Rímere

According to the Old English Dictionary:

es;

rímere

m. A computer, reckoner, calculator :-- Betwux ðisre sprǽce sceal se rímre geþencean, ðæt hé gedó ðæt Februarius mónþ ðý geáre hæbbe þrittig nihta ealdne mónan, Anglia viii. 307, 34. rimere