image digest: single read, cache file read & hash

For image digests cache file read & hash, one read per image
file

Fix issue: images referenced from several objects were read and
hashed once per reference.

  [as an example in the sisu-manual sm_tux.png accounts for 6 of
  the document's image records (12 across the sample set), each
  of them a fresh read of the same bytes, and source_pod.d hashes
  the file again for digests.txt.]

Memoise on the full path within safeComputeHashAndSize. static is
thread-local in D, so documents processed in parallel each get
their own cache and no synchronisation is involved. Files are not
expected to change during a run.

Output is unchanged: identical .ssp output. (An image that is
missing or unreadable now warns once to stdout rather than once
per reference).

(assisted by Claude-Code)

1 files changed, 11 insertions, 1 deletions

diff --git a/src/sisudoc/ocda/meta/metadoc_from_src_functions.d b/src/sisudoc/ocda/meta/metadoc_from_src_functions.d
index 6a000ea..cc79a67 100644
--- a/src/sisudoc/ocda/meta/metadoc_from_src_functions.d
+++ b/src/sisudoc/ocda/meta/metadoc_from_src_functions.d
@@ -3190,7 +3190,16 @@ template docAbstractionFunctions() {
     M                    manifested,
   ) {
     ST_file_name_hash_size_ safeComputeHashAndSize(N,P)(N filename, P file_path) @trusted {
-      auto file_with_path = file_path ~ filename;
+      string file_with_path = (file_path ~ filename).to!string;
+      /+ ↓ read & hash each image file once: an image referenced from several
+         objects (or from several documents sharing an image directory) was
+         otherwise re-read and re-hashed for every reference. static is
+         thread-local in D, so the cache needs no synchronisation
+      +/
+      static ST_file_name_hash_size_[string] _image_file_info_cache;
+      if (auto _cached = file_with_path in _image_file_info_cache) {
+        return *_cached;
+      }
       ST_file_name_hash_size_ _file_info;
       _file_info.fileName = filename;
       /+ ↓ a referenced image that is absent or unreadable is recorded as
@@ -3226,6 +3235,7 @@ template docAbstractionFunctions() {
           }
         }
       }
+      _image_file_info_cache[file_with_path] = _file_info;
       return _file_info;
     }
     obj.metainfo.sha256.text = obj.text.sha256Of;