source_map_decoder.py

import json
import sys


BASE64_ALPHABET = ("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
                   "0123456789+/")
BASE64_TABLE = dict(zip(BASE64_ALPHABET, range(64)))


def decode_vlq(s):
    octets = [BASE64_TABLE[c] for c in s]

    i = 0
    res = []

    while i < len(octets):
        cur = 0
        cont = True
        shift = 0

        while cont:
            cont = octets[i] & 0x20
            val = octets[i] & 0x1f

            cur |= val << shift
            i += 1
            shift += 5

        neg = cur & 1
        cur >>= 1

        if neg:
            res.append(-cur)
        else:
            res.append(cur)

    print(s, octets, res)

    return res


def main():
    with open(sys.argv[1]) as f:
        source_map = json.load(f)
        mappings = source_map["mappings"]
        sources = source_map["sources"]

        mapping = [0] * 5

        for line, line_mapping in enumerate(mappings.split(";")):
            mapping[0] = 0

            for col_mapping in line_mapping.split(","):
                res = decode_vlq(col_mapping)
                res += [0] * (5 - len(res))

                mapping = [x + y for x, y in zip(mapping, res)]

                print(f"{line}:{mapping[0]} -> "
                      f"{sources[mapping[1]]}:{mapping[2]}:{mapping[3]}")

            print("-----------------------")


if __name__ == "__main__":
    main()