var fs = require('fs');
var path = require('path');
var cst = require('../../constants.js')

// XP's system default value for `PATHEXT` system variable, just in case it's not
// set on Windows.
var XP_DEFAULT_PATHEXT = '.com;.exe;.bat;.cmd;.vbs;.vbe;.js;.jse;.wsf;.wsh';

// For earlier versions of NodeJS that doesn't have a list of constants (< v6)
var FILE_EXECUTABLE_MODE = 1;

function statFollowLinks() {
  return fs.statSync.apply(fs, arguments);
}

function isWindowsPlatform() {
  return cst.IS_WINDOWS;
}

// Cross-platform method for splitting environment `PATH` variables
function splitPath(p) {
  return p ? p.split(path.delimiter) : [];
}

// Tests are running all cases for this func but it stays uncovered by codecov due to unknown reason
/* istanbul ignore next */
function isExecutable(pathName) {
  try {
    // TODO(node-support): replace with fs.constants.X_OK once remove support for node < v6
    fs.accessSync(pathName, FILE_EXECUTABLE_MODE);
  } catch (err) {
    return false;
  }
  return true;
}

function checkPath(pathName) {
  return fs.existsSync(pathName) && !statFollowLinks(pathName).isDirectory()
    && (isWindowsPlatform() || isExecutable(pathName));
}

//@
//@ ### which(command)
//@
//@ Examples:
//@
//@ ```javascript
//@ var nodeExec = which('node');
//@ ```
//@
//@ Searches for `command` in the system's `PATH`. On Windows, this uses the
//@ `PATHEXT` variable to append the extension if it's not already executable.
//@ Returns a [ShellString](#shellstringstr) containing the absolute path to
//@ `command`.
function _which(cmd) {
  if (!cmd) console.error('must specify command');

  var options = {}

  var isWindows = isWindowsPlatform();
  var pathArray = splitPath(process.env.PATH);

  var queryMatches = [];

  // No relative/absolute paths provided?
  if (cmd.indexOf('/') === -1) {
    // Assume that there are no extensions to append to queries (this is the
    // case for unix)
    var pathExtArray = [''];
    if (isWindows) {
      // In case the PATHEXT variable is somehow not set (e.g.
      // child_process.spawn with an empty environment), use the XP default.
      var pathExtEnv = process.env.PATHEXT || XP_DEFAULT_PATHEXT;
      pathExtArray = splitPath(pathExtEnv.toUpperCase());
    }

    // Search for command in PATH
    for (var k = 0; k < pathArray.length; k++) {
      // already found it
      if (queryMatches.length > 0 && !options.all) break;

      var attempt = path.resolve(pathArray[k], cmd);

      if (isWindows) {
        attempt = attempt.toUpperCase();
      }

      var match = attempt.match(/\.[^<>:"/|?*.]+$/);
      if (match && pathExtArray.indexOf(match[0]) >= 0) { // this is Windows-only
        // The user typed a query with the file extension, like
        // `which('node.exe')`
        if (checkPath(attempt)) {
          queryMatches.push(attempt);
          break;
        }
      } else { // All-platforms
        // Cycle through the PATHEXT array, and check each extension
        // Note: the array is always [''] on Unix
        for (var i = 0; i < pathExtArray.length; i++) {
          var ext = pathExtArray[i];
          var newAttempt = attempt + ext;
          if (checkPath(newAttempt)) {
            queryMatches.push(newAttempt);
            break;
          }
        }
      }
    }
  } else if (checkPath(cmd)) { // a valid absolute or relative path
    queryMatches.push(path.resolve(cmd));
  }

  if (queryMatches.length > 0) {
    return options.all ? queryMatches : queryMatches[0];
  }
  return options.all ? [] : null;
}

module.exports = _which;