cbz2ebook: Use mt map for opening image and zip

This apparently solves some weird issues with reading the zip file like

zlib.error: Error -3 while decompressing data: invalid block type
zlib.error: Error -3 while decompressing data: invalid distance too far back

Though it reduced the amount of times this happened, some files still
persistently refuse to be read by zipfile in parallel. In those cases,
using `-s` to force single-threading should be sufficient.

cbz2ebook.py

@@ -5,8 +5,6 @@ from concurrent.futures import ThreadPoolExecutor
 from tqdm import tqdm
 from zipfile import ZipFile
 import argparse
-import sys
-import zlib
 
 
 def should_scale(in_width, in_height, out_width, out_height):
@@ -48,31 +46,6 @@ def process_image(im):
     return im
 
 
-def process_archive(srczip, dstzip, multithread=True):
-    executor = ThreadPoolExecutor()
-
-    if multithread:
-        mapper = executor.map
-    else:
-        mapper = map
-
-    srcfiles = sorted(
-        map(
-            lambda zi: zi.filename,
-            filter(lambda zi: not zi.is_dir(), srczip.infolist()),
-        )
-    )
-
-    for idx, im in enumerate(
-        tqdm(
-            mapper(process_image, mapper(Image.open, mapper(srczip.open, srcfiles))),
-            total=len(srcfiles),
-        )
-    ):
-        with dstzip.open(f"{idx+1:04d}.jpg", "w") as outpagefile:
-            im.save(outpagefile, format="JPEG", quality=92)
-
-
 parser = argparse.ArgumentParser()
 parser.add_argument("infile")
 parser.add_argument("outfile")
@@ -84,14 +57,22 @@ args = parser.parse_args()
 srczip = ZipFile(args.infile)
 dstzip = ZipFile(args.outfile, "x")
 
+srcfiles = sorted(
+    map(lambda zi: zi.filename, filter(lambda zi: not zi.is_dir(), srczip.infolist(),),)
+)
+
+executor = ThreadPoolExecutor()
+
 if args.singlethreaded:
-    process_archive(srczip, dstzip, multithread=False)
+    mapper = map
 else:
-    try:
-        process_archive(srczip, dstzip, multithread=True)
-    except zlib.error as e:
-        print(f"zlib choked ({e}), retrying in single threaded mode", file=sys.stderr)
-        # replace partial archive with new empty one
-        dstzip.close()
-        dstzip = ZipFile(args.outfile, "w")
-        process_archive(srczip, dstzip, multithread=False)
+    mapper = executor.map
+
+for idx, im in enumerate(
+    tqdm(
+        mapper(process_image, mapper(Image.open, mapper(srczip.open, srcfiles)),),
+        total=len(srcfiles),
+    )
+):
+    with dstzip.open(f"{idx+1:04d}.jpg", "w") as outpagefile:
+        im.save(outpagefile, format="JPEG", quality=92)