select.js

// of several indexes available,
// select the one most suitable for a given query.

// select the index that matches the most exact fields in the query
// starting from the left, then moving on to range fields.

var Q = require('map-filter-reduce/util')
var u = require('./util')

function _max (ary, score) {
  var j = -1; var M = 0
  for (var i = 0; i < ary.length; i++) {
    var m = score(ary[i])
    if (m > M) {
      M = m; j = i
    }
  }
  return ary[j]
}

module.exports = function select (indexes, query) {
  function score (k) {
    var v = u.get(k, query)
    return u.has(k, query) ? (
      Q.isExact(v) ? 3
        : Q.isRange(v) ? 2
          : 1
    ) : 0
  }

  return _max(indexes, function (index) {
    var s = 0; var _s
    for (var i = 0; i < index.value.length; i++) {
      _s = score(index.value[i])
      if (!_s) return s // stop counting when one thing doesn't match
      s = s * s + _s
    }
    return s
  })
}