
https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
Pull up input bits, check that output bits represent the count.
LEDs and resistors
| # | Input | Output |
|---|---|---|
| 0 | bit0 | count0 |
| 1 | bit1 | count1 |
| 2 | bit2 | count2 |
| 3 | bit3 | count3 |
| 4 | bit4 | count4 |
| 5 | bit5 | count5 |
| 6 | bit6 | count6 |
| 7 | bit7 | count7 |