var path = require('path')
// path.isAbsolute shim for Node.js 0.10 support
path.isAbsolute = (path.isAbsolute) ? path.isAbsolute : require('path-is-absolute')
var fs = require('graceful-fs')

/**
 * Function that returns two types of paths, one relative to symlink, and one
 * relative to the current working directory. Checks if path is absolute or
 * relative. If the path is relative, this function checks if the path is
 * relative to symlink or relative to current working directory. This is an
 * initiative to find a smarter `srcpath` to supply when building symlinks.
 * This allows you to determine which path to use out of one of three possible
 * types of source paths. The first is an absolute path. This is detected by
 * `path.isAbsolute()`. When an absolute path is provided, it is checked to
 * see if it exists. If it does it's used, if not an error is returned
 * (callback)/ thrown (sync). The other two options for `srcpath` are a
 * relative url. By default Node's `fs.symlink` works by creating a symlink
 * using `dstpath` and expects the `srcpath` to be relative to the newly
 * created symlink. If you provide a `srcpath` that does not exist on the file
 * system it results in a broken symlink. To minimize this, the function
 * checks to see if the 'relative to symlink' source file exists, and if it
 * does it will use it. If it does not, it checks if there's a file that
 * exists that is relative to the current working directory, if does its used.
 * This preserves the expectations of the original fs.symlink spec and adds
 * the ability to pass in `relative to current working direcotry` paths.
 */

function symlinkPaths (srcpath, dstpath, callback) {
  if (path.isAbsolute(srcpath)) {
    return fs.lstat(srcpath, function (err, stat) {
      if (err) {
        err.message = err.message.replace('lstat', 'ensureSymlink')
        return callback(err)
      }
      return callback(null, {
        'toCwd': srcpath,
        'toDst': srcpath
      })
    })
  } else {
    var dstdir = path.dirname(dstpath)
    var relativeToDst = path.join(dstdir, srcpath)
    return fs.exists(relativeToDst, function (exists) {
      if (exists) {
        return callback(null, {
          'toCwd': relativeToDst,
          'toDst': srcpath
        })
      } else {
        return fs.lstat(srcpath, function (err, stat) {
          if (err) {
            err.message = err.message.replace('lstat', 'ensureSymlink')
            return callback(err)
          }
          return callback(null, {
            'toCwd': srcpath,
            'toDst': path.relative(dstdir, srcpath)
          })
        })
      }
    })
  }
}

function symlinkPathsSync (srcpath, dstpath) {
  var exists
  if (path.isAbsolute(srcpath)) {
    exists = fs.existsSync(srcpath)
    if (!exists) throw new Error('absolute srcpath does not exist')
    return {
      'toCwd': srcpath,
      'toDst': srcpath
    }
  } else {
    var dstdir = path.dirname(dstpath)
    var relativeToDst = path.join(dstdir, srcpath)
    exists = fs.existsSync(relativeToDst)
    if (exists) {
      return {
        'toCwd': relativeToDst,
        'toDst': srcpath
      }
    } else {
      exists = fs.existsSync(srcpath)
      if (!exists) throw new Error('relative srcpath does not exist')
      return {
        'toCwd': srcpath,
        'toDst': path.relative(dstdir, srcpath)
      }
    }
  }
}

module.exports = {
  'symlinkPaths': symlinkPaths,
  'symlinkPathsSync': symlinkPathsSync
}