Coding - Decoding
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Letter shift coding
Shift list
\text{code}_i = \text{letter}_i + d_i \pmod{26}
d is the shift for place i, found from the sample. Above 26, subtract 26.
Decoding
\text{letter}_i = \text{code}_i - d_i \pmod{26}
To decode, subtract the same shifts. At 0 or below, add 26.
Number coding
Position sum
\text{code} = \sum p_i
p = position of a letter, A = 1 … Z = 26.
Sum with an extra step
\text{code} = k \cdot \sum p_i + r
Find k (multiply) or r (add) from the sample. Check with a second sample if one is given.
Weighted sum
\text{code} = \sum i \cdot p_i
i = place of the letter in the word (1st, 2nd, 3rd …).
Reverse positions
p^{\text{rev}} = 27 - p
Z counts as 1 and A as 26.