Map exchange string format

This is a technical description of the map exchange string format, used to share map generating configurations with other users.

The outer layer of the map exchange string format includes three angle brackets on either side: ">>>" and "<<<", which must be present for Factorio to accept the string. Between those two tokens is the map exchange data, encoded using base 64 as defined by RFC 4648 (or 3548, 2535, 2045, 1421, et al.) All numerical values are stored in unsigned little-endian format. This document will use the terms "int" to refer to 4 byte numbers and "short" to refer to 2 byte numbers. Strings are Pascal-style int length prefixed strings. The format described below is valid at least for Factorio 0.14.x, other versions may use a different version scheme.

short[4]	The version string of Factorio that generated this string, used to determine encoding format.
byte	Water frequency.
byte	Water size.
ore_def[int]	An array of ore definitions (described later). The array length is stored as an integer prefix. Factorio doesn’t care what order these are in, but it always alphabetizes during export. Vanilla has six ores: coal, copper-ore, crude-oil, enemy-base, iron-ore, and stone. Unknown ores are ignored and missing ores are set to their defaults.
int	Map seed.
int	Map width.
int	Map height.
byte	Starting area size.
byte	Peaceful mode. Enabled if set to 1, disabled otherwise.
int	CRC32 checksum of all preceding data, as defined by ANSI X3.66 / FIPS 71 / ITU-T V.42 (the same one used by zlib, ethernet, etc.)