'use strict';

var Promise = require('./../deps/promise');
var explain404 = require('./../deps/ajax/explain404');
var pouchCollate = require('pouchdb-collate');
var collate = pouchCollate.collate;

var CHECKPOINT_VERSION = 1;
var REPLICATOR = "pouchdb";
// This is an arbitrary number to limit the
// amount of replication history we save in the checkpoint.
// If we save too much, the checkpoing docs will become very big,
// if we save fewer, we'll run a greater risk of having to
// read all the changes from 0 when checkpoint PUTs fail
// CouchDB 2.0 has a more involved history pruning,
// but let's go for the simple version for now.
var CHECKPOINT_HISTORY_SIZE = 5;
var LOWEST_SEQ = 0;

function updateCheckpoint(db, id, checkpoint, session, returnValue) {
  return db.get(id).catch(function (err) {
    if (err.status === 404) {
      if (db.type() === 'http') {
        explain404('PouchDB is just checking if a remote checkpoint exists.');
      }
      return {
        session_id: session,
        _id: id,
        history: [],
        replicator: REPLICATOR,
        version: CHECKPOINT_VERSION
      };
    }
    throw err;
  }).then(function (doc) {
    if (returnValue.cancelled) {
      return;
    }
    // Filter out current entry for this replication
    doc.history = (doc.history || []).filter(function (item) {
      return item.session_id !== session;
    });

    // Add the latest checkpoint to history
    doc.history.unshift({
      last_seq: checkpoint,
      session_id: session
    });

    // Just take the last pieces in history, to
    // avoid really big checkpoint docs.
    // see comment on history size above
    doc.history = doc.history.slice(0, CHECKPOINT_HISTORY_SIZE);

    doc.version = CHECKPOINT_VERSION;
    doc.replicator = REPLICATOR;

    doc.session_id = session;
    doc.last_seq = checkpoint;

    return db.put(doc).catch(function (err) {
      if (err.status === 409) {
        // retry; someone is trying to write a checkpoint simultaneously
        return updateCheckpoint(db, id, checkpoint, session, returnValue);
      }
      throw err;
    });
  });
}

function Checkpointer(src, target, id, returnValue) {
  this.src = src;
  this.target = target;
  this.id = id;
  this.returnValue = returnValue;
}

Checkpointer.prototype.writeCheckpoint = function (checkpoint, session) {
  var self = this;
  return this.updateTarget(checkpoint, session).then(function () {
    return self.updateSource(checkpoint, session);
  });
};

Checkpointer.prototype.updateTarget = function (checkpoint, session) {
  return updateCheckpoint(this.target, this.id, checkpoint,
      session, this.returnValue);
};

Checkpointer.prototype.updateSource = function (checkpoint, session) {
  var self = this;
  if (this.readOnlySource) {
    return Promise.resolve(true);
  }
  return updateCheckpoint(this.src, this.id, checkpoint,
      session, this.returnValue)
    .catch(function (err) {
      var isForbidden = typeof err.status === 'number' &&
        Math.floor(err.status / 100) === 4;
      if (isForbidden) {
        self.readOnlySource = true;
        return true;
      }
      throw err;
    });
};

var comparisons = {
  "undefined": function(targetDoc, sourceDoc) {
    // This is the previous comparison function
    if (collate(targetDoc.last_seq, sourceDoc.last_seq) === 0) {
      return sourceDoc.last_seq;
    }

    return 0;
  },
  "1": function(targetDoc, sourceDoc) {
    // This is the comparison function ported from CouchDB
    return compareReplicationLogs(sourceDoc, targetDoc).last_seq;
  }
};

Checkpointer.prototype.getCheckpoint = function () {
  var self = this;
  return self.target.get(self.id).then(function (targetDoc) {
    return self.src.get(self.id).then(function (sourceDoc) {
      // Since we can't migrate an old version doc to a new one
      // (no session id), we just go with the lowest seq in this case
      if (targetDoc.version !==
         sourceDoc.version) {
        return LOWEST_SEQ;
      }

      var version;
      if (targetDoc.version) {
        version = targetDoc.version.toString();
      } else {
        version = "undefined";
      }

      if (version in comparisons) {
        return comparisons[version](targetDoc, sourceDoc);
      }

      return LOWEST_SEQ;
    }, function (err) {
      if (err.status === 404 && targetDoc.last_seq) {
        return self.src.put({
          _id: self.id,
          last_seq: LOWEST_SEQ
        }).then(function () {
          return LOWEST_SEQ;
        }, function (err) {
          if (err.status === 401) {
            self.readOnlySource = true;
            return targetDoc.last_seq;
          }
          return LOWEST_SEQ;
        });
      }
      throw err;
    });
  }).catch(function (err) {
    if (err.status !== 404) {
      throw err;
    }
    return LOWEST_SEQ;
  });
};
// This checkpoint comparison is ported from CouchDBs source
// they come from here:
// https://github.com/apache/couchdb-couch-replicator/blob/master/src/couch_replicator.erl#L863-L906

function compareReplicationLogs (srcDoc, tgtDoc) {
  if (srcDoc.session_id === tgtDoc.session_id) {
    return {
      last_seq: srcDoc.last_seq,
      history: srcDoc.history || []
    };
  }

  var sourceHistory = srcDoc.history || [];
  var targetHistory = tgtDoc.history || [];
  return compareReplicationHistory(sourceHistory, targetHistory);
}

function compareReplicationHistory (sourceHistory, targetHistory) {
  // the erlang loop via function arguments is not so easy to repeat in JS
  // therefore, doing this as recursion
  var S = sourceHistory[0];
  var sourceRest = sourceHistory.slice(1);
  var T = targetHistory[0];
  var targetRest = targetHistory.slice(1);

  if (!S || targetHistory.length === 0) {
    return {
      last_seq: LOWEST_SEQ,
      history: []
    };
  }

  var sourceId = S.session_id;
  if (hasSessionId(sourceId, targetHistory)) {
    return {
      last_seq: S.last_seq,
      history: sourceHistory
    };
  }

  var targetId = T.session_id;
  if (hasSessionId(targetId, sourceRest)) {
    return {
      last_seq: T.last_seq,
      history: targetRest
    };
  }

  return compareReplicationHistory(sourceRest, targetRest);
}

function hasSessionId (sessionId, history) {
  var props = history[0];
  var rest = history.slice(1);

  if (!sessionId || history.length === 0) {
    return false;
  }

  if (sessionId === props.session_id) {
    return true;
  }

  return hasSessionId(sessionId, rest);
}


module.exports = Checkpointer;