#CJam, 129
26mr:D;q~:B;{B,I8m<f+[0_X256]ff+[Z6]2*f.{\1{2md7\#\j_**2G#)%}j\K+%D+26%'A+}:R[1f=R3f=R2f%]"\//\X"2/.f*'|f*Sf*~\_'{f>"-+"f=}fI;]N* [Try it online][1]
Uses only letters. I can explain… later.
[1]: http://cjam.aditsu.net/#code=26mr%3AD%3Bq~%3AB%3B%7BB%2CI8m%3Cf%2B%5B0_X256%5Dff%2B%5BZ6%5D2*f.%7B%5C1%7B2md7%5C%23%5Cj_**2G%23)%25%7Dj%5CK%2B%25D%2B26%25'A%2B%7D%3AR%5B1f%3DR3f%3DR2f%25%5D%22%5C%2F%2F%5CX%222%2F.f*'%7CfSf~%5C_'%7Bf%3E%22-%2B%22f%3D%7DfI%3B%5DN*&input=20%2012