/**
 *  Copyright (c) 2019 GraphQL Contributors
 *  All rights reserved.
 *
 *  This source code is licensed under the BSD-style license found in the
 *  LICENSE file in the root directory of this source tree. An additional grant
 *  of patent rights can be found in the PATENTS file in the same directory.
 */

/**
 * This JSON parser simply walks the input, generating an AST. Use this in lieu
 * of JSON.parse if you need character offset parse errors and an AST parse tree
 * with location information.
 *
 * If an error is encountered, a SyntaxError will be thrown, with properties:
 *
 *   - message: string
 *   - start: int - the start inclusive offset of the syntax error
 *   - end: int - the end exclusive offset of the syntax error
 *
 */
export default function jsonParse(str) {
  string = str;
  strLen = str.length;
  start = end = lastEnd = -1;
  ch();
  lex();
  const ast = parseObj();
  expect('EOF');
  return ast;
}

let string;
let strLen;
let start;
let end;
let lastEnd;
let code;
let kind;

function parseObj() {
  const nodeStart = start;
  const members = [];
  expect('{');
  if (!skip('}')) {
    do {
      members.push(parseMember());
    } while (skip(','));
    expect('}');
  }
  return {
    kind: 'Object',
    start: nodeStart,
    end: lastEnd,
    members,
  };
}

function parseMember() {
  const nodeStart = start;
  const key = kind === 'String' ? curToken() : null;
  expect('String');
  expect(':');
  const value = parseVal();
  return {
    kind: 'Member',
    start: nodeStart,
    end: lastEnd,
    key,
    value,
  };
}

function parseArr() {
  const nodeStart = start;
  const values = [];
  expect('[');
  if (!skip(']')) {
    do {
      values.push(parseVal());
    } while (skip(','));
    expect(']');
  }
  return {
    kind: 'Array',
    start: nodeStart,
    end: lastEnd,
    values,
  };
}

function parseVal() {
  switch (kind) {
    case '[':
      return parseArr();
    case '{':
      return parseObj();
    case 'String':
    case 'Number':
    case 'Boolean':
    case 'Null':
      const token = curToken();
      lex();
      return token;
  }
  return expect('Value');
}

function curToken() {
  return { kind, start, end, value: JSON.parse(string.slice(start, end)) };
}

function expect(str) {
  if (kind === str) {
    lex();
    return;
  }

  let found;
  if (kind === 'EOF') {
    found = '[end of file]';
  } else if (end - start > 1) {
    found = '`' + string.slice(start, end) + '`';
  } else {
    const match = string.slice(start).match(/^.+?\b/);
    found = '`' + (match ? match[0] : string[start]) + '`';
  }

  throw syntaxError(`Expected ${str} but found ${found}.`);
}

function syntaxError(message) {
  return { message, start, end };
}

function skip(k) {
  if (kind === k) {
    lex();
    return true;
  }
}

function ch() {
  if (end < strLen) {
    end++;
    code = end === strLen ? 0 : string.charCodeAt(end);
  }
}

function lex() {
  lastEnd = end;

  while (code === 9 || code === 10 || code === 13 || code === 32) {
    ch();
  }

  if (code === 0) {
    kind = 'EOF';
    return;
  }

  start = end;

  switch (code) {
    // "
    case 34:
      kind = 'String';
      return readString();
    // -, 0-9
    case 45:
    case 48:
    case 49:
    case 50:
    case 51:
    case 52:
    case 53:
    case 54:
    case 55:
    case 56:
    case 57:
      kind = 'Number';
      return readNumber();
    // f
    case 102:
      if (string.slice(start, start + 5) !== 'false') {
        break;
      }
      end += 4;
      ch();

      kind = 'Boolean';
      return;
    // n
    case 110:
      if (string.slice(start, start + 4) !== 'null') {
        break;
      }
      end += 3;
      ch();

      kind = 'Null';
      return;
    // t
    case 116:
      if (string.slice(start, start + 4) !== 'true') {
        break;
      }
      end += 3;
      ch();

      kind = 'Boolean';
      return;
  }

  kind = string[start];
  ch();
}

function readString() {
  ch();
  while (code !== 34 && code > 31) {
    if (code === 92) {
      // \
      ch();
      switch (code) {
        case 34: // "
        case 47: // /
        case 92: // \
        case 98: // b
        case 102: // f
        case 110: // n
        case 114: // r
        case 116: // t
          ch();
          break;
        case 117: // u
          ch();
          readHex();
          readHex();
          readHex();
          readHex();
          break;
        default:
          throw syntaxError('Bad character escape sequence.');
      }
    } else if (end === strLen) {
      throw syntaxError('Unterminated string.');
    } else {
      ch();
    }
  }

  if (code === 34) {
    ch();
    return;
  }

  throw syntaxError('Unterminated string.');
}

function readHex() {
  if (
    (code >= 48 && code <= 57) || // 0-9
    (code >= 65 && code <= 70) || // A-F
    (code >= 97 && code <= 102) // a-f
  ) {
    return ch();
  }
  throw syntaxError('Expected hexadecimal digit.');
}

function readNumber() {
  if (code === 45) {
    // -
    ch();
  }

  if (code === 48) {
    // 0
    ch();
  } else {
    readDigits();
  }

  if (code === 46) {
    // .
    ch();
    readDigits();
  }

  if (code === 69 || code === 101) {
    // E e
    ch();
    if (code === 43 || code === 45) {
      // + -
      ch();
    }
    readDigits();
  }
}

function readDigits() {
  if (code < 48 || code > 57) {
    // 0 - 9
    throw syntaxError('Expected decimal digit.');
  }
  do {
    ch();
  } while (code >= 48 && code <= 57); // 0 - 9
}