Eudc is an extension that provides functions for character set conversion between UTF8 and SJIS, UTF8 and EUC_JP, including user defined characters.
Eudc was developed by Itagaki Takahiro of NTT as part of the textsearch-ja project, but maintenance seems to have stopped since around 2012. This code is taken over and adapted for recent PostgreSQL versions.
For more information about using eudc, see doc/eudc.md