#!/usr/bin/env python3

import math
import re
import decimal
from typing import Set

si_table = {
    -8: ("yocto", "y", "y", 10e-24),
    -7: ("zepto", "z", "z", 10e-21),
    -6: ("atto", "a", "a", 10e-18),
    -5: ("femto", "f", "f", 10e-15),
    -4: ("pico", "p", "p", 10e-12),
    -3: ("nano", "n", "n", 10e-9),
    -2: ("micro", "u", "μ", 10e-6),
    -1: ("milli", "m", "m", 10e-3),
    0: ("", "", "", 0),
    1: ("kilo", "k", "k", 10e3),
    2: ("mega", "M", "M", 10e6),
    3: ("giga", "G", "G", 10e9),
    4: ("tera", "T", "T", 10e12),
    5: ("peta", "P", "P", 10e15),
    6: ("exa", "E", "E", 10e18),
    7: ("zetta", "Z", "Z", 10e21),
    8: ("yotta", "Y", "Y", 10e24),
}

_si_letters_set: Set[str] = set()
for n in si_table:
    _si_letters_set.add(si_table[n][1])
    _si_letters_set.add(si_table[n][2])

si_letters = "".join(sorted(_si_letters_set))


def prefix_tuple(value, utf=True):
    value = float(value)
    ex = math.log(abs(value * 1.0000001), 10)
    nex = math.floor(ex) // 3
    nval = value / 10 ** (nex * 3)
    if nex not in si_table:
        raise ValueError("No suitable SI prefix found")
    if utf == True:
        px = si_table[nex][2]
    else:
        px = si_table[nex][1]
    return (nval, px)


def prefix(value, utf=True, replace_separator=True):
    v, px = prefix_tuple(value, utf)
    if px == "":
        return "{:1.2f}".format(v).rstrip("0").rstrip(".")
    elif replace_separator == True:
        s = "{:1.2f}".format(v).replace(".", px).rstrip("0").rstrip(".")
        return s
    else:
        s = "{:1.2f}".format(v).rstrip("0").rstrip(".")
        return s + " {:s}".format(px)


def power_by_prefix(string):
    for v in si_table:
        if string in si_table[v]:
            return v
    raise ValueError()


def decode_infix(string):
    np = "^(([1-9][0-9]*)?[0-9](\.[0-9]+)?) ?([{:s}])?$".format(si_letters)
    ni = "^(([1-9][0-9]*)?[0-9](([{:s}])[0-9]+)?)$".format(si_letters)
    rp = re.match(np, string)
    ri = re.match(ni, string)
    if rp is not None:
        if rp.groups()[3] is not None:
            power = power_by_prefix(rp.groups()[3])
        else:
            power = 0
        base = decimal.Decimal(rp.groups()[0])
        return base * decimal.Decimal("10") ** (power * 3)
    elif ri is not None:
        base = decimal.Decimal(str(ri.groups()[0]).replace(ri.groups()[3], "."))
        power = power_by_prefix(ri.groups()[3])
        return base * decimal.Decimal("10") ** (power * 3)
    else:
        raise ValueError()


if __name__ == "__main__":
    print(prefix(1.5632e11))
    print(prefix(12.32e-9))
    print(prefix(47000))
    print(prefix(4700))
    print(prefix(470))

    print(si_letters)
    print(decode_infix("0.23"))
    print(decode_infix("470M"))
    print(prefix(float(decode_infix("0k23"))))