fix(dash): add SIDX parsing for SegmentBase manifests and deduplicate multi-period segments

Multi-period DASH manifests using SegmentBase with shared BaseURLs were downloading the entire file once per period, causing massive file size inflation. Parse the SIDX box to extract proper per-segment byte ranges and deduplicate identical segments across periods.

unshackle/core/manifests/dash.py

@@ -6,6 +6,7 @@ import logging
 import math
 import re
 import shutil
+import struct
 import sys
 from copy import copy
 from functools import partial
@@ -330,7 +331,7 @@ class DASH:
         period_count = len(content_periods)
 
         if period_count > 1:
-            log.info(f"Multi-period manifest detected with {period_count} content periods")
+            log.debug(f"Multi-period manifest detected with {period_count} content periods")
 
         for period_idx, content_period in enumerate(content_periods):
             # Find the matching representation in this period
@@ -371,7 +372,9 @@ class DASH:
                 if p_kid and track_kid and p_kid != track_kid:
                     log.debug(f"Period {content_period.get('id', period_idx)} has different KID: {p_kid}")
 
-            segments.extend(p_segments)
+            for seg in p_segments:
+                if seg not in segments:
+                    segments.append(seg)
             segment_durations.extend(p_durations)
 
         if not segments:
@@ -568,6 +571,49 @@ class DASH:
 
         progress(downloaded="Downloaded")
 
+    @staticmethod
+    def _parse_sidx(data: bytes, index_range: str) -> list[str]:
+        """Parse a SIDX box to extract per-segment byte ranges."""
+        # Find the sidx box in the data
+        offset = 0
+        while offset < len(data) - 8:
+            box_size = struct.unpack(">I", data[offset:offset + 4])[0]
+            if box_size < 8 or data[offset + 4:offset + 8] != b"sidx":
+                offset += max(box_size, 8)
+                continue
+
+            pos = offset + 8
+            version = data[pos]
+            pos += 4  # version + flags
+            pos += 4  # reference_ID
+            pos += 4  # timescale
+
+            if version == 0:
+                first_offset = struct.unpack(">I", data[pos + 4:pos + 8])[0]
+                pos += 8
+            else:
+                first_offset = struct.unpack(">Q", data[pos + 8:pos + 16])[0]
+                pos += 16
+
+            pos += 2  # reserved
+            reference_count = struct.unpack(">H", data[pos:pos + 2])[0]
+            pos += 2
+
+            idx_end = int(index_range.split("-")[1])
+            current_offset = idx_end + 1 + first_offset
+            segments = []
+
+            for _ in range(reference_count):
+                ref_size = struct.unpack(">I", data[pos:pos + 4])[0] & 0x7FFFFFFF
+                pos += 12  # ref_info + subseg_duration + SAP fields
+                seg_end = current_offset + ref_size - 1
+                segments.append(f"{current_offset}-{seg_end}")
+                current_offset = seg_end + 1
+
+            return segments
+
+        return []
+
     @staticmethod
     def _is_content_period(period: Element, filtered_period_ids: list[str]) -> bool:
         """Check if a period is a valid content period (not an ad, not filtered, not trick mode)."""
@@ -768,7 +814,19 @@ class DASH:
                 if total_size:
                     media_range = f"{len(init_data)}-{total_size}"
 
-            segments.append((rep_base_url, media_range))
+            # Parse SIDX box from indexRange to get per-segment byte ranges
+            index_range = segment_base.get("indexRange")
+            if index_range:
+                sidx_res = session.get(url=rep_base_url, headers={"Range": f"bytes={index_range}"})
+                sidx_res.raise_for_status()
+                sidx_segments = DASH._parse_sidx(sidx_res.content, index_range)
+                if sidx_segments:
+                    for seg_range in sidx_segments:
+                        segments.append((rep_base_url, seg_range))
+                else:
+                    segments.append((rep_base_url, media_range))
+            else:
+                segments.append((rep_base_url, media_range))
         elif rep_base_url:
             segments.append((rep_base_url, None))