a10

  ECE8473xxxxx.....

  EC E8 47 3? ?? .....

  h[0] == 0xEC
  h[1] == 0xE8
  h[2] == 0x47
  (h[3] & 0xF0) == 0x30
  or: (h[3] >> 4) == 0x3
  or: h[3] == 0x30 || h[3] == 0x31 || ... || h[3] == 0x3F
  or: h[3] >= 0x30 && h[3] <= 0x3F

---

logic values: 0 is false, non-zero is true

if( 1.9) ... true

!3.79 -> 0
!0 -> 1

---

$ GCC -O2 -o p1 pthread.c -pthread
$ GCC -O2 -o p4 pthread2.c -pthread
$ lscpu | grep '^CPU(s)'
CPU(s):                1
$ time ./p1
main got: hi from f
main got: f x 03a0f637 ans abcdef

real    0m6.643s
user    0m6.635s
sys     0m0.000s
$ time ./p4
main got: f 1 1073741824 2147483647
main got: f 2 2147483648 3221225471
main got: f 3 3221225472 4294967295
main got: f 0 0 1073741823
main got: f x 40836e91 ans abcdef

real    0m3.414s
user    0m3.408s
sys     0m0.000s
$

---

Using inverse of FNV prime to run hash in reverse:

$ python3
>>> p=16777619			# FNV prime
>>> n=1<<32			# modulus
>>> t=1<<31			# totient
>>> pow(p,t,n)			# Euler
1
>>> pi=pow(p,t-1,n)		# p inverse, MOD n
>>> pi
899433627
>>> (p*pi)%n
1
>>> mask=0xffffffff		# (x AND mask) same as (x MOD n)
>>> (p*pi)&mask
1
>>> init=2166136261		# FNV offset basis
>>> data=123456			# random data
>>> h=((init^data)*p) & mask	# hash
>>> h
3329211999
>>> ((h*pi)^data) & mask	# reverse hash, back to init
2166136261
>>>