@fabriek it feels almost like one of those assembler optimisations but for people
(Division is slow so invert and multiply):(Subtraction is slow so invert and add)
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subtraction by radix complement
@derwinmcgeary @fabriek That's because this is the decimal version of how subtraction is commonly done in hardware. For binary, replace 9 with 1 and 10 with 2 (two's complement). More info: https://en.wikipedia.org/wiki/Method_of_complements
