systemjs/lib/extension-map.js

/*
  SystemJS map support
  
  Provides map configuration through
    System.map['jquery'] = 'some/module/map'

  As well as contextual map config through
    System.map['bootstrap'] = {
      jquery: 'some/module/map2'
    }

  Note that this applies for subpaths, just like RequireJS

  jquery      -> 'some/module/map'
  jquery/path -> 'some/module/map/path'
  bootstrap   -> 'bootstrap'

  Inside any module name of the form 'bootstrap' or 'bootstrap/*'
    jquery    -> 'some/module/map2'
    jquery/p  -> 'some/module/map2/p'

  Maps are carefully applied from most specific contextual map, to least specific global map
*/
function map(loader) {
  loader.map = loader.map || {};

  // return if prefix parts (separated by '/') match the name
  // eg prefixMatch('jquery/some/thing', 'jquery') -> true
  //    prefixMatch('jqueryhere/', 'jquery') -> false
  function prefixMatch(name, prefix) {
    if (name.length < prefix.length)
      return false;
    if (name.substr(0, prefix.length) != prefix)
      return false;
    if (name[prefix.length] && name[prefix.length] != '/')
      return false;
    return true;
  }

  // get the depth of a given path
  // eg pathLen('some/name') -> 2
  function pathLen(name) {
    var len = 1;
    for (var i = 0, l = name.length; i < l; i++)
      if (name[i] === '/')
        len++;
    return len;
  }

  function doMap(name, matchLen, map) {
    return map + name.substr(matchLen);
  }

  // given a relative-resolved module name and normalized parent name,
  // apply the map configuration
  function applyMap(name, parentName, loader) {
    var curMatch, curMatchLength = 0;
    var curParent, curParentMatchLength = 0;
    var tmpParentLength, tmpPrefixLength;
    var subPath;
    var nameParts;
    
    // first find most specific contextual match
    if (parentName) {
      for (var p in loader.map) {
        var curMap = loader.map[p];
        if (typeof curMap != 'object')
          continue;

        // most specific parent match wins first
        if (!prefixMatch(parentName, p))
          continue;

        tmpParentLength = pathLen(p);
        if (tmpParentLength <= curParentMatchLength)
          continue;

        for (var q in curMap) {
          // most specific name match wins
          if (!prefixMatch(name, q))
            continue;
          tmpPrefixLength = pathLen(q);
          if (tmpPrefixLength <= curMatchLength)
            continue;

          curMatch = q;
          curMatchLength = tmpPrefixLength;
          curParent = p;
          curParentMatchLength = tmpParentLength;
        }
      }
    }

    // if we found a contextual match, apply it now
    if (curMatch)
      return doMap(name, curMatch.length, loader.map[curParent][curMatch]);

    // now do the global map
    for (var p in loader.map) {
      var curMap = loader.map[p];
      if (typeof curMap != 'string')
        continue;

      if (!prefixMatch(name, p))
        continue;

      var tmpPrefixLength = pathLen(p);

      if (tmpPrefixLength <= curMatchLength)
        continue;

      curMatch = p;
      curMatchLength = tmpPrefixLength;
    }

    if (curMatch)
      return doMap(name, curMatch.length, loader.map[curMatch]);

    return name;
  }

  var loaderNormalize = loader.normalize;
  loader.normalize = function(name, parentName, parentAddress) {
    var loader = this;
    if (!loader.map)
      loader.map = {};

    var isPackage = false;
    if (name.substr(name.length - 1, 1) == '/') {
      isPackage = true;
      name += '#';
    }

    return Promise.resolve(loaderNormalize.call(loader, name, parentName, parentAddress))
    .then(function(name) {
      name = applyMap(name, parentName, loader);

      // Normalize "module/" into "module/module"
      // Convenient for packages
      if (isPackage) {
        var nameParts = name.split('/');
        nameParts.pop();
        var pkgName = nameParts.pop();
        nameParts.push(pkgName);
        nameParts.push(pkgName);
        name = nameParts.join('/');
      }

      return name;
    });
  }
}