python 3 only version cuz I'm too lazy to make backwards compatible, use https://gist.github.com/NWPlayer123/05dd55dd1cce6eb5457741ce14f48587 for 2

yaz0dec3.py

from struct import unpack
import sys

with open(sys.argv[1], "rb") as f:
    base = ".".join(sys.argv[1].split(".")[:-1])
    with open(base + ".sarc", "wb+") as o:
        assert f.read(4) == b"Yaz0"
        unc_size = unpack(">I", f.read(4))[0]
        o.write(b"\x00" * unc_size) #pre-allocate
        o.seek(0);f.seek(16)
        mask = 0
        while unc_size != 0:
            if not mask: #read new flag byte
                mask = 0x80
                flag = ord(f.read(1))
            if flag & mask: #read one byte
                o.write(f.read(1))
                unc_size -= 1
            else: #RLE stuff
                code = (ord(f.read(1)) << 8) | ord(f.read(1))
                back = (code & 0xFFF) + 1
                if code >> 12:
                    size = (code >> 12) + 2
                else: #read third byte
                    size = ord(f.read(1)) + 0x12
                unc_size -= size
                if size > back: #overlap, cache it
                    o.seek(back * -1, 1)
                    buff = list(o.read(size + back))
                    for i in range(size):
                        buff[back + i] = buff[i]
                    o.seek((size + back) * -1, 1)
                    o.write(bytes(buff))
                else:
                    o.seek(back * -1, 1)
                    data = o.read(size)
                    o.seek(back - size, 1)
                    o.write(data)
            mask >>= 1