const key_rule =
        sortedProps.length === 0 ? this._addPrimitive('string', PRIMITIVE_RULES['string'])
        : this._addRule(`${subName}-k`, this._notStrings(sortedProps));

      propKvRuleNames['*'] = this._addRule(
        `${subName}-kv`,
        `${key_rule} ":" space ${valueRule}`);
      optionalProps.push('*');
    }

    let rule = '"{" space ';
    rule += requiredProps.map(k => propKvRuleNames[k]).join(' "," space ');

    if (optionalProps.length > 0) {
      rule += ' (';
      if (requiredProps.length > 0) {
        rule += ' "," space ( ';
      }

      const getRecursiveRefs = (ks, firstIsOptional) => {
        const [k, ...rest] = ks;
        const kvRuleName = propKvRuleNames[k];
        let res;
        const commaRef = `( "," space ${kvRuleName} )`;
        if (firstIsOptional) {
          res = commaRef + (k === '*' ? '*' : '?');
        } else {
          res = kvRuleName + (k === '*' ? ' ' + commaRef + '*' : '');
        }
        if (rest.length > 0) {
          res += ' ' + this._addRule(
            `${name ?? ''}${name ? '-' : ''}${k}-rest`,
            getRecursiveRefs(rest, true)
          );
        }
        return res;
      };

      rule += optionalProps.map((_, i) => getRecursiveRefs(optionalProps.slice(i), false)).join(' | ');
      if (requiredProps.length > 0) {
        rule += ' )';
      }
      rule += ' )?';
    }

    rule += ' "}" space';

    return rule;
  }

  formatGrammar() {
    let grammar = '';
    for (const [name, rule] of Object.entries(this._rules).sort(([a], [b]) => a.localeCompare(b))) {
      grammar += `${name} ::= ${rule}\n`;
    }
    return grammar;
  }
}

// Helper function to group elements by a key function
function* groupBy(iterable, keyFn) {
  let lastKey = null;
  let group = [];
  for (const element of iterable) {
    const key = keyFn(element);
    if (lastKey !== null && key !== lastKey) {
      yield [lastKey, group];
      group = [];
    }
    group.push(element);
    lastKey = key;
  }
  if (group.length > 0) {
    yield [lastKey, group];
  }
}