Program compressor

Question

Choose a language and write a program in the language that:

• Take a valid program of the language as input,
• Output a string(bytearray, etc.). It needn't be a valid program.
• Only finite valid inputs result in longer/same-length(in bytes) output.
• Different valid input, even if behave same, lead to different output.

Let length of your code as \$L\$ and number of inputs with longer/equal-length output as \$n\$. To win, you should be lowest on at least one of:

• \$L\$,
• \$N\$,
• \$L+N\$,
• \$L+\log_2(N+1)\$.

Notes

• Solution exist so long as illegal substring exist.

Answer 1

Python 3.8 (pre-release), \$L=79\$, \$N\approx255^{1500}\$

^{-3 bytes thanks to @Jonathan Allan}

f=lambda b,s=0,l=0:b and f(b[1:],255*s+b[0],l+1)or s.to_bytes(l-(l>1500),"big")

Try it online!

Python source code can't contain null bytes. This converts the input (bytestring) to base 255 and then back to bytes. After 1500 removes a redundant byte.