lib/private/base64.bzl

"""Base64 encoder/decoder utility for Starlark

Handles string only and not binary data

Implementation based on https://gist.github.com/trondhumbor/ce57c0c2816bb45a8fbb but adapted to use
the subset of python available in Starlark.
"""

load(":strings.bzl", "chr", "ord")

def decode(data):
    """Decode a Base64 encoded string.

    See https://en.wikipedia.org/wiki/Base64.

    Args:
        data: base64 encoded string

    Returns:
        A string containing the decoded data
    """
    padding = data.count("=")
    data = data.replace("=", "A")

    binstring = ""
    for i in range(len(data)):
        index = BASE64_CHARS.find(data[i])
        if index == -1:
            fail("expected a base64 encoded string")
        binstring += _int_to_binary(index, 6)

    eight_chunks = _chunk(binstring, 8)

    outstring = ""
    for chunk in eight_chunks:
        outstring += chr(int(chunk, 2))

    return outstring if padding == 0 else outstring[:-padding]

def encode(data):
    """Base64 encode a string.

    See https://en.wikipedia.org/wiki/Base64.

    Args:
        data: string to encode

    Returns:
        The base64 encoded string
    """
    padding = 0
    if len(data) % 3 != 0:
        padding = (len(data) + 3 - len(data) % 3) - len(data)
    data += "\0" * padding

    three_chunks = _chunk(data, 3)

    binstring = ""
    for chunk in three_chunks:
        for i in range(len(chunk)):
            binstring += _int_to_binary(ord(chunk[i]))

    six_chunks = _chunk(binstring, 6)

    outstring = ""
    for element in six_chunks:
        outstring += BASE64_CHARS[int(element, 2)]

    return outstring if padding == 0 else outstring[:-padding] + "=" * padding

def _chunk(data, length):
    return [data[i:i + length] for i in range(0, len(data), length)]

BASE64_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

# Starlark support octal values that we can leverage for this conversion.
# Generated by lib/private/string.py utility script.
INT_TO_BINARY = [
    "00000000",
    "00000001",
    "00000010",
    "00000011",
    "00000100",
    "00000101",
    "00000110",
    "00000111",
    "00001000",
    "00001001",
    "00001010",
    "00001011",
    "00001100",
    "00001101",
    "00001110",
    "00001111",
    "00010000",
    "00010001",
    "00010010",
    "00010011",
    "00010100",
    "00010101",
    "00010110",
    "00010111",
    "00011000",
    "00011001",
    "00011010",
    "00011011",
    "00011100",
    "00011101",
    "00011110",
    "00011111",
    "00100000",
    "00100001",
    "00100010",
    "00100011",
    "00100100",
    "00100101",
    "00100110",
    "00100111",
    "00101000",
    "00101001",
    "00101010",
    "00101011",
    "00101100",
    "00101101",
    "00101110",
    "00101111",
    "00110000",
    "00110001",
    "00110010",
    "00110011",
    "00110100",
    "00110101",
    "00110110",
    "00110111",
    "00111000",
    "00111001",
    "00111010",
    "00111011",
    "00111100",
    "00111101",
    "00111110",
    "00111111",
    "01000000",
    "01000001",
    "01000010",
    "01000011",
    "01000100",
    "01000101",
    "01000110",
    "01000111",
    "01001000",
    "01001001",
    "01001010",
    "01001011",
    "01001100",
    "01001101",
    "01001110",
    "01001111",
    "01010000",
    "01010001",
    "01010010",
    "01010011",
    "01010100",
    "01010101",
    "01010110",
    "01010111",
    "01011000",
    "01011001",
    "01011010",
    "01011011",
    "01011100",
    "01011101",
    "01011110",
    "01011111",
    "01100000",
    "01100001",
    "01100010",
    "01100011",
    "01100100",
    "01100101",
    "01100110",
    "01100111",
    "01101000",
    "01101001",
    "01101010",
    "01101011",
    "01101100",
    "01101101",
    "01101110",
    "01101111",
    "01110000",
    "01110001",
    "01110010",
    "01110011",
    "01110100",
    "01110101",
    "01110110",
    "01110111",
    "01111000",
    "01111001",
    "01111010",
    "01111011",
    "01111100",
    "01111101",
    "01111110",
    "01111111",
    "10000000",
    "10000001",
    "10000010",
    "10000011",
    "10000100",
    "10000101",
    "10000110",
    "10000111",
    "10001000",
    "10001001",
    "10001010",
    "10001011",
    "10001100",
    "10001101",
    "10001110",
    "10001111",
    "10010000",
    "10010001",
    "10010010",
    "10010011",
    "10010100",
    "10010101",
    "10010110",
    "10010111",
    "10011000",
    "10011001",
    "10011010",
    "10011011",
    "10011100",
    "10011101",
    "10011110",
    "10011111",
    "10100000",
    "10100001",
    "10100010",
    "10100011",
    "10100100",
    "10100101",
    "10100110",
    "10100111",
    "10101000",
    "10101001",
    "10101010",
    "10101011",
    "10101100",
    "10101101",
    "10101110",
    "10101111",
    "10110000",
    "10110001",
    "10110010",
    "10110011",
    "10110100",
    "10110101",
    "10110110",
    "10110111",
    "10111000",
    "10111001",
    "10111010",
    "10111011",
    "10111100",
    "10111101",
    "10111110",
    "10111111",
    "11000000",
    "11000001",
    "11000010",
    "11000011",
    "11000100",
    "11000101",
    "11000110",
    "11000111",
    "11001000",
    "11001001",
    "11001010",
    "11001011",
    "11001100",
    "11001101",
    "11001110",
    "11001111",
    "11010000",
    "11010001",
    "11010010",
    "11010011",
    "11010100",
    "11010101",
    "11010110",
    "11010111",
    "11011000",
    "11011001",
    "11011010",
    "11011011",
    "11011100",
    "11011101",
    "11011110",
    "11011111",
    "11100000",
    "11100001",
    "11100010",
    "11100011",
    "11100100",
    "11100101",
    "11100110",
    "11100111",
    "11101000",
    "11101001",
    "11101010",
    "11101011",
    "11101100",
    "11101101",
    "11101110",
    "11101111",
    "11110000",
    "11110001",
    "11110010",
    "11110011",
    "11110100",
    "11110101",
    "11110110",
    "11110111",
    "11111000",
    "11111001",
    "11111010",
    "11111011",
    "11111100",
    "11111101",
    "11111110",
    "11111111",
]

def _int_to_binary(i, digits = 8):
    if i < 0 or i > 255:
        fail("expected a int between 0 and 255 (inclusive)")
    if digits < 1 or digits > 8:
        fail("expected digits to be between 1 and 8 (inclusive)")
    return INT_TO_BINARY[i][8 - digits:]