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)

4 files changed, 22 insertions, 4 deletions

diff --git a/org/ocda_functions.org b/org/ocda_functions.org
index 71d1b97..0e42cfb 100644
--- a/org/ocda_functions.org
+++ b/org/ocda_functions.org
@@ -3430,7 +3430,16 @@ SHA_256_Hashes_ obj_digest(M)(
   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
@@ -3466,6 +3475,7 @@ SHA_256_Hashes_ obj_digest(M)(
         }
       }
     }
+    _image_file_info_cache[file_with_path] = _file_info;
     return _file_info;
   }
   obj.metainfo.sha256.text = obj.text.sha256Of;
diff --git a/org/out_src_abstraction_ocda_ssp.org b/org/out_src_abstraction_ocda_ssp.org
index 64af5cc..a14773e 100644
--- a/org/out_src_abstraction_ocda_ssp.org
+++ b/org/out_src_abstraction_ocda_ssp.org
@@ -283,7 +283,6 @@ template spineAbstractionTxt() {
               ~ " missing:true"
             : ".image: "          ~ i.fileName
               ~ " sha256:"        ~ i.fileHash_sha256.toHexString.to!string
-              ~ " sha256:"        ~ i.fileHash_sha256.toHexString.to!string
               ~ " bytes:"         ~ i.fileSize.to!string
               ~ ((i.imageWidth > 0 && i.imageHeight > 0)
                 ? " px:"          ~ i.imageWidth.to!string
diff --git a/src/sisudoc/ocda/abstraction/ssp.d b/src/sisudoc/ocda/abstraction/ssp.d
index 25cc467..1d1ea22 100644
--- a/src/sisudoc/ocda/abstraction/ssp.d
+++ b/src/sisudoc/ocda/abstraction/ssp.d
@@ -301,7 +301,6 @@ template spineAbstractionTxt() {
               ~ " missing:true"
             : ".image: "          ~ i.fileName
               ~ " sha256:"        ~ i.fileHash_sha256.toHexString.to!string
-              ~ " sha256:"        ~ i.fileHash_sha256.toHexString.to!string
               ~ " bytes:"         ~ i.fileSize.to!string
               ~ ((i.imageWidth > 0 && i.imageHeight > 0)
                 ? " px:"          ~ i.imageWidth.to!string
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;