This is a compression format used by EA Games

The header takes up the first 0x15 bytes of the compressed file

ovln_parse.py

def bytes_to_int24(arr, offset):
    return (arr[offset + 0] << 16) | (arr[offset + 1] << 8) | arr[offset + 2]
 
# get 4 bytes from the array and arrange them in little endian order.
def bytes_to_int_reversed(arr, offset):
    arr_boundary = len(arr) - offset # for safety
    if arr_boundary >= 4:
        return (arr[offset + 3] << 24) | (arr[offset + 2] << 16) | (arr[offset + 1] << 8) | arr[offset]
    elif arr_boundary == 3:
        return (arr[offset + 2] << 16) | (arr[offset + 1] << 8) | arr[offset]
    elif arr_boundary == 2:
        return (arr[offset + 1] << 8) | arr[offset]
    else:
        return arr[offset]
 
def ovln_memset(dst, dst_offset, value, amount):
    for i in range(amount):
        dst[dst_offset + i] = value
 
def ovln_memcpy(dst, dst_offset, dst_backwards_offset, amount):
    src_offset = dst_offset - dst_backwards_offset
    for i in range(amount):
        dst[dst_offset + i] = dst[src_offset + i]
 
def ovln_fetch_bytes(dst, dst_offset, dst_backwards_offset, amount):
    if dst_backwards_offset > 1:
        ovln_memcpy(dst, dst_offset, dst_backwards_offset, amount)
    else:
        ovln_memset(dst, dst_offset, dst[dst_offset - dst_backwards_offset], amount)
    return dst_offset + amount
 
def ovln_copy_literal(dst, dst_offset, src, src_offset, amount):
    for i in range(amount):
        dst[dst_offset + i] = src[src_offset + i]
 
# Input: src = compressed data list
# Output: returns decompressed data list
def ovln_parse(src):
    decompressed_size = bytes_to_int24(src, 0x12)
    dst = [0] * decompressed_size # Decompressed data list
    src_offset = 0x15
    dst_offset = 0
    while True:
        cmd = bytes_to_int_reversed(src, src_offset)
        if cmd & 0x80 == 0: # Covers commands 00 to 7F
            src_offset += 2
            # Copy bytes from the compressed data array
            amount_to_copy = cmd & 0x3
            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
            src_offset += amount_to_copy
            dst_offset += amount_to_copy
            # Fetch bytes from previous decompressed data
            backward_fetch_offset = ((cmd << 3) & 0x300) + ((cmd >> 8) & 0xFF) + 1
            amount_of_bytes_to_fetch = ((cmd >> 2) & 0x7) + 3
            dst_offset = ovln_fetch_bytes(dst, dst_offset, backward_fetch_offset, amount_of_bytes_to_fetch)
            continue
        if cmd & 0x40 == 0: # Covers commands 0x80 to 0xBF
            # Copy bytes from the compressed data array
            src_offset += 3
            amount_to_copy = ((cmd >> 8) >> 6) & 0x3
            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
            src_offset += amount_to_copy
            dst_offset += amount_to_copy
            # Fetch bytes from previous decompressed data
            backward_fetch_offset = (((cmd >> 8) << 8) & 0x3F00) + ((cmd >> 16) & 0xFF) + 1
            amount_of_bytes_to_fetch = (cmd & 0x3F) + 4
            dst_offset = ovln_fetch_bytes(dst, dst_offset, backward_fetch_offset, amount_of_bytes_to_fetch)
            continue
        if cmd & 0x20 == 0: # Covers commands 0xC0 to 0xDF
            src_offset += 4
            amount_to_copy = cmd & 0x3
            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
            src_offset += amount_to_copy
            dst_offset += amount_to_copy
            # Fetch bytes from previous decompressed data
            backward_fetch_offset = ((cmd << 12) & 0x10000) + (((cmd >> 8) << 8) & 0xFF00) + 1 + ((cmd >> 16) & 0xFF)
            amount_of_bytes_to_fetch = ((cmd << 6) & 0x300) + ((cmd >> 24) & 0xFF) + 5
            dst_offset = ovln_fetch_bytes(dst, dst_offset, backward_fetch_offset, amount_of_bytes_to_fetch)
            continue
        src_offset += 1
        if (cmd & 0xFF) < 0xFC: # Covers commands 0xE0 to 0xFB
            # Copy bytes from the compressed data array
            amount_to_copy = ((cmd & 0x1F) + 1) * 4
            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
            src_offset += amount_to_copy
            dst_offset += amount_to_copy
            continue
        if cmd & 3 != 0: # Copy any left-over bytes
            # Copy bytes from the compressed data array
            amount_to_copy = cmd & 0x3
            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
            src_offset += amount_to_copy
            dst_offset += amount_to_copy
        break # A command of 0xFC, 0xFD, 0xFE, or 0xFF will end the decompression routine.
    return dst