test.py

def normalize(sign, v, modulus):
    """Compute sign*v mod modulus, where v in (-2*modulus,modulus); output in [0,modulus)."""
    ans = (sign*v) % modulus
    v += modulus & (v >> 256) # I think this line is wrong when v has 257th bit set and 256 unset
    c = (sign - 1) >> 1
    v = (v ^ c) - c
    v += modulus & (v >> 256)
    assert ans == (v % modulus), print(ans, v)
    return v

def normalize_var(sign, v, modulus):
    """Compute sign*v mod modulus, where v in (-2*modulus,modulus); output in [0,modulus)."""
    assert sign == 1 or sign == -1
    # v in (-2*modulus,modulus)
    if v < 0:
        v += modulus
    # v in (-modulus,modulus)
    if sign == -1:
        v =-v
    if v < 0:
        v += modulus
    # v in [0,modulus)
    # assert v >=0 and v < modulas
    return v

sign = 1
v = -136834439519537824422704808331812250261221278517316523901929417126851299120761
secp_mod = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141

assert -2* secp_mod < v < secp_mod, "Initial condition not satisfied"
assert normalize(sign, v, secp_mod) == normalize_var(sign, v, secp_mod), "Normalizations must be equal"