var _ = require('../lodash')

module.exports = tarjan

function tarjan (g) {
  let index = 0
  const stack = []
  const visited = {} // node id -> { onStack, lowlink, index }
  const results = []

  function dfs (v) {
    var entry = visited[v] = {
      onStack: true,
      lowlink: index,
      index: index++
    }
    stack.push(v)

    g.successors(v).forEach(function (w) {
      if (!_.has(visited, w)) {
        dfs(w)
        entry.lowlink = Math.min(entry.lowlink, visited[w].lowlink)
      } else if (visited[w].onStack) {
        entry.lowlink = Math.min(entry.lowlink, visited[w].index)
      }
    })

    if (entry.lowlink === entry.index) {
      const cmpt = []
      let w
      do {
        w = stack.pop()
        visited[w].onStack = false
        cmpt.push(w)
      } while (v !== w)
      results.push(cmpt)
    }
  }

  g.nodes().forEach(function (v) {
    if (!_.has(visited, v)) {
      dfs(v)
    }
  })

  return results
}