irresistible.py

# released under CC0: https://creativecommons.org/publicdomain/zero/1.0/

from decimal import Decimal


DEFAULT_TOLERANCE = "20"
COLOR_ALIASES = {
    # alternative names/spellings
    "grey": "gray",
    "purple": "violet",

    # shortcuts
    # (black/blue/brown is annoying)
    "pk": "pink",
    "s": "silver",
    "gd": "gold",
    "bk": "black",
    "br": "brown",
    "bn": "brown",
    "r": "red",
    "o": "orange",
    "y": "yellow",
    "gn": "green",
    "bu": "blue",
    "be": "blue",
    "v": "violet",
    "pu": "violet", # purple
    "gy": "gray",
    "w": "white",
}
IMPRECISE_COLOR_ALIASES = {
    "b": 'use "bk" for black, "br" or "bn" for brown, "bu" or "be" for blue',
    "g": 'use "gn" for green, "gy" for gray, "gd" for gold',
    "p": 'use "pu" for purple (or "v" for violet), "pk" for pink',
}
COLOR_TO_DIGIT = {
    "black": 0,
    "brown": 1,
    "red": 2,
    "orange": 3,
    "yellow": 4,
    "green": 5,
    "blue": 6,
    "violet": 7,
    "gray": 8,
    "white": 9,
}
COLOR_TO_POWER = {
    "pink": -3,
    "silver": -2,
    "gold": -1,
    "black": 0,
    "brown": 1,
    "red": 2,
    "orange": 3,
    "yellow": 4,
    "green": 5,
    "blue": 6,
    "violet": 7,
    "gray": 8,
    "white": 9,
}
COLOR_TO_TOLERANCE = {
    "silver": "10",
    "gold": "5",
    "brown": "1",
    "red": "2",
    "orange": "0.05",
    "yellow": "0.02",
    "green": "0.5",
    "blue": "0.25",
    "violet": "0.1",
    "gray": "0.01",
}
INDEX_TO_BAND_ORDINAL = [
    "1st",
    "2nd",
    "3rd",
    "4th",
    "5th",
]
SI_PREFIXES_POSITIVE = ["", "k", "M", "G", "T", "P", "E", "Z", "Y"]
SI_PREFIXES_NEGATIVE = ["", "m", "μ", "n", "p", "f", "a", "z", "y"]


def decode_resistor(spec: str) -> str:
    # lowercase the input, split on spaces (removing empty words)
    spec = spec.lower()
    colors = [color for color in spec.split(" ") if color]

    # map color aliases to their canonical name
    for i, color in enumerate(colors):
        unaliased_color = COLOR_ALIASES.get(color, None)
        if unaliased_color is not None:
            colors[i] = unaliased_color
            continue

        annoyed_message = IMPRECISE_COLOR_ALIASES.get(color, None)
        if annoyed_message is not None:
            raise ValueError(annoyed_message)

    # ensure correct count (also handle single black band)
    if len(colors) == 1 and colors[0] == "black":
        return "0 Ω"
    if len(colors) not in (4, 5):
        raise ValueError("need 4 or 5 colors, got {0}".format(len(colors)))

    # assemble the digits
    value = Decimal(0)
    for i, color in enumerate(colors[:-2]):
        try:
            digit = COLOR_TO_DIGIT[color]
        except KeyError as ex:
            msg = "{0} band (digit) has unknown color {1}".format(
                INDEX_TO_BAND_ORDINAL[i], repr(color)
            )
            raise ValueError(msg) from ex

        value *= Decimal(10)
        value += Decimal(digit)

    # multiply by the power of 10
    try:
        power = COLOR_TO_POWER[colors[-2]]
    except KeyError as ex:
        i = len(colors) - 2
        msg = "{0} band (multiplier) has unknown color {1}".format(
            INDEX_TO_BAND_ORDINAL[i], repr(colors[-2])
        )
        raise ValueError(msg) from ex

    value *= Decimal(10) ** Decimal(power)

    # get tolerance
    try:
        tolerance = COLOR_TO_TOLERANCE[colors[-1]]
    except KeyError as ex:
        i = len(colors) - 1
        msg = "{0} band (tolerance) has unknown color {1}".format(
            INDEX_TO_BAND_ORDINAL[i], repr(colors[-1])
        )
        raise ValueError(msg) from ex

    # simplify to the nearest SI prefix
    si_prefix = 0
    if not value.is_zero:
        thousand = Decimal(1000)
        thousandth = Decimal("0.001")

        while value >= thousand:
            value /= thousand
            si_prefix += 1

        while value <= thousandth:
            value *= thousand
            si_prefix -= 1

    # assemble the final text
    return "{0} {1}Ω (± {2}%)".format(
        value,
        SI_PREFIXES_POSITIVE[si_prefix] if si_prefix >= 0 else SI_PREFIXES_NEGATIVE[-si_prefix],
        tolerance,
    )


if __name__ == "__main__":
    import sys
    for color_string in sys.argv[1:]:
        try:
            decoded = decode_resistor(color_string)
        except ValueError as ex:
            decoded = ex.args[0]
        print("{0}: {1}".format(color_string, decoded))